[Cin] Some tests on blending

[Georgy Salnikov]

On Mon, 31 Mar 2025, Andrea paz wrote:

> With:
> printf ("%g\n", R(0));
> gives me a very long series of \u201c1's\u201d; perhaps a result for each pixel
> of the 1920x1080 frame?

Yes, it is what I meant. The function is called repeatedly for each pixel of
the whole frame. Moreover, if 'parallelize' is switched on in the blend
program dialog, and there is 'PARALLEL_SAFE' in the function, then the order
of printed lines is unpredictable.

> With:
> if (PIX_X == 0 && PIX_Y == 0) printf ("%g\n", R(0));
> It gives me a simple 1 for all plugins, with very few exceptions
> when the frame is replaced by other colors for which the different
> value is shown (always under 1).

It is clipping.

> I can't interpret the results: I always get 1, even for Choma Key
> (Avid) and for Blender Algebra itself. I have tried all the internal

To be completely sure, you can do the following.

Deactivate (or detach) the other plugin which was placed between the two
BlendPrograms.

In the first BlendProgram (with R(0)=2.0;) add the line just after the
assignment of 2.0:

if (PIX_X == 0 && PIX_Y == 0) printf ("It was set: %g\n", R(0));

So you will see what was assigned to R(0) (with the first program) as well
as what was read back (with the second one).

Recheck that both functions have COLORSPACE_RGB (not YUV, not HSV).

Recheck that your project is RGBA_FLOAT (not 8 bit) and that clipping is
switched off in the dialogs of both BlendPrograms. So BlendPrograms
themselves should not introduce any color transformations.

Then test once again what values come out.

Actually it might be that some base function clips in between, such as
read_frame().
_______________________________________________________________________________

Georgy Salnikov
NMR Group
Novosibirsk Institute of Organic Chemistry
Lavrentjeva, 9, 630090 Novosibirsk, Russia
Phone   +7-383-3307864
Email   sge at nmr.nioch.nsc.ru
_______________________________________________________________________________