Image Compression Using Laplacian Pyramid Encoding

Listing 3: The Expand function

void LP::Expand (unsigned level)
{
    if (level >= _levels || level + 1 >= _levels)
        throw "Level is out of range";

    unsigned char *src = _r_images[level + 1];
    unsigned char *dest = _l_images[level];

    unsigned width = _width >> level;
    unsigned height = _height >> level;

    for (int y = 0; y < (int) (height - 3); y += 2)
    {
        for (int x = 0; x < (int) (width - 3); x += 2)
        {
            // Expand argument is the top left corner
            // of the sample block

            Expand3x3 (src[((y / 2) * (width / 2)) + x / 2],
                       &dest[y * width + x], width);
        }
    }

    // Fix the edges

    // If the width is a multiple of two, then the reduced 
    // image has a column on the right that was not expanded.


    if (!(width & 1))
    {
        for (int y = 0; y < (int) (height - 3); y += 2)
        {
            Expand2x3 (src[((y/2) * (width/2)) + (width - 2) / 2],
                       &dest[y * width + (width - 2)], width);
        }
    }

    // If the height is a multiple of two, then the reduced 
    // image has a row on the bottom that was not expanded.

    if (!(height & 1))
    {
        for (int x = 0; x < (int) (width - 3); x += 2)
        {
            Expand3x2 (src[(((height - 2)/2) * (width/2)) + x/2],
                       &dest[(height - 2) * width + x], width);
        }
    }

    // If both the width and height are multiples of two, then
    // the bottom right corner was never expanded.

    if (!(width & 1) && !(height & 1))
    {
        Expand2x2 (src[(((height-2)/2) * (width/2)) + (width-2)/2],
                   &dest[(height-2) * width + (width-2)], width);
    }

    // When expanding, the outer edges need a little more
    // fixing because they have not been interpolated enough.

    // The top row only got interpolated with half the number 
    // of pixels needed.

    for (int x = 0; x < (int) (width); x++)
        dest[x] *= 2;

    // Same with the left column.  The top left corner only got 1/4
    // the number of pixels it needed, but we're multiplying it
    // by 2 twice, so it's OK.

    for (y = 0; y < (int) (height); y++)
        dest[y * width] *= 2;

    // If the width is an odd number, the number of pixels needed
    // to interpolate the right column is 1/2.  If it's an even
    // number, then it's OK.

    if (width & 1)
        for (int y = 1; y <= (int) (height); y++)
            dest[y * width - 1] *= 2;

    // If the height is an odd number, then the bottom row needs
    // twice as many pixels to interpolate correctly.

    if (height & 1)
        for (int x = 0; x < (int) (width); x++)
            dest[(height - 1) * width + x] *= 2;

    // Notice that the NE, SE, and SW corners are OK -- they
    // were multiplied by 2 or 4 automatically when the sides
    // were being fixed.
}
//End of File