ReduceAlphaChannel -- reduce alpha channel intensity (V6.0)
ReduceAlphaChannel(id, ratio)
This function can be used to reduce the intensity of the alpha channel associated with the specified brush. Every alpha pixel is multiplied by the ratio you pass in argument 2. This ratio must be between 0 and 255. A ratio of 255 means 1.0 or 100% whereas 0 means 0.0 or 0%. Thus, if you want to reduce the alpha transparency of all pixels by 50%, you would have to pass 128 in the ratio parameter.

ratio can also be a string containing a percent specification, e.g. "50%".

brush whose alpha channel should be modified
value between 0 and 255 that specifies the intensity of the reduction operation or a percent specification

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