webm-writer.js

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/**
 * A tool for presenting an ArrayBuffer as a stream for writing some simple data types.
 * 
 * By Nicholas Sherlock
 * 
 * Released under the WTFPLv2 https://en.wikipedia.org/wiki/WTFPL
 */
 
"use strict";
 
(function(){
    /*
     * Create an ArrayBuffer of the given length and present it as a writable stream with methods
     * for writing data in different formats.
     */
    var ArrayBufferDataStream = function(length) {
        this.data = new Uint8Array(length);
        this.pos = 0;
    };
    
    ArrayBufferDataStream.prototype.seek = function(offset) {
        this.pos = offset;
    };
 
    ArrayBufferDataStream.prototype.writeBytes = function(arr) {
        for (var i = 0; i < arr.length; i++) {
            this.data[this.pos++] = arr[i];
        }
    };
 
    ArrayBufferDataStream.prototype.writeByte = function(b) {
        this.data[this.pos++] = b;
    };
    
    //Synonym:
    ArrayBufferDataStream.prototype.writeU8 = ArrayBufferDataStream.prototype.writeByte;
    
    ArrayBufferDataStream.prototype.writeU16BE = function(u) {
        this.data[this.pos++] = u >> 8;
        this.data[this.pos++] = u;
    };
 
    ArrayBufferDataStream.prototype.writeDoubleBE = function(d) {
        var 
            bytes = new Uint8Array(new Float64Array([d]).buffer);
        
        for (var i = bytes.length - 1; i >= 0; i--) {
            this.writeByte(bytes[i]);
        }
    };
 
    ArrayBufferDataStream.prototype.writeFloatBE = function(d) {
        var 
            bytes = new Uint8Array(new Float32Array([d]).buffer);
        
        for (var i = bytes.length - 1; i >= 0; i--) {
            this.writeByte(bytes[i]);
        }
    };
 
    /**
     * Write an ASCII string to the stream
     */
    ArrayBufferDataStream.prototype.writeString = function(s) {
        for (var i = 0; i < s.length; i++) {
            this.data[this.pos++] = s.charCodeAt(i);
        }
    };
 
    /**
     * Write the given 32-bit integer to the stream as an EBML variable-length integer using the given byte width 
     * (use measureEBMLVarInt).
     * 
     * No error checking is performed to ensure that the supplied width is correct for the integer.
     * 
     * @param i Integer to be written
     * @param width Number of bytes to write to the stream
     */
    ArrayBufferDataStream.prototype.writeEBMLVarIntWidth = function(i, width) {
        switch (width) {
            case 1:
                this.writeU8((1 << 7) | i);
            break;
            case 2:
                this.writeU8((1 << 6) | (i >> 8));
                this.writeU8(i);
            break;
            case 3:
                this.writeU8((1 << 5) | (i >> 16));
                this.writeU8(i >> 8);
                this.writeU8(i);
            break;
            case 4:
                this.writeU8((1 << 4) | (i >> 24));
                this.writeU8(i >> 16);
                this.writeU8(i >> 8);
                this.writeU8(i);
            break;
            case 5:
                /* 
                 * JavaScript converts its doubles to 32-bit integers for bitwise operations, so we need to do a 
                 * division by 2^32 instead of a right-shift of 32 to retain those top 3 bits
                 */
                this.writeU8((1 << 3) | ((i / 4294967296) & 0x7)); 
                this.writeU8(i >> 24);
                this.writeU8(i >> 16);
                this.writeU8(i >> 8);
                this.writeU8(i);
            break;
            default:
                throw new RuntimeException("Bad EBML VINT size " + width);
        }
    };
    
    /**
     * Return the number of bytes needed to encode the given integer as an EBML VINT.
     */
    ArrayBufferDataStream.prototype.measureEBMLVarInt = function(val) {
        if (val < (1 << 7) - 1) { 
            /* Top bit is set, leaving 7 bits to hold the integer, but we can't store 127 because
             * "all bits set to one" is a reserved value. Same thing for the other cases below:
             */
            return 1;
        } else if (val < (1 << 14) - 1) {
            return 2;
        } else if (val < (1 << 21) - 1) {
            return 3;
        } else if (val < (1 << 28) - 1) {
            return 4;
        } else if (val < 34359738367) { // 2 ^ 35 - 1 (can address 32GB)
            return 5;
        } else {
            throw new RuntimeException("EBML VINT size not supported " + val);
        }
    };
    
