FIGURE 10. Diagram showing the principles of the channel (or layer) blending modes “Subtract” and “Difference” in Adobe Photoshop. Each bar in the histograms represents one pixel in an image, the height of the bar corresponding to the pixel value, from 0 (black) to 255 (white). The right-hand column of the blue diagrams (D, F, H, J) shows the result of a level adjustment to extend the range of values from 5 (near-black) to 250 (near-white). A–B. Two versions of an image, with slight nuance differences, mainly in the central part. C–D. “Subtract” mode, with no offset. Note negative values in C, representing an area where B is brighter than A, which will be rendered as 0 (5 after level adjustment in D). E–F. Same as C–D, except that offset value (the value added to the result of the subtraction) has been set to 100. Note that the negative values in C are now positive, and the information in that part of the pictures will be retained. G–H. Same as E–F, but with A subtracted from B rather than the opposite. Note that the resulting curves are negatives of those in E–F. I–J. “Difference” mode. Note that the valley (dark) in F is now rendered as a peak (bright).