How to change color / transparency of the bodies

[kkkmail]

I want to make a static transparent cylinder so that only the mesh is visible and all objects inside a cylinder are also visible. How do I do that?

As a example, how would I make a planet in PlanetDemo (from Demos.sln) transparent?

Thanks.

[kkkmail]

The main issue seems to be in HLSL files, which do not handle transparency. For example, to enable mesh transparency PSMain in RenderMeshes.hlsl should be changed to:

// TRANSPARENCY CHANGE: Modified return type from float3 to float4 to include alpha
float4 PSMain(PSInput input) : SV_Target0
{
    float3 dpdx = ddx(input.ToAABB);
    float3 dpdy = ddy(input.ToAABB);
    float3 normal = normalize(cross(dpdy, dpdx));
    float3 color = ShadeSurface(
       input.ToAABB, normal, UnpackR11G11B10_UNorm(input.PackedColor), dpdx, dpdy,
       input.InstancePosition, input.Orientation);

    // TRANSPARENCY CHANGE: Extract alpha from PackedColor and return float4 instead of float3
    // For RGBA format (alpha in low 8 bits, matching sphere shader):
    float packedAlpha = (input.PackedColor & 0xFF) / 255.0;
    return float4(color, packedAlpha);

    // TRANSPARENCY CHANGE: To disable transparency, comment out the above 2 lines and uncomment this:
    // return float4(color, 1.0);
}

I've never heard about HLSL until like about 3 days ago, so all the credit goes to Claude 4. Then the Renderer from DemoRenderer can be tweaked to make all or some meshes as transparent as necessary. The same can be done with spheres.

[RossNordby]

Sorry for the slow reply, somehow totally missed this post:
You will indeed need to update the relevant shaders, but also the blend modes on the CPU invocation side. The default blend mode is opaque, so all the samples covered by a new object fully replace what was there before.

Raytraced shapes have a different behavior; they rely on Alpha to Coverage to handle antialiasing with MSAA enabled. Notably, while Alpha to Coverage isn't the same thing as true transparency, you can sorta-kinda do transparency with it, you just get very few alpha levels (proportional to your MSAA depth sample count). You'd also need to be able to inform the shader about how much transparency you'd want for each shape with some additional uploaded data.

Notably, transparent blend modes that don't route through alpha to coverage will end up 'wrong' when layered because the renderer doesn't care about depth sorting. The final composed result in the framebuffer will be dependent on the order of batch renders, and further, even if you do sort instances, there will remain corner cases where there's no 'correct' order at the instance level (only at the pixel level).

Alpha to coverage is kinda nice because it just uses depth samples for 'transparency,' so you get render order independence by default.

I don't plan to add any graphical features to the demos, though; it's a very narrow renderer meant for doing only one thing reasonably quickly without too much complexity.