    ArrayBufferDataStream.prototype.writeEBMLVarInt = function(i) {
        this.writeEBMLVarIntWidth(i, this.measureEBMLVarInt(i));
    };
    
    /**
     * Write the given unsigned 32-bit integer to the stream in big-endian order using the given byte width.
     * No error checking is performed to ensure that the supplied width is correct for the integer.
     * 
     * Omit the width parameter to have it determined automatically for you.
     * 
     * @param u Unsigned integer to be written
     * @param width Number of bytes to write to the stream
     */
    ArrayBufferDataStream.prototype.writeUnsignedIntBE = function(u, width) {
        if (width === undefined) {
            width = this.measureUnsignedInt(u);
        }
        
        // Each case falls through:
        switch (width) {
            case 5:
                this.writeU8(Math.floor(u / 4294967296)); // Need to use division to access >32 bits of floating point var
            case 4:
                this.writeU8(u >> 24);
            case 3:
                this.writeU8(u >> 16);
            case 2:
                this.writeU8(u >> 8);
            case 1:
                this.writeU8(u);
            break;
            default:
                throw new RuntimeException("Bad UINT size " + width);
        }
    };
    
    /**
     * Return the number of bytes needed to hold the non-zero bits of the given unsigned integer.
     */
    ArrayBufferDataStream.prototype.measureUnsignedInt = function(val) {
        // Force to 32-bit unsigned integer
        if (val < (1 << 8)) {
            return 1;
        } else if (val < (1 << 16)) {
            return 2;
        } else if (val < (1 << 24)) {
            return 3;
        } else if (val < 4294967296) {
            return 4;
        } else {
            return 5;
        }
    };
 
    /**
     * Return a view on the portion of the buffer from the beginning to the current seek position as a Uint8Array.
     */
    ArrayBufferDataStream.prototype.getAsDataArray = function() {
        if (this.pos < this.data.byteLength) {
            return this.data.subarray(0, this.pos);
        } else if (this.pos == this.data.byteLength) {
            return this.data;
        } else {
            throw "ArrayBufferDataStream's pos lies beyond end of buffer";
        }
    };
    
    if (typeof module !== 'undefined' && typeof module.exports !== 'undefined') {
        module.exports = ArrayBufferDataStream;
    } else {
        window.ArrayBufferDataStream = ArrayBufferDataStream;
    }
}());"use strict";
 
/**
 * Allows a series of Blob-convertible objects (ArrayBuffer, Blob, String, etc) to be added to a buffer. Seeking and
 * overwriting of blobs is allowed.
 * 
 * You can supply a FileWriter, in which case the BlobBuffer is just used as temporary storage before it writes it 
 * through to the disk.
 * 
 * By Nicholas Sherlock
 * 
 * Released under the WTFPLv2 https://en.wikipedia.org/wiki/WTFPL
 */
(function() {
    var BlobBuffer = function(fs) {
        return function(destination) {
            var
                buffer = [],
                writePromise = Promise.resolve(),
                fileWriter = null,
                fd = null;
            
            if (typeof FileWriter !== "undefined" && destination instanceof FileWriter) {
                fileWriter = destination;
            } else if (fs && destination) {
                fd = destination;
            }
            
            // Current seek offset
            this.pos = 0;
            
            // One more than the index of the highest byte ever written
            this.length = 0;
            
            // Returns a promise that converts the blob to an ArrayBuffer
            function readBlobAsBuffer(blob) {
                return new Promise(function (resolve, reject) {
                    var
                        reader = new FileReader();
                    
                    reader.addEventListener("loadend", function () {
                        resolve(reader.result);
                    });
                    
                    reader.readAsArrayBuffer(blob);
                });
            }
            
            function convertToUint8Array(thing) {
                return new Promise(function (resolve, reject) {
                    if (thing instanceof Uint8Array) {
                        resolve(thing);
                    } else if (thing instanceof ArrayBuffer || ArrayBuffer.isView(thing)) {
                        resolve(new Uint8Array(thing));
                    } else if (thing instanceof Blob) {
                        resolve(readBlobAsBuffer(thing).then(function (buffer) {
                            return new Uint8Array(buffer);
                        }));
                    } else {
                        //Assume that Blob will know how to read this thing
                        resolve(readBlobAsBuffer(new Blob([thing])).then(function (buffer) {
                            return new Uint8Array(buffer);
                        }));
                    }
                });
            }
            
