jit.expr fills your matrix based on a formula. in this case you have a one plane matrix only: so if you have black its just the number "0", if you have pure white it is "255".

If you want to replace all the black pixels (= zeros), you could first transform the matrix in a 4 plane matrix (jit.matrix 4 char 200 100), then you could use f.e. jit.chromakey to replace black with another color, or you could use jit.op...

Thank you very much.

So, can't I create the same effect with a different 'color background' inside 'jit.expr' (changing, of course the matrix to 4 planes)? I tried to change the expression, but no success…

mat

since you are simply trying to replace black (or 0), as simple [ jit.op @op + ] will do the trick. one way of many:

Thank you Robert,

now it's clearer.