With: printf ("%g\n", R(0)); gives me a very long series of “1's”; perhaps a result for each pixel of the 1920x1080 frame? With: if (PIX_X == 0 && PIX_Y == 0) printf ("%g\n", R(0)); It gives me a simple “1” for all plugins, with very few exceptions when the frame is replaced by other colors for which the different value is shown (always under “1”). For example, “Color Swatch” replaces the solid red with its own palette and then the result depends on the new color of the pixel (0,0). Another example is “Lens” which causes the red frame to shrink to a central part leaving the rest (including the pixel (0,0)) black. In fact the result is “0” and not “1”). However, by changing the Blend Program function and setting X=960 and Y=540 (i.e., the center pixel, where Lens still produces solid red) the result is still “1”. Other exceptions are HistEQ which turns the whole frame black and thus the result is “0,” Histogram Bezier which gives 0.999996 (which is “1” anyway) and Posterize which gives 0.99678. I can't interpret the results: I always get 1, even for Choma Key (Avid) and for Blender Algebra itself. I have tried all the internal plugins of CinGG, even those of only geometric transformations (Scale; Transform; Lens; DownSample, etc) and the result is always “1”. Maybe something limits (clips) the signal regardless of the plugin used? Perhaps the print_hdr.bp function is not suitable for checking the actual pixel value? I think the issue is too complicated so it's better to give up. However, I better understood Blend Algebra and Blend Program and their tremendous possibilities.