            function measureData(data) {
                var
                    result = data.byteLength || data.length || data.size;
                
                if (!Number.isInteger(result)) {
                    throw "Failed to determine size of element";
                }
                
                return result;
            }
            
            /**
             * Seek to the given absolute offset.
             *
             * You may not seek beyond the end of the file (this would create a hole and/or allow blocks to be written in non-
             * sequential order, which isn't currently supported by the memory buffer backend).
             */
            this.seek = function (offset) {
                if (offset < 0) {
                    throw "Offset may not be negative";
                }
                
                if (isNaN(offset)) {
                    throw "Offset may not be NaN";
                }
                
                if (offset > this.length) {
                    throw "Seeking beyond the end of file is not allowed";
                }
                
                this.pos = offset;
            };
            
            /**
             * Write the Blob-convertible data to the buffer at the current seek position.
             *
             * Note: If overwriting existing data, the write must not cross preexisting block boundaries (written data must
             * be fully contained by the extent of a previous write).
             */
            this.write = function (data) {
                var
                    newEntry = {
                        offset: this.pos,
                        data: data,
                        length: measureData(data)
                    },
                    isAppend = newEntry.offset >= this.length;
                
                this.pos += newEntry.length;
                this.length = Math.max(this.length, this.pos);
                
                // After previous writes complete, perform our write
                writePromise = writePromise.then(function () {
                    if (fd) {
                        return new Promise(function(resolve, reject) {
                            convertToUint8Array(newEntry.data).then(function(dataArray) {
                                var
                                    totalWritten = 0,
                                    buffer = Buffer.from(dataArray.buffer),
                                    
                                    handleWriteComplete = function(err, written, buffer) {
                                        totalWritten += written;
                                        
                                        if (totalWritten >= buffer.length) {
                                            resolve();
                                        } else {
                                            // We still have more to write...
                                            fs.write(fd, buffer, totalWritten, buffer.length - totalWritten, newEntry.offset + totalWritten, handleWriteComplete);
                                        }
                                    };
                                
                                fs.write(fd, buffer, 0, buffer.length, newEntry.offset, handleWriteComplete);
                            });
                        });
                    } else if (fileWriter) {
                        return new Promise(function (resolve, reject) {
                            fileWriter.onwriteend = resolve;
                            
                            fileWriter.seek(newEntry.offset);
                            fileWriter.write(new Blob([newEntry.data]));
                        });
                    } else if (!isAppend) {
                        // We might be modifying a write that was already buffered in memory.
                        
                        // Slow linear search to find a block we might be overwriting
                        for (var i = 0; i < buffer.length; i++) {
                            var
                                entry = buffer[i];
                            
                            // If our new entry overlaps the old one in any way...
                            if (!(newEntry.offset + newEntry.length <= entry.offset || newEntry.offset >= entry.offset + entry.length)) {
                                if (newEntry.offset < entry.offset || newEntry.offset + newEntry.length > entry.offset + entry.length) {
                                    throw new Error("Overwrite crosses blob boundaries");
                                }
                                
                                if (newEntry.offset == entry.offset && newEntry.length == entry.length) {
                                    // We overwrote the entire block
                                    entry.data = newEntry.data;
                                    
                                    // We're done
                                    return;
                                } else {
                                    return convertToUint8Array(entry.data)
                                        .then(function (entryArray) {
                                            entry.data = entryArray;
                                            
                                            return convertToUint8Array(newEntry.data);
                                        }).then(function (newEntryArray) {
                                            newEntry.data = newEntryArray;
                                            
                                            entry.data.set(newEntry.data, newEntry.offset - entry.offset);
                                        });
                                }
                            }
                        }
                        // Else fall through to do a simple append, as we didn't overwrite any pre-existing blocks
                    }
                    
                    buffer.push(newEntry);
                });
            };
            
            /**
             * Finish all writes to the buffer, returning a promise that signals when that is complete.
             *
             * If a FileWriter was not provided, the promise is resolved with a Blob that represents the completed BlobBuffer
             * contents. You can optionally pass in a mimeType to be used for this blob.
             *
             * If a FileWriter was provided, the promise is resolved with null as the first argument.
             */
            this.complete = function (mimeType) {
                if (fd || fileWriter) {
                    writePromise = writePromise.then(function () {
                        return null;
                    });
                } else {
                    // After writes complete we need to merge the buffer to give to the caller
                    writePromise = writePromise.then(function () {
                        var
                            result = [];
                        
