Expert: Cindy Myers - 10/22/2004

Question
Hi Cindy, I have a problem with some maths that I would really appreciate some help with. I have a range of formulas which are from Photoshop software. The formulas take 2 variables ( pixel colours as  number values) and combine them to produce a result colour value. I would like to be able to re-work the formulas so that if I know one of the variables and the result value I want, I can calculate the other variable. Some are quite straight forward, but there are a few which I really have no idea where to begin ! any help you may have would be v. appreciated

thanks

Simo

e.g. formula.:

result = (2*(a*b)) + (a squared*( 2*b -1) )

The values for a and b are the variable pixel brightnesses,  the values are between 0 (black) and 1(white) in 256 steps, hence 0.5 = mid grey.

here, if I know the value for a, and the result I want, how would I work out the value for b ?

Any help you can give will I am sure be really useful in solving the remaining formulas I have to re-work !


Answer
This is very difficult to do as a formula. But it should be much easier when actual numbers are available. Would you send me two examples: one with the result and value of a; the other with the result and value of b?

I'll do my best to show the procedure once I can work with real numbers. I cannot make examples myself because I am not familiar with what results are reasonable.

Cindy M