In the Operands sections it says "When used in a setCombineAlpha() call, RGB is automatically aggregated into grayscale.", except this is not at all true. Instead it throws an error that the operand is not suitable for use as an alpha operand. Which is too bad, since I really wanted this behavior. Does the underlying rendering support this behavior and just the parameter check need to be relaxed? Does the underlying rendering need to be improved to support this behavior (is is possible?)? Or does the documentation just need to correct to reflect actual behavior?