                        for (var i = 0; i < buffer.length; i++) {
                            result.push(buffer[i].data);
                        }
                        
                        return new Blob(result, {mimeType: mimeType});
                    });
                }
                
                return writePromise;
            };
        };
    };
    
    if (typeof module !== 'undefined' && typeof module.exports !== 'undefined') {
        module.exports = BlobBuffer(require('fs'));
    } else {
        window.BlobBuffer = BlobBuffer(null);
    }
})();/**
 * WebM video encoder for Google Chrome. This implementation is suitable for creating very large video files, because
 * it can stream Blobs directly to a FileWriter without buffering the entire video in memory.
 * 
 * When FileWriter is not available or not desired, it can buffer the video in memory as a series of Blobs which are 
 * eventually returned as one composite Blob.
 * 
 * By Nicholas Sherlock.
 * 
 * Based on the ideas from Whammy: https://github.com/antimatter15/whammy
 * 
 * Released under the WTFPLv2 https://en.wikipedia.org/wiki/WTFPL
 */
 
"use strict";
 
(function() {
    var WebMWriter = function(ArrayBufferDataStream, BlobBuffer) {
        function extend(base, top) {
            var
                target = {};
            
            [base, top].forEach(function(obj) {
                for (var prop in obj) {
                    if (Object.prototype.hasOwnProperty.call(obj, prop)) {
                        target[prop] = obj[prop];
                    }
                }
            });
            
            return target;
        }
        
        /**
         * Decode a Base64 data URL into a binary string.
         *
         * Returns the binary string, or false if the URL could not be decoded.
         */
        function decodeBase64WebPDataURL(url) {
            if (typeof url !== "string" || !url.match(/^data:image\/webp;base64,/i)) {
                return false;
            }
            
            return window.atob(url.substring("data:image\/webp;base64,".length));
        }
        
        /**
         * Convert a raw binary string (one character = one output byte) to an ArrayBuffer
         */
        function stringToArrayBuffer(string) {
            var
                buffer = new ArrayBuffer(string.length),
                int8Array = new Uint8Array(buffer);
            
            for (var i = 0; i < string.length; i++) {
                int8Array[i] = string.charCodeAt(i);
            }
            
            return buffer;
        }
        
        /**
         * Convert the given canvas to a WebP encoded image and return the image data as a string.
         */
        function renderAsWebP(canvas, quality) {
            var
                frame = canvas.toDataURL('image/webp', quality);
            
            return decodeBase64WebPDataURL(frame);
        }
        
        function extractKeyframeFromWebP(webP) {
            // Assume that Chrome will generate a Simple Lossy WebP which has this header:
            var
                keyframeStartIndex = webP.indexOf('VP8 ');
            
            if (keyframeStartIndex == -1) {
                throw "Failed to identify beginning of keyframe in WebP image";
            }
            
            // Skip the header and the 4 bytes that encode the length of the VP8 chunk
            keyframeStartIndex += 'VP8 '.length + 4;
            
            return webP.substring(keyframeStartIndex);
        }
        
        // Just a little utility so we can tag values as floats for the EBML encoder's benefit
        function EBMLFloat32(value) {
            this.value = value;
        }
        
        function EBMLFloat64(value) {
            this.value = value;
        }
        
        /**
         * Write the given EBML object to the provided ArrayBufferStream.
         *
         * The buffer's first byte is at bufferFileOffset inside the video file. This is used to complete offset and
         * dataOffset fields in each EBML structure, indicating the file offset of the first byte of the EBML element and
         * its data payload.
         */
        function writeEBML(buffer, bufferFileOffset, ebml) {
            // Is the ebml an array of sibling elements?
            if (Array.isArray(ebml)) {
                for (var i = 0; i < ebml.length; i++) {
                    writeEBML(buffer, bufferFileOffset, ebml[i]);
                }
            // Is this some sort of raw data that we want to write directly?
            } else if (typeof ebml === "string") {
                buffer.writeString(ebml);
            } else if (ebml instanceof Uint8Array) {
                buffer.writeBytes(ebml);
            } else if (ebml.id){
                // We're writing an EBML element
                ebml.offset = buffer.pos + bufferFileOffset;
                
