Question

I have the next standard code for JPEG image decompression which is based on libjpeg.

jpeg_decompress_struct cinfo;
// ...Set error manager and data source...
jpeg_read_header(&cinfo, TRUE);
jpeg_start_decompress(&cinfo);
while (cinfo.output_scanline < cinfo.output_height) {
    JSAMPLE* scanlines[1];
    // ...Set target pointer for scanline...
    jpeg_read_scanlines(&cinfo, scanlines, 1);
}
jpeg_destroy_decompress(&cinfo);

I want to read a part of the image, cropped by a rectangle:

// struct RECT {
//     int left;
//     int top;
//     int right;
//     int bottom;
// };
RECT cropRect; // Coordinates of the crop rectangle relative to the output image size

What should I modify in the code below to tell libjpeg to crop the image immediately?

This is how I can implement it:

1. Ignore first top - 1 lines;
2. For each of the next bottom - top lines:
   1) Read scanline to temporary buffer;
   2) Copy pixels from column range [left, right) from temporary buffer to the target buffer.
3. Abort the decompression.

But this code is redundant.

Answer 1

Performance-wise, especially if the original image is high resolution and you need a relatively small part of it, you should perhaps first crop/trim the image losslessly without decompressing it, which is possible at 16×16 px (8×8?) granularity and fast, and then decompress skipping just a few lines and pixels off the margins. You might also like this approach for smaller amount of memory in use for the operation.

If you are cropping just a bit, then the original plan to start decompressing in full is perhaps the best. There is almost no redundancy here.