There are many ways you could create a filter with a red hue. One way is to consider the overall brightness of a pixel and map it to the same brightness on a red-hue scale. This red scale has white and black at 100% brightness and 0% brightness respectively, but in the middle, where the grayscale color would be rgb(128,128,128), this scale has a red color rgb(255,0,0). You can see that the red (R) value increases with brightness up to 50%, then it stays 255. Green (G) and blue (B) values are zero up to 50% and then increase to 255 at 100%.
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