Wunderbart's post worked for me combined with statler's improvements. Adding a few more comments and syntax cleanup, and also passing back the orientation value and I have the following code feel free to use. Just call readImageFile() function below and you get back the transformed image and the original orientation.
const JpegOrientation = [
"NOT_JPEG",
"NORMAL",
"FLIP-HORIZ",
"ROT180",
"FLIP-HORIZ-ROT180",
"FLIP-HORIZ-ROT270",
"ROT270",
"FLIP-HORIZ-ROT90",
"ROT90"
];
//Provided a image file, determines the orientation of the file based on the EXIF information.
//Calls the "callback" function with an index into the JpegOrientation array.
//If the image is not a JPEG, returns 0. If the orientation value cannot be read (corrupted file?) return -1.
function getOrientation(file, callback) {
const reader = new FileReader();
reader.onload = (e) => {
const view = new DataView(e.target.result);
if (view.getUint16(0, false) !== 0xFFD8) {
return callback(0); //NOT A JPEG FILE
}
const length = view.byteLength;
let offset = 2;
while (offset < length) {
if (view.getUint16(offset+2, false) <= 8) //unknown?
return callback(-1);
const marker = view.getUint16(offset, false);
offset += 2;
if (marker === 0xFFE1) {
if (view.getUint32(offset += 2, false) !== 0x45786966)
return callback(-1); //unknown?
const little = view.getUint16(offset += 6, false) === 0x4949;
offset += view.getUint32(offset + 4, little);
const tags = view.getUint16(offset, little);
offset += 2;
for (var i = 0; i < tags; i++) {
if (view.getUint16(offset + (i * 12), little) === 0x0112) {
return callback(view.getUint16(offset + (i * 12) + 8, little)); //found orientation code
}
}
}
else if ((marker & 0xFF00) !== 0xFF00) {
break;
}
else {
offset += view.getUint16(offset, false);
}
}
return callback(-1); //unknown?
};
reader.readAsArrayBuffer(file);
}
//Takes a jpeg image file as base64 and transforms it back to original, providing the
//transformed image in callback. If the image is not a jpeg or is already in normal orientation,
//just calls the callback directly with the source.
//Set type to the desired output type if transformed, default is image/jpeg for speed.
function resetOrientation(srcBase64, srcOrientation, callback, type = "image/jpeg") {
if (srcOrientation <= 1) { //no transform needed
callback(srcBase64);
return;
}
const img = new Image();
img.onload = () => {
const width = img.width;
const height = img.height;
const canvas = document.createElement('canvas');
const ctx = canvas.getContext("2d");
// set proper canvas dimensions before transform & export
if (4 < srcOrientation && srcOrientation < 9) {
canvas.width = height;
canvas.height = width;
} else {
canvas.width = width;
canvas.height = height;
}
// transform context before drawing image
switch (srcOrientation) {
//case 1: normal, no transform needed
case 2:
ctx.transform(-1, 0, 0, 1, width, 0);
break;
case 3:
ctx.transform(-1, 0, 0, -1, width, height);
break;
case 4:
ctx.transform(1, 0, 0, -1, 0, height);
break;
case 5:
ctx.transform(0, 1, 1, 0, 0, 0);
break;
case 6:
ctx.transform(0, 1, -1, 0, height, 0);
break;
case 7:
ctx.transform(0, -1, -1, 0, height, width);
break;
case 8:
ctx.transform(0, -1, 1, 0, 0, width);
break;
default:
break;
}
// draw image
ctx.drawImage(img, 0, 0);
//export base64
callback(canvas.toDataURL(type), srcOrientation);
};
img.src = srcBase64;
};
//Read an image file, providing the returned data to callback. If the image is jpeg
//and is transformed according to EXIF info, transform it first.
//The callback function receives the image data and the orientation value (index into JpegOrientation)
export function readImageFile(file, callback) {
getOrientation(file, (orientation) => {
console.log("Read file \"" + file.name + "\" with orientation: " + JpegOrientation[orientation]);
const reader = new FileReader();
reader.onload = () => { //when reading complete
const img = reader.result;
resetOrientation(img, orientation, callback);
};
reader.readAsDataURL(file); //start read
});
}
