Hi, this Jim, the subject of the article. The Atlantic article omits that I'm normalizing the result (otherwise, the histogram is clustered in the middle and you get a dirt-brown color -- the hue is orange, but it's very desaturated). Here is pseudo-code in Processing showing what I'm doing:

To avoid floating point rounding issues, I process the pixels in integer space. For each channel (R,G,B) I create an 1D array of integers (one integer for each pixel) and initialize them to zero.

smaxPixels = width*height; srmap = new int[smaxPixels]; sgmap = new int[smaxPixels]; sbmap = new int[smaxPixels]; for (int i = 0; i < smaxPixels; ++i) { srmap[i] = 0; sgmap[i] = 0; sbmap[i] = 0; }

For each pixel in each image, I add the pixel value to the sum for the corresponding pixel (in the below code, 'offset' is typically zero).

  for (int i = 0; i < smaxPixels; ++i) {
    int px = sImage.pixels[i];
    srmap[i] += red(px);
    sgmap[i] += green(px);
    sbmap[i] += blue(px);
  }

I then produce a normalized image by finding the minimum and maximum components, and mapping all the pixel sums to that range.

for (int i = 0; i < smaxPixels; ++i) { minValue = min(minValue, srmap[i]); maxValue = max(maxValue, srmap[i]); minValue = min(minValue, sgmap[i]); maxValue = max(maxValue, sgmap[i]); minValue = min(minValue, sbmap[i]); maxValue = max(maxValue, sbmap[i]); }

  pg.beginDraw();  // pg is an offscreen Image buffer I am going to draw into...
  pg.loadPixels();
  for (int i = 0; i < smaxPixels; ++i) 
  {
    if (maxImagesPerTile == 1) 
      pg.pixels[i] = color(srmap[i],sgmap[i],sbmap[i]);
    else
      pg.pixels[i] = color( map(srmap[i],minValue,maxValue,0,255),
                       map(sgmap[i],minValue,maxValue,0,255),
                      map(sbmap[i],minValue,maxValue,0,255));
  }
  pg.updatePixels();
  pg.endDraw();

 

That's the essence of it. The bright "orange" is a normalization effect. Note that if I were to simply "average" the images without the normalization step, the resulting color is generally a dirt-brown. Normalizing the image has the effect of increasing the saturation and the contrast, without affecting the hue in an HSL sense. I discuss this in my paper, although the Atlantic article omitted this important point.