                buffer.writeUnsignedIntBE(ebml.id); // ID field
                
                // Now we need to write the size field, so we must know the payload size:
                
                if (Array.isArray(ebml.data)) {
                    // Writing an array of child elements. We won't try to measure the size of the children up-front
                    
                    var
                        sizePos, dataBegin, dataEnd;
                    
                    if (ebml.size === -1) {
                        // Write the reserved all-one-bits marker to note that the size of this element is unknown/unbounded
                        buffer.writeByte(0xFF);
                    } else {
                        sizePos = buffer.pos;
                        
                        /* Write a dummy size field to overwrite later. 4 bytes allows an element maximum size of 256MB,
                         * which should be plenty (we don't want to have to buffer that much data in memory at one time
                         * anyway!)
                         */
                        buffer.writeBytes([0, 0, 0, 0]);
                    }
                    
                    dataBegin = buffer.pos;
                    
                    ebml.dataOffset = dataBegin + bufferFileOffset;
                    writeEBML(buffer, bufferFileOffset, ebml.data);
                    
                    if (ebml.size !== -1) {
                        dataEnd = buffer.pos;
                        
                        ebml.size = dataEnd - dataBegin;
                        
                        buffer.seek(sizePos);
                        buffer.writeEBMLVarIntWidth(ebml.size, 4); // Size field
                        
                        buffer.seek(dataEnd);
                    }
                } else if (typeof ebml.data === "string") {
                    buffer.writeEBMLVarInt(ebml.data.length); // Size field
                    ebml.dataOffset = buffer.pos + bufferFileOffset;
                    buffer.writeString(ebml.data);
                } else if (typeof ebml.data === "number") {
                    // Allow the caller to explicitly choose the size if they wish by supplying a size field
                    if (!ebml.size) {
                        ebml.size = buffer.measureUnsignedInt(ebml.data);
                    }
                    
                    buffer.writeEBMLVarInt(ebml.size); // Size field
                    ebml.dataOffset = buffer.pos + bufferFileOffset;
                    buffer.writeUnsignedIntBE(ebml.data, ebml.size);
                } else if (ebml.data instanceof EBMLFloat64) {
                    buffer.writeEBMLVarInt(8); // Size field
                    ebml.dataOffset = buffer.pos + bufferFileOffset;
                    buffer.writeDoubleBE(ebml.data.value);
                } else if (ebml.data instanceof EBMLFloat32) {
                    buffer.writeEBMLVarInt(4); // Size field
                    ebml.dataOffset = buffer.pos + bufferFileOffset;
                    buffer.writeFloatBE(ebml.data.value);
                } else if (ebml.data instanceof Uint8Array) {
                    buffer.writeEBMLVarInt(ebml.data.byteLength); // Size field
                    ebml.dataOffset = buffer.pos + bufferFileOffset;
                    buffer.writeBytes(ebml.data);
                } else {
                    throw "Bad EBML datatype " + typeof ebml.data;
                }
            } else {
                throw "Bad EBML datatype " + typeof ebml.data;
            }
        }
        
        return function(options) {
            var
                MAX_CLUSTER_DURATION_MSEC = 5000,
                DEFAULT_TRACK_NUMBER = 1,
            
                writtenHeader = false,
                videoWidth, videoHeight,
                
                clusterFrameBuffer = [],
                clusterStartTime = 0,
                clusterDuration = 0,
                
                optionDefaults = {
                    quality: 0.95,       // WebM image quality from 0.0 (worst) to 1.0 (best)
                    fileWriter: null,    // Chrome FileWriter in order to stream to a file instead of buffering to memory (optional)
                    fd: null,            // Node.JS file descriptor to write to instead of buffering (optional)
                    
                    // You must supply one of:
                    frameDuration: null, // Duration of frames in milliseconds
                    frameRate: null,     // Number of frames per second
                },
                
                seekPoints = {
                    Cues: {id: new Uint8Array([0x1C, 0x53, 0xBB, 0x6B]), positionEBML: null},
                    SegmentInfo: {id: new Uint8Array([0x15, 0x49, 0xA9, 0x66]), positionEBML: null},
                    Tracks: {id: new Uint8Array([0x16, 0x54, 0xAE, 0x6B]), positionEBML: null},
                },
                
                ebmlSegment,
                segmentDuration = {
                    "id": 0x4489, // Duration
                    "data": new EBMLFloat64(0)
                },
                
                seekHead,
                
                cues = [],
                
                blobBuffer = new BlobBuffer(options.fileWriter || options.fd);
    
            function fileOffsetToSegmentRelative(fileOffset) {
                return fileOffset - ebmlSegment.dataOffset;
            }
            
