How does the luminance manipulate the color? (PTM-LRGB)

[Erxio]

Hello everyone! 

I got interested in the PTM format and wanted to "play around" with it a little bit and tried to write a PTM-reader in Java on my own.

I was able to get all the rgb-values and the luminance coefficences and I implemented the formular for calculating C (also regarding bias and scale) but from here on I really don't know how to proceed. How does the luminance C manipulate the colorvalues?

Am I using the formular " C = s_i * ( C_raw - b_i) " correctly this way?:

C = (((a₁ * l_u²) - b₁ ) * s₁)

+ (((a₂ * l_v²) - b₂ ) * s₂)

+ (((a₃ * l_v * l_u) - b₃ ) * s₃) 

+ (((a₄ * l_u) - b₄ ) * s₄) 

+ (((a₅ * l_v) - b₅ ) * s₅) 

+  ((a₆ - b₆) * s₆) 

Or is my formula itself wrong?

 

 

 

Hope someone can help me.