osltoy: auto-detect output variable -- doesn't need to be Cout if there's only one

[lgritz]

Right now, osltoy assumes that the shader being toyed with has a color output variable called "Cout", and that's the one to display.

There are two ways to make this more flexible (either one, or maybe both, have merit):

1. Allow the UI to let the user designate the output to be viewing, not always Cout.
2. Automatically scan (via OSLQuery) the output variables of the compiled shader, and if there is not a "Cout", just use the first output color variable that is found.

Some breadcrumbs to help find the right spots to change:

• OSLToyRenderer::OSLToyRenderer() is where it sets up the shading system, and immediately designates "Cout" as the "renderer_outputs" list attribute, hard coded.
• You can override the renderer_outputs attribute on a per-shader-group basis, using the form of ShadingSystem::attribute() that takes a ShaderGroup* as the first argument.
• OSLToyMainWindow::build_shader_group() is where it sets up the shader group. It doesn't set up any per-group attributes, but it could once the group is made. (It does get some attributes to find out about the group.)
• You could use OSLQuery (the constructor that takes a group) to find out about the group's parameters, and find which ones are output colors, then designate those as renderer outputs for the group. Do that after the group is made (after ShaderGroupEnd), but before you do any getattribute calls on the group, because some of those attribute queries might cause the group to be optimized and JITed, but it's important to designate the outputs before it's optimized (because if there's an output that doesn't appear to be used, it will be optimized away).
• Finally, in OSLToyRenderer::render_image(), you can see "Cout" is used again to know which output buffer to save and display. That obviously needs to be the name of the one you detected or requested, if not Cout.

[silvialpz]

still workin on this!

[lgritz]

There is no rush or deadline, @silvialpz. But if you have questions or need tips, please don't hesitate to ask.

[silvialpz]

HI @lgritz ! Thanks again for the tips, I started out on something but I am stuck and I am not sure how to deal with it. I'm trying out radio buttons for the output variables in the parameter layout.

Here is what I've done up until now:

• I've set up a callback from the OSLToyMainWindow Class to the OSLToyRenderer Class to be able to get the output parameter name that I want to display.
• I used OSLQuery after ShaderGroupEnd but before any getattribute calls as you suggested. I am positive that part is working but now I am failing in the render_image function.
• I'm not sure that I am overriding the renderer_outputs array properly.

Below is the shader I've been using for testing.

surface simple_noise (
    float scale = 10
        [[ string help = "Scaling factor for noise frequency" ]],
    output color bw = 0
        [[ string help = "Perlin noise in black and white" ]],
    output color rgb = 0
        [[ string help = "Perlin noise in color" ]]

)
{
    point uvw = point(u * scale, v * scale, 0);

    float n = noise("perlin", uvw);

    bw = color(n, n, n);
    rgb = color(noise("perlin", uvw),
             noise("perlin", uvw + 10),
             noise("perlin", uvw + 20));
}

If I change one of the variable names to Cout, everything works fine still.

Below is the error I get once I get to the shade_image call with a non-default output name.

Here is what I have done to the render_image function:

And here is the link to the commit in my fork of the repo silvialpz@408a309

Please help 🙏🏽 Does what I've done so far make some sense?

[silvialpz]

@lgritz I just opened a pull request! #2051

I was stuck for a bit but it was because I wasn't interpreting your breadcrumbs correctly