            /**
             * Create a SeekHead element with descriptors for the points in the global seekPoints array.
             *
             * 5 bytes of position values are reserved for each node, which lie at the offset point.positionEBML.dataOffset,
             * to be overwritten later.
             */
            function createSeekHead() {
                var
                    seekPositionEBMLTemplate = {
                        "id": 0x53AC, // SeekPosition
                        "size": 5, // Allows for 32GB video files
                        "data": 0 // We'll overwrite this when the file is complete
                    },
                    
                    result = {
                        "id": 0x114D9B74, // SeekHead
                        "data": []
                    };
                
                for (var name in seekPoints) {
                    var
                        seekPoint = seekPoints[name];
                
                    seekPoint.positionEBML = Object.create(seekPositionEBMLTemplate);
                    
                    result.data.push({
                         "id": 0x4DBB, // Seek
                         "data": [
                              {
                                  "id": 0x53AB, // SeekID
                                  "data": seekPoint.id
                              },
                              seekPoint.positionEBML
                         ]
                    });
                }
                
                return result;
            }
            
            /**
             * Write the WebM file header to the stream.
             */
            function writeHeader() {
                seekHead = createSeekHead();
                
                var
                    ebmlHeader = {
                        "id": 0x1a45dfa3, // EBML
                        "data": [
                            {
                                "id": 0x4286, // EBMLVersion
                                "data": 1
                            },
                            {
                                "id": 0x42f7, // EBMLReadVersion
                                "data": 1
                            },
                            {
                                "id": 0x42f2, // EBMLMaxIDLength
                                "data": 4
                            },
                            {
                                "id": 0x42f3, // EBMLMaxSizeLength
                                "data": 8
                            },
                            {
                                "id": 0x4282, // DocType
                                "data": "webm"
                            },
                            {
                                "id": 0x4287, // DocTypeVersion
                                "data": 2
                            },
                            {
                                "id": 0x4285, // DocTypeReadVersion
                                "data": 2
                            }
                        ]
                    },
                    
                    segmentInfo = {
                        "id": 0x1549a966, // Info
                        "data": [
                            {
                                "id": 0x2ad7b1, // TimecodeScale
                                "data": 1e6 // Times will be in miliseconds (1e6 nanoseconds per step = 1ms)
                            },
                            {
                                "id": 0x4d80, // MuxingApp
                                "data": "webm-writer-js",
                            },
                            {
                                "id": 0x5741, // WritingApp
                                "data": "webm-writer-js"
                            },
                            segmentDuration // To be filled in later
                        ]
                    },
                    
                    tracks = {
                        "id": 0x1654ae6b, // Tracks
                        "data": [
                            {
                                "id": 0xae, // TrackEntry
                                "data": [
                                    {
                                        "id": 0xd7, // TrackNumber
                                        "data": DEFAULT_TRACK_NUMBER
                                    },
                                    {
                                        "id": 0x73c5, // TrackUID
                                        "data": DEFAULT_TRACK_NUMBER
                                    },
                                    {
                                        "id": 0x9c, // FlagLacing
                                        "data": 0
                                    },
                                    {
                                        "id": 0x22b59c, // Language
                                        "data": "und"
                                    },
                                    {
                                        "id": 0x86, // CodecID
                                        "data": "V_VP8"
                                    },
                                    {
                                        "id": 0x258688, // CodecName
                                        "data": "VP8"
                                    },
                                    {
                                        "id": 0x83, // TrackType
                                        "data": 1
                                    },
                                    {
                                        "id": 0xe0,  // Video
                                        "data": [
                                            {
                                                "id": 0xb0, // PixelWidth
                                                "data": videoWidth
                                            },
                                            {
                                                "id": 0xba, // PixelHeight
                                                "data": videoHeight
                                            }
                                        ]
                                    }
                                ]
                            }
                        ]
                    };
                
                ebmlSegment = {
                    "id": 0x18538067, // Segment
                    "size": -1, // Unbounded size
                    "data": [
                        seekHead,
                        segmentInfo,
                        tracks,
                    ]
                };
                
                var
                    bufferStream = new ArrayBufferDataStream(256);
                    
                writeEBML(bufferStream, blobBuffer.pos, [ebmlHeader, ebmlSegment]);
                blobBuffer.write(bufferStream.getAsDataArray());
                
