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jpeglibjpeg

Why does libjpeg store my image array incorrectly?

I use libjpg to save an image to disk. I have an pixel array, but the resulting image looks not as I thought.

The array is 8x8 pixels and each pixel has 3 components. Can anyone explain what I might do wrong?

uint32 data[] = {255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
    255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0, 255, 0, 0,
};
write_JPEG_file("foo.jpg", 100, width, height, (JSAMPROW)data);

Here is the original output:

enter image description here

Upscaled to 128x128:

enter image description here

I use the default write function from the examples:

int write_JPEG_file (const char * filename, int quality, int width, int height, JSAMPROW raw_image)
{
    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr jerr;
    
    JSAMPROW row_pointer[1];
    FILE *outfile = fopen( filename, "wb" );
    
    if ( !outfile )
    {
        printf("Error opening output jpeg file %s\n!", filename );
        return -1;
    }
    cinfo.err = jpeg_std_error( &jerr );
    jpeg_create_compress(&cinfo);
    jpeg_stdio_dest(&cinfo, outfile);


    cinfo.image_width = width;
    cinfo.image_height = height;
    cinfo.input_components = 3;
    cinfo.in_color_space = JCS_RGB;

    jpeg_set_defaults( &cinfo );

    jpeg_start_compress( &cinfo, TRUE );

    while( cinfo.next_scanline < cinfo.image_height )
    {
        row_pointer[0] = &raw_image[ cinfo.next_scanline * cinfo.image_width *  cinfo.input_components];
        jpeg_write_scanlines( &cinfo, row_pointer, 1 );
    }

    jpeg_finish_compress( &cinfo );
    jpeg_destroy_compress( &cinfo );
    fclose( outfile );

    return 1;
}
Solution

  • I accidentally used uint32 per component instead of unsigned char.