Neal Johnson

I am trying to modify one video stream based on another video stream, and I am trying to use jit.expr. What I am trying to so is:

If the "/" is a "+" then I get output, the "/" gives me black pixels. I basically want to stay in integer land which "+" does but I know that "/" gets me into floats.

I checked my math in excel and it works. What I am trying to do is:

The math is to modify the "reflected light" seen based on the color of the background. I am assuming that the RGB values fall into 0 to 255. Are there some float versus integer/char things going on? Do I have to force conversions? I looked all over that's why I am here now...

video1 = a colored light source
video2 = a colored background

reflecting-colored-light-jit-expr-question-what-else-float-and-int-issues

You don't need to divide by 255 in this case. Jit.expr natively processes everything as float32, so char values are converted to 0.-1. before running the expression. In the case of char input, jit.expr will convert back to char at the output as well.

Hi Neal,
You don't need to divide by 255 in this case.  Jit.expr natively processes everything as float32, so char values are converted to 0.-1. before running the expression.  In the case of char input, jit.expr will convert back to char at the output as well.


Thanks for the feedback... What I am trying to do is calculate the color of a reflected colored light from a colored surface. So I want to know how much red is in the surface pixel so I divide the red amount R by 255 to get a percentage. I then multiply the incoming light by this percentage to know how much red to reflect back. I am doing this for each R G B channel. My math seems to return black if I do it to all the channels.

output pixel red = video1 pixel red * (video2 pixel red / 255)

output pixel green = video1 pixel green * (video2 pixel green / 255)

output pixel blue = video1 pixel blue * (video2 pixel blue / 255)

If I do only it to the blue I get video but darker. If I add green the video is even darker, add red, just black. Are my math assumptions wrong?

output pixel red  = video1 pixel red * (video2 pixel red / 255)
output pixel green  = video1 pixel green * (video2 pixel green / 255)
output pixel blue  = video1 pixel blue * (video2 pixel blue / 255)

If I do only it to the blue I get video but darker. If I add green the video is even darker, add red, just black. Are my math assumptions wrong?


Reflecting colored light... jit.expr question, what else? float and int issues?