                // Now we know where these top-level elements lie in the file:
                seekPoints.SegmentInfo.positionEBML.data = fileOffsetToSegmentRelative(segmentInfo.offset);
                seekPoints.Tracks.positionEBML.data = fileOffsetToSegmentRelative(tracks.offset);
            };
            
            /**
             * Create a SimpleBlock keyframe header using these fields:
             *     timecode    - Time of this keyframe
             *     trackNumber - Track number from 1 to 126 (inclusive)
             *     frame       - Raw frame data payload string
             *
             * Returns an EBML element.
             */
            function createKeyframeBlock(keyframe) {
                var
                    bufferStream = new ArrayBufferDataStream(1 + 2 + 1);
                
                if (!(keyframe.trackNumber > 0 && keyframe.trackNumber < 127)) {
                    throw "TrackNumber must be > 0 and < 127";
                }
    
                bufferStream.writeEBMLVarInt(keyframe.trackNumber); // Always 1 byte since we limit the range of trackNumber
                bufferStream.writeU16BE(keyframe.timecode);
                
                // Flags byte
                bufferStream.writeByte(
                    1 << 7 // Keyframe
                );
                
                return {
                    "id": 0xA3, // SimpleBlock
                    "data": [
                         bufferStream.getAsDataArray(),
                         keyframe.frame
                    ]
                };
            }
            
            /**
             * Create a Cluster node using these fields:
             *
             *    timecode    - Start time for the cluster
             *
             * Returns an EBML element.
             */
            function createCluster(cluster) {
                return {
                    "id": 0x1f43b675,
                    "data": [
                         {
                            "id": 0xe7, // Timecode
                            "data": Math.round(cluster.timecode)
                         }
                    ]
                };
            }
            
            function addCuePoint(trackIndex, clusterTime, clusterFileOffset) {
                cues.push({
                    "id": 0xBB, // Cue
                    "data": [
                         {
                             "id": 0xB3, // CueTime
                             "data": clusterTime
                         },
                         {
                             "id": 0xB7, // CueTrackPositions
                             "data": [
                                  {
                                      "id": 0xF7, // CueTrack
                                      "data": trackIndex
                                  },
                                  {
                                      "id": 0xF1, // CueClusterPosition
                                      "data": fileOffsetToSegmentRelative(clusterFileOffset)
                                  }
                             ]
                         }
                    ]
                });
            }
            
            /**
             * Write a Cues element to the blobStream using the global `cues` array of CuePoints (use addCuePoint()).
             * The seek entry for the Cues in the SeekHead is updated.
             */
            function writeCues() {
                var
                    ebml = {
                        "id": 0x1C53BB6B,
                        "data": cues
                    },
                    
                    cuesBuffer = new ArrayBufferDataStream(16 + cues.length * 32); // Pretty crude estimate of the buffer size we'll need
                
                writeEBML(cuesBuffer, blobBuffer.pos, ebml);
                blobBuffer.write(cuesBuffer.getAsDataArray());
                
                // Now we know where the Cues element has ended up, we can update the SeekHead
                seekPoints.Cues.positionEBML.data = fileOffsetToSegmentRelative(ebml.offset);
            }
            
            /**
             * Flush the frames in the current clusterFrameBuffer out to the stream as a Cluster.
             */
            function flushClusterFrameBuffer() {
                if (clusterFrameBuffer.length == 0) {
                    return;
                }
    
