alpha blend image

imgblend

void imgBlend::blendImage(int posX, int posY, ofImage &imgfg, ofImage &imgbg, ofImage &imgdst)
{
    int imgFgWidth = imgfg.getWidth();
    int imgFgHeight = imgfg.getHeight();

    int imgBgWidth = imgbg.getWidth();
    int imgBgHeight = imgbg.getHeight();

    unsigned char * pix_bg = imgbg.getPixelsRef().getPixels();
    unsigned char * pix_fg = imgfg.getPixelsRef().getPixels();

    imgdst.getPixelsRef().setFromPixels(pix_bg,imgBgWidth,imgBgHeight, OF_IMAGE_COLOR ); //copy bg image to dst, auto allocate
    imgdst.setImageType(OF_IMAGE_COLOR_ALPHA); //add alpha channel to imgdst

    unsigned char * pix_dst = imgdst.getPixelsRef().getPixels();

    int channel_dst = imgdst.getPixelsRef().getBytesPerPixel();
    int channel_bg  = imgbg.getPixelsRef().getBytesPerPixel();
    int channel_fg  = imgfg.getPixelsRef().getBytesPerPixel();

    for (int x = 0 ; x < imgFgWidth; x++)
    {
        for (int y = 0 ; y < imgFgHeight; y++)
        {
            int arrpos_fg = (y*channel_fg*imgFgWidth) + x*channel_fg;
            int arrpos_dst = ((y+posY)*channel_dst*imgBgWidth) + (x+posX)*channel_dst;

            //output = alpha * foreground + (1-alpha) * background

            float alpha = pix_fg[arrpos_fg+3]/255.0;
            float Rp = alpha * pix_fg[arrpos_fg+0] + (1.0-alpha) * pix_dst[arrpos_dst+0];
            float Gp = alpha * pix_fg[arrpos_fg+1] + (1.0-alpha) * pix_dst[arrpos_dst+1];
            float Bp = alpha * pix_fg[arrpos_fg+2] + (1.0-alpha) * pix_dst[arrpos_dst+2];

            pix_dst[arrpos_dst]   = (int) Rp;
            pix_dst[arrpos_dst+1] = (int) Gp;
            pix_dst[arrpos_dst+2] = (int) Bp;
            pix_dst[arrpos_dst+3] = 255;
        }
    }

    imgdst.update();
}