Thread: fast image crop

It's not going to be quite that easy. You'll need to read the image, convert it to an array of RGB values, crop it, and then convert back to whatever format the image was originally in. Not especially difficult if you have a good image manipulation library, of course, but not trivial either.

Assuming the source and destination images are packed 24-bit RGB, and you have two open file handles to these images, and the headers are properly formed, try this:

Code:

void cropRGB( FILE *srcImg, unsigned int srcDataOffset, unsigned int sw, unsigned int sh,
              FILE *dstImg, unsigned int dstDataOffset, unsigned int dw, unsigned int dh,
              unsigned int cropX, unsigned int cropY )
{
    unsigned int row;
    unsigned char *rowBuffer = malloc( 3 * dw );
    if( dstDataOffset > 0 )
        fseek( dstImg, dstDataOffset, SEEK_SET );
    for( row = cropY; row < cropY + dh; ++row )
    {
        fseek( srcImg, srcDataOffset + 3 * ( row * sw + cropX ), SEEK_SET );
        fread ( rowBuffer, 1, 3 * dw, srcImg );
        fwrite( rowBuffer, 1, 3 * dw, dstImg );
    }
    free( rowBuffer );
}

This crops the source image of dimension (sw x sh) starting at pixel (cropX, cropY) to the destination image of dimension (dw x dh).

srcDataOffset and dstDataOffset point past the image headers to the first byte of where the image data starts for both the input and output images.

if the destination is not seekable (for instance, it's a pipe or stdout) then you can pass 0 for the dstDataOffset and the data will be written where the file pointer is positioned.

Originally Posted by stabu

@brewbuck, you're just too excellent.
@sebastiani, libgd hasn't got a bad name for this ... cleaned up their documentation alot recently.
Thank you!

It might not necessarily be more efficient to seek. Seeking clears the stdio buffers. Another approach might be to simply read the excess data but discard it (which means you'd allocate a rowBuffer big enough for an entire SOURCE row instead of an entire DESTINATION row).

Code:

void cropRGB( FILE *srcImg, unsigned int srcDataOffset, unsigned int sw, unsigned int sh,
              FILE *dstImg, unsigned int dstDataOffset, unsigned int dw, unsigned int dh,
              unsigned int cropX, unsigned int cropY )
{
    unsigned int row;
    unsigned char *rowBuffer = malloc( 3 * sw );
    fseek( srcImg, srcDataOffset + 3 * ( row * sw ), SEEK_SET );
    if( dstDataOffset > 0 )
        fseek( dstImg, dstDataOffset, SEEK_SET );
    for( row = cropY; row < cropY + dh; ++row )
    {
        fread ( rowBuffer, 1, 3 * sw, srcImg );
        fwrite( rowBuffer + 3 * cropX, 1, 3 * dw, dstImg );
    }
    free( rowBuffer );
}

You could try it either way to find out, but you'd probably need some fairly large images to be able to notice the difference.