I’ve been playing more with OpenCV and I think I’m missing something. I can’t do any manipulations on the image data without really screwing it up. The only thing that doesn’t seem to wash out the data out is moving pixels around without changing them. Not sure what I’m missing. Here’s the few different manipulations and what they look like when they wash out.

Monochrome

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for(int i = 0; i < frame->height; i++) {
  int offset = i*frame->width*3;
  for(int j = 0; j < frame->width; j++) {
    uchar temp = frame->imageData[offset+(j*3)]*0.114
                    + frame->imageData[offset+(j*3)+1]*0.587
                    + frame->imageData[offset+(j*3)+2]*0.299;
    frame->imageData[offset+(j*3)] = temp;
    frame->imageData[offset+(j*3)+1] = temp;
    frame->imageData[offset+(j*3)+2] = temp;
  }
}

Memory
This one just keeps five frames and then adds them in to create a faded composite, should be simple.

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if(0 == memory_frameCounter)
  memory_frames[0] = cvCloneImage(frame);
else if(2 == memory_frameCounter)
  memory_frames[1] = cvCloneImage(frame);
else if(4 == memory_frameCounter)
  memory_frames[2] = cvCloneImage(frame);
else if(6 == memory_frameCounter)
  memory_frames[3] = cvCloneImage(frame);
else if(8 == memory_frameCounter)
  memory_frames[4] = cvCloneImage(frame);
else if(10 <= memory_frameCounter)
  memory_frameCounter = -1;
 
memory_frameCounter++;
 
for(int i = 0; i < frame->height; i++) {
  for(int j =0; j < frame->width*3; j++) {
    memory_agg = frame->imageData[(i*frame->width*3)+j];
    for(int k = 0; k < 5; k++) {
      memory_agg = (memory_agg + memory_frames[k]->imageData[(i*frame->width*3)+j])/2;
    }
    frame->imageData[(i*frame->width*3)+j] = memory_agg;
  }
}

I just can’t figure out what I’m doing wrong here.

3 Responses to “Screwy OpenCV Manipulations”

1. [OpenCV] Washed out images : OpenCV Says:
   March 3rd, 2008 at 7:42 am

   [...] I have some code and images here: http://www.velvetcache.org/2008/03/02/screwy-opencv-manipulations [...]

2. giulio Says:
   March 3rd, 2008 at 12:27 pm

   try using cvSet2D and cvGet2D, or using img->widthstep instead of (img->width)*3.. often it’s not the same

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   IplImage* img = cvLoadImage(.....); //your image int width = img->width; int height = img->height; for (int j=0; j < height; j++){ for (int i=0; i < width; i++){   CvScalar rgb = cvGet2D(img, j, i); int gray = rgb[0]*0.114 + rgb[1]*0.587 + rgb[2]*0.299; } }

3. john Says:
   March 4th, 2008 at 11:16 am

   Works like a charm! Here’s the code I ended up with for it.

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   int width = frame->width; int height = frame->height; for (int j=0; j < height; j++){ for (int i=0; i < width; i++){ rgb = cvGet2D(frame, j, i); int gray = rgb.val[0]*0.114 + rgb.val[1]*0.587 + rgb.val[2]*0.299; rgb.val[0] = gray; rgb.val[1] = gray; rgb.val[2] = gray; cvSet2D(frame,j,i,rgb); } }