Adding transparency support?

[gh2k]

I'd like to render a stack of transparent shapes, and I'm wondering how easy this would be to achieve.

Currently, I'm rendering shape-by-shape and then alpha-compositing them in separate tensorflow operations, but this is slower than I'd like and I'm finding I'm debugging a number of problems where tf is trying to move things back and forth between the CPU and GPU.

I had a go at hacking together support, but I couldn't get it working. I don't really know opengl programming, so that is probably part of the problem.

Here's what I tried:

• In shaders.cpp convert vec3 to vec4 for colour_in and colous_out in the forward shaders: https://github.com/pmh47/dirt/blob/45119bb8c9e53830c76bde400c8ccfa7e776ec3c/csrc/shaders.cpp#L16-L43
• In rasterise_egl.cpp allow import of RGBA colours: https://github.com/pmh47/dirt/blob/45119bb8c9e53830c76bde400c8ccfa7e776ec3c/csrc/rasterise_egl.cpp#L308-L309
• Change the call to glVertexAttribPointer to take objects of size 4 instead of objects.channels: https://github.com/pmh47/dirt/blob/45119bb8c9e53830c76bde400c8ccfa7e776ec3c/csrc/rasterise_egl.cpp#L208
• Added glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); at the end of initialise_per_thread_objects: https://github.com/pmh47/dirt/blob/45119bb8c9e53830c76bde400c8ccfa7e776ec3c/csrc/rasterise_egl.cpp#L214

This still draws the triangles that I expect in the colours that I expect, but they are still opaque, not transparent. (The triangles I'm feeding in have random numbers for their vertex colour alpha components.)

Any ideas what else I might need to change to make this work?

[edit]: I was incorrect about the triangle colours:

If I pass an alpha of <1, the triangles are rendered in a lighter colour. So:

• If I render a triangle with colours [1., 0., 0., 0.5], [1., 0., 0., 0.5], [1., 0., 0., 0.5] then I get a pink triangle. which is what I would expect.
• If I add another triangle that crosses the pink one, with colours [0., 0., 1., 0.5], [0., 0., 1., 0.5], [0., 0., 1., 0.5] then it is rendered in light blue
• But, where they intersect, the blue colour overwrites the pink instead of merging with it. I would expect to see a purple colour, but instead, the blue triangle is a uniform colour and occludes the pink one.

[gh2k]

Ah, never mind I got it working. I think it was the depth buffer getting in the way, so I disabled it.

I'll tidy this up and make a PR.

[Frank-Dz]

Ah, never mind I got it working. I think it was the depth buffer getting in the way, so I disabled it.

I'll tidy this up and make a PR.

Hi~ May I ask you how to render a picture with a transparent background? All my results are like:

I know I should adjust the background_attributes, but I do not know how to set it.

pixels = dirt.rasterise_deferred(
        vertices=cube_vertices_clip,
        vertex_attributes=tf.concat([
            tf.ones_like(cube_vertices_object[:, :1]),  # mask
            cube_uvs,  # texture coordinates
            cube_normals_world  # normals
        ], axis=1),
        faces=cube_faces,
        background_attributes=(tf.ones([frame_height, frame_width, 6])*255),
        shader_fn=shader_fn,
        shader_additional_inputs=[texture, light_direction]
    )

Thanks for any help and guidance! Best, Frank