qt - Blur QImage alpha channel -

i'm trying blur qimage alpha channel. current implementation use deprecated 'alphachannel' method , works slow.

qimage blurimage(const qimage & image, double radius) {   qimage newimage = image.converttoformat(qimage::format_argb32);    qimage alpha = newimage.alphachannel();   qimage blurredalpha = alpha;   (int x = 0; x < alpha.width(); x++)   {     (int y = 0; y < alpha.height(); y++)     {       uint color = calculateaveragealpha(x, y, alpha, radius);       blurredalpha.setpixel(x, y, color);     }   }   newimage.setalphachannel(blurredalpha);    return newimage; } 

i trying implement using qgraphicsblureffect, doesn't affect alpha.

what proper way blur qimage alpha channel?

i have faced similar question pixel read\write access :

1. invert loops. image laid out in memory succession of rows. should access first height width
2. use qimage::scanline access data, rather expensives qimage::pixel , qimage::setpixel. pixels in scan (aka row) guaranteed consecutive.

your code :

for (int ii = 0; ii < image.height(); ii++) {     uchar* scan = image.scanline(ii);     int depth =4;     (int jj = 0; jj < image.width(); jj++) {         //it in fact rgba         qrgb* rgbpixel = reinterpret_cast<qrgb*>(scan + jj*depth);         qcolor color(*rgbpixel);         int alpha = calculateaveragealpha(ii, jj, color, image);         color.setalpha(alpha);          //write         *rgbpixel = color.rgba();     } } 

you can go further , optimize computation of alpha average. lets @ sum of pixel in radius. sum of alpha value @ (x,y) in radius s(x,y). when move 1 pixel in either direction, single line added while single line removed. lets move horizontally. if l(x,y) sum of vertical line of length 2*radius centered around (x,y), have

  s(x + 1, y) = s(x, y) + l(x + r + 1, y) - l(x - r, y) 

which allow efficiently compute matrix of sum (then average, dividing number of pixel) in first pass.

i suspect kind of optimization implemented in better way in libraries such opencv. encourage use existing opencv functions if wish save time.