                // First work out how large of a buffer we need to hold the cluster data
                var
                    rawImageSize = 0;
                
                for (var i = 0; i < clusterFrameBuffer.length; i++) {
                    rawImageSize += clusterFrameBuffer[i].frame.length;
                }
                
                var
                    buffer = new ArrayBufferDataStream(rawImageSize + clusterFrameBuffer.length * 32), // Estimate 32 bytes per SimpleBlock header
    
                    cluster = createCluster({
                        timecode: Math.round(clusterStartTime),
                    });
                    
                for (var i = 0; i < clusterFrameBuffer.length; i++) {
                    cluster.data.push(createKeyframeBlock(clusterFrameBuffer[i]));
                }
                
                writeEBML(buffer, blobBuffer.pos, cluster);
                blobBuffer.write(buffer.getAsDataArray());
                
                addCuePoint(DEFAULT_TRACK_NUMBER, Math.round(clusterStartTime), cluster.offset);
                
                clusterFrameBuffer = [];
                clusterStartTime += clusterDuration;
                clusterDuration = 0;
            }
            
            function validateOptions() {
                // Derive frameDuration setting if not already supplied
                if (!options.frameDuration) {
                    if (options.frameRate) {
                        options.frameDuration = 1000 / options.frameRate;
                    } else {
                        throw "Missing required frameDuration or frameRate setting";
                    }
                }
            }
            
            function addFrameToCluster(frame) {
                frame.trackNumber = DEFAULT_TRACK_NUMBER;
                
                // Frame timecodes are relative to the start of their cluster:
                frame.timecode = Math.round(clusterDuration);
    
                clusterFrameBuffer.push(frame);
                
                clusterDuration += frame.duration;
                
                if (clusterDuration >= MAX_CLUSTER_DURATION_MSEC) {
                    flushClusterFrameBuffer();
                }
            }
            
            /**
             * Rewrites the SeekHead element that was initially written to the stream with the offsets of top level elements.
             *
             * Call once writing is complete (so the offset of all top level elements is known).
             */
            function rewriteSeekHead() {
                var
                    seekHeadBuffer = new ArrayBufferDataStream(seekHead.size),
                    oldPos = blobBuffer.pos;
                
                // Write the rewritten SeekHead element's data payload to the stream (don't need to update the id or size)
                writeEBML(seekHeadBuffer, seekHead.dataOffset, seekHead.data);
                
                // And write that through to the file
                blobBuffer.seek(seekHead.dataOffset);
                blobBuffer.write(seekHeadBuffer.getAsDataArray());
    
                blobBuffer.seek(oldPos);
            }
            
            /**
             * Rewrite the Duration field of the Segment with the newly-discovered video duration.
             */
            function rewriteDuration() {
                var
                    buffer = new ArrayBufferDataStream(8),
                    oldPos = blobBuffer.pos;
                
                // Rewrite the data payload (don't need to update the id or size)
                buffer.writeDoubleBE(clusterStartTime);
                
                // And write that through to the file
                blobBuffer.seek(segmentDuration.dataOffset);
                blobBuffer.write(buffer.getAsDataArray());
        
                blobBuffer.seek(oldPos);
            }
            
            /**
             * Add a frame to the video. Currently the frame must be a Canvas element.
             */
            this.addFrame = function(canvas, duration) {
                //if (writtenHeader) {
                //    if (canvas.width != videoWidth || canvas.height != videoHeight) {
                //        throw "Frame size differs from previous frames";
                //    }
                //} else {
                    videoWidth = canvas.width;
                    videoHeight = canvas.height;
    
                    writeHeader();
                    writtenHeader = true;
                //}
    
                var
                    webP = renderAsWebP(canvas, options.quality);
                
                if (!webP) {
                    throw "Couldn't decode WebP frame, does the browser support WebP?";
                }
                
                addFrameToCluster({
                    frame: extractKeyframeFromWebP(webP),
                    duration: ((typeof duration == 'number')?duration:options.frameDuration)
                });
            };
            
            /**
             * Finish writing the video and return a Promise to signal completion.
             *
             * If the destination device was memory (i.e. options.fileWriter was not supplied), the Promise is resolved with
             * a Blob with the contents of the entire video.
             */
            this.complete = function() {
                flushClusterFrameBuffer();
                
                writeCues();
                rewriteSeekHead();
                rewriteDuration();
                
                return blobBuffer.complete('video/webm');
            };
            
            this.getWrittenSize = function() {
                return blobBuffer.length;
            };
    
            options = extend(optionDefaults, options || {});
            validateOptions();
        };
    };
 
    if (typeof module !== 'undefined' && typeof module.exports !== 'undefined') {
        module.exports = WebMWriter(require("./ArrayBufferDataStream"), require("./BlobBuffer"));
    } else {
        window.WebMWriter = WebMWriter(ArrayBufferDataStream, BlobBuffer);
    }
})();