File size: 3,850 Bytes
15c3607 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 | import {
EXIF_SCAN_BYTE_LIMIT,
JPEG_SOI_MARKER,
APP1_MARKER,
SOS_MARKER,
EXIF_SIGNATURE,
TIFF_LITTLE_ENDIAN,
TIFF_MAGIC,
EXIF_ORIENTATION_TAG,
IFD_ENTRY_SIZE
} from '$lib/constants/jpeg-exif';
import { MimeTypeImage } from '$lib/enums';
/**
* Read the EXIF orientation tag from a JPEG base64 data URL
*
* Only a bounded prefix of the base64 payload is decoded, the APP1 segment
* always sits near the start of the file.
* @param base64UrlJpeg - The JPEG base64 data URL to inspect
* @returns The orientation value (1 to 8), or 1 when absent or unreadable
*/
export function getJpegOrientationFromDataURL(base64UrlJpeg: string): number {
try {
const payloadStart = base64UrlJpeg.indexOf(',') + 1;
if (payloadStart <= 0) {
return 1;
}
// Keep the slice a multiple of 4 characters so atob accepts it
const charLimit = Math.ceil(EXIF_SCAN_BYTE_LIMIT / 3) * 4;
const slice = base64UrlJpeg.slice(payloadStart, payloadStart + charLimit);
const binary = atob(slice.slice(0, slice.length - (slice.length % 4)));
const bytes = new Uint8Array(binary.length);
for (let i = 0; i < binary.length; i++) {
bytes[i] = binary.charCodeAt(i);
}
return findExifOrientation(new DataView(bytes.buffer));
} catch {
return 1;
}
}
/**
* Walk the JPEG segments of a header buffer looking for the APP1 EXIF block
* @param view - DataView over the JPEG header bytes
* @returns The orientation value (1 to 8), or 1 when absent or malformed
*/
function findExifOrientation(view: DataView): number {
if (view.byteLength < 4 || view.getUint16(0) !== JPEG_SOI_MARKER) {
return 1;
}
let offset = 2;
while (offset + 4 <= view.byteLength) {
if (view.getUint8(offset) !== 0xff) {
return 1;
}
const marker = view.getUint8(offset + 1);
// Compressed image data starts here: no EXIF past this point
if (marker === SOS_MARKER) {
return 1;
}
const segmentLength = view.getUint16(offset + 2);
if (marker === APP1_MARKER) {
return parseExifOrientation(view, offset + 4, segmentLength);
}
offset += 2 + segmentLength;
}
return 1;
}
/**
* Parse the orientation tag from an APP1 EXIF payload
* @param view - DataView over the JPEG header bytes
* @param start - Offset of the APP1 payload, right after the segment length
* @param segmentLength - Declared APP1 segment length
* @returns The orientation value (1 to 8), or 1 when absent or malformed
*/
function parseExifOrientation(view: DataView, start: number, segmentLength: number): number {
const end = Math.min(start + segmentLength, view.byteLength);
// The payload opens with the "Exif\0\0" signature
if (
start + 6 > end ||
view.getUint32(start) !== EXIF_SIGNATURE ||
view.getUint16(start + 4) !== 0
) {
return 1;
}
const tiff = start + 6;
if (tiff + 8 > end) {
return 1;
}
const littleEndian = view.getUint16(tiff) === TIFF_LITTLE_ENDIAN;
if (view.getUint16(tiff + 2, littleEndian) !== TIFF_MAGIC) {
return 1;
}
const ifdOffset = view.getUint32(tiff + 4, littleEndian);
if (tiff + ifdOffset + 2 > end) {
return 1;
}
const entryCount = view.getUint16(tiff + ifdOffset, littleEndian);
// Scan IFD0 entries for the orientation tag
for (let i = 0; i < entryCount; i++) {
const entry = tiff + ifdOffset + 2 + i * IFD_ENTRY_SIZE;
if (entry + IFD_ENTRY_SIZE > end) {
return 1;
}
if (view.getUint16(entry, littleEndian) === EXIF_ORIENTATION_TAG) {
const orientation = view.getUint16(entry + 8, littleEndian);
return orientation >= 1 && orientation <= 8 ? orientation : 1;
}
}
return 1;
}
/**
* Check if a MIME type represents a JPEG
* @param mimeType - The MIME type to check
* @returns True if the MIME type is a JPEG variant
*/
export function isJpegMimeType(mimeType: string): boolean {
return mimeType === MimeTypeImage.JPEG || mimeType === MimeTypeImage.JPG;
}
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