问题描述
我将几个photoshop元素映射到CIFilter，唯一一个我遇到问题的是这个级别调整：
I'm mapping several photoshop elements to CIFilter, the only one I'm having trouble with is this Levels Adjustment:
哪个CI过滤器（或过滤器的组合）可以让我利用16,1.73,239&第一个例子中的39/245或者第二个例子中的31,1.25,255 30/255。我相信这是一种阴影/黑白水平调整。
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.
任何帮助表示感谢。
推荐答案
通过调整此页面中的公式：，我相信你可以使用 CIColorMatrix ， CIGammaAdjust的组合来做到这一点和另一个 CIColorMatrix 。
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 CIColorMatrix, CIGammaAdjust and another CIColorMatrix.
让我们调用输入级别 inBlack ， inGamma 和 inWhite ，输出等级 outBlack 和 outWhite 。请注意，Photoshop颜色介于0到255之间，而CI颜色介于0和1之间，因此您需要将Photoshop值（除了 inGamma ！）除以255，然后再将它们放入以下公式。
Let's call the input levels inBlack, inGamma and inWhite respectively, and the output levels outBlack and 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.
输入映射是 pixel =（inPixel-inBlack）/（inWhite-inBlack），其中意味着你的第一个矩阵将是
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]
然后使用 CIGammaAdjust 和 inGamma 编号应用伽马校正（我必须使用反算 1 / inGamma 进行计算时，也试试！）。
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!).
最后输出映射是pixel = gammaCorrectedPixel *（outWhite - outBlack）+ outBlack ，为您提供最终矩阵
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]
我实际上并没有尝试使用CoreImage，但计算效果很好！
I haven't actually tried this using CoreImage, but the calculations work out nicely!
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