Split channels from RGB array in C++ without OpenCV

Question

I'm working in C++ with an array of unsigned char representing pixels in an image. Each pixel has 3 channel (R,G,B). The image is represented linearly, sort of like

RGBRGBRGBRGB.....

How do I split each of the R,G and B, into separate arrays efficiently?

I tried:

  for(int pos = 0; pos < srcWidth * srcHeight; pos++) {
      int rgbPos = pos * 3;
      splitChannels[0][pos] = rgbSrcData[rgbPos];
      splitChannels[1][pos] = rgbSrcData[rgbPos + 1];
      splitChannels[2][pos] = rgbSrcData[rgbPos + 2];
  }

But this is surprisingly slow.

Thanks!

Answer 1

My attempt: load and store the bytes four by four. Byte scrambling will be tedious but possibly throughput will improve.

// Load 4 interleaved pixels
unsigned int RGB0= ((int*)rgbSrcData)[i];
unsigned int RGB1= ((int*)rgbSrcData)[i + 1];
unsigned int RGB2= ((int*)rgbSrcData)[i + 2];

// Rearrange and store 4 unpacked pixels
((int*)splitChannels[0])[j]= 
    (RGB0 & 0xFF) | (RGB0 >> 24) | (RGB1 & 0xFF0000) | ((RGB2 & 0xFF00) << 16);
((int*)splitChannels[1])[j]= 
    ((RGB0 & 0xFF00) >> 8) | (RGB1 & 0xFF) | (RGB1 >> 24) | (RGB2 & 0xFF0000) >> 16;
((int*)splitChannels[2])[j]= 
    ((RGB0 & 0xFF0000) >> 16) | (RGB1 & 0xFF00) | ((RGB2 & 0xFF) >> 16) | (RGB2 & 0xFF000000);

(CAUTION: not unchecked !) A shift-only version is also possible.

An SSE solution would be more complex (the stride 3 does not get along with powers of 2).

Answer 2

A great technique to use to make it run faster is loop unwinding. You can read about it here: http://en.wikipedia.org/wiki/Loop_unwinding