70 lines
1.8 KiB
JavaScript
70 lines
1.8 KiB
JavaScript
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'use strict';
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// based on https://developers.google.com/speed/webp/docs/riff_container
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function isWebP (buffer) {
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var riffHeader = 'RIFF' === buffer.toString('ascii', 0, 4);
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var webpHeader = 'WEBP' === buffer.toString('ascii', 8, 12);
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var vp8Header = 'VP8' === buffer.toString('ascii', 12, 15);
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return (riffHeader && webpHeader && vp8Header);
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}
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function calculate (buffer) {
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var chunkHeader = buffer.toString('ascii', 12, 16);
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buffer = buffer.slice(20, 30);
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// Extended webp stream signature
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if (chunkHeader === 'VP8X') {
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var extendedHeader = buffer[0];
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var validStart = (extendedHeader & 0xc0) === 0;
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var validEnd = (extendedHeader & 0x01) === 0;
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if (validStart && validEnd) {
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return calculateExtended(buffer);
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} else {
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return false;
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}
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}
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// Lossless webp stream signature
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if (chunkHeader === 'VP8 ' && buffer[0] !== 0x2f) {
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return calculateLossy(buffer);
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}
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// Lossy webp stream signature
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var signature = buffer.toString('hex', 3, 6);
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if (chunkHeader === 'VP8L' && signature !== '9d012a') {
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return calculateLossless(buffer);
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}
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return false;
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}
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function calculateExtended (buffer) {
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return {
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'width': 1 + buffer.readUIntLE(4, 3),
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'height': 1 + buffer.readUIntLE(7, 3)
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}
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}
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function calculateLossless (buffer) {
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return {
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'width': 1 + (((buffer[2] & 0x3F) << 8) | buffer[1]),
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'height': 1 + (((buffer[4] & 0xF) << 10) | (buffer[3] << 2) |
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((buffer[2] & 0xC0) >> 6))
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};
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}
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function calculateLossy (buffer) {
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// `& 0x3fff` returns the last 14 bits
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// TO-DO: include webp scaling in the calculations
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return {
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'width': buffer.readInt16LE(6) & 0x3fff,
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'height': buffer.readInt16LE(8) & 0x3fff
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};
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}
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module.exports = {
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'detect': isWebP,
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'calculate': calculate
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};
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