I'm mapping several photoshop elements to CIFilter, the only one I'm having trouble with is this Levels Adjustment:
Which CI Filter (or combination of filters) would let me utilize the 16, 1.73, 239 & 39/245 above in the first example or the 31, 1.25, 255 30/255 in the second example. I believe this is a kind of shadow/black and white level adjustment.
Any help appreciated.
By adapting the formula from this page: http://http.developer.nvidia.com/GPUGems/gpugems_ch22.html, I believe you can do this using a combination of
CIGammaAdjust and another
Let's call the input levels
inWhite respectively, and the output levels
outWhite. Note that Photoshop color are between 0 and 255 while CI colors are between 0 and 1 so you need to divide the Photoshop values (except
inGamma!) by 255 before putting them into the following formulas.
The input mapping is
pixel = (inPixel-inBlack)/(inWhite-inBlack), which means your first matrix will be
red = [1/(inWhite-inBlack) 0 0 0] green = [0 1/(inWhite-inBlack) 0 0] blue = [0 0 1/(inWhite-inBlack) 0] alpha = [0 0 0 1] bias = [-inBlack/(inWhite-inBlack), -inBlack/(inWhite-inBlack),-inBlack/(inWhite-inBlack), 0]
Then you apply gamma correction using
CIGammaAdjust and the
inGamma number (I had to use the inverse
1/inGamma when doing my calculations, try that too!).
Finally the output mapping is pixel =
gammaCorrectedPixel * (outWhite - outBlack) + outBlack, giving you the final matrix
red = [(outWhite - outBlack) 0 0 0] green = [0 (outWhite - outBlack) 0 0] blue = [0 0 (outWhite - outBlack) 0] alpha = [0 0 0 1] bias = [outBlack outBlack outBlack 0]
I haven't actually tried this using CoreImage, but the calculations work out nicely!