How to setup a zbias and alpha threshold for rendering decals

Community Hydra
1

Hi, i am setting up an HdxRenderTask with the following code:

auto decoRenderTaskId = GetControllerId().AppendChild(TfToken(“renderTask_decorations”));
GetRenderIndex()->InsertTask(&delegate, decoRenderTaskId);

HdxRenderTaskParams renderParams;
renderParams.camera = freeCameraSceneDelegate->GetCameraId();
renderParams.viewport = viewport;
renderParams.framing = framing;
renderParams.overrideWindowPolicy = overrideWindowPolicy;

renderParams.enableLighting = true;
renderParams.depthFunc = HdCmpFuncLEqual;
renderParams.cullStyle = HdCullStyleBack;
renderParams.alphaThreshold = 0.5f;

renderParams.depthBiasUseDefault = false;
renderParams.depthBiasEnable = true;
renderParams.depthBiasConstantFactor = 0.01f;
renderParams.depthBiasSlopeFactor = 1.f;

renderParams.blendEnable = false;
renderParams.depthMaskEnable = true;
renderParams.enableAlphaToCoverage = true;
renderParams.useAovMultiSample = true;

HdRprimCollection coll(HdTokens->geometry,
                       HdReprSelector(HdReprTokens->refined),
                       SdfPath::AbsoluteRootPath().AppendChild(TfToken("DecorationMeshes")));
coll.SetMaterialTag(HdStMaterialTagTokens->masked);

TfTokenVector renderTags = {HdRenderTagTokens->geometry};

delegate.SetParameter(decoRenderTaskId, HdTokens->params, renderParams);
delegate.SetParameter(decoRenderTaskId, HdTokens->collection, coll);
delegate.SetParameter(decoRenderTaskId, HdTokens->renderTags, renderTags

The collection is rendered , but without zbias and alpha masking applied. I debugged through hydra and veryfied that even the gl call is issued:
glPolygonOffset(_depthBiasSlopeFactor, _depthBiasConstantFactor); is called with the correct values.
Maybe i am missing something important to set this up? If i play around with other render task parameters they are reflected in the rendered scene. Only zbias and alpha treshold doesn’t seem to do anything. Why could this be?

Cheers,
Robert

2

Sorry i found the reason. My fault: i needed negative bias values instead of positive. However the alpha threshold still not seems to do anything. How do i setup alpha masking? I suppose that is done in the preview shader?

1 Like
3

Alright! I found the reason now why i don’t see alpha. It was because i forgot to connect the alpha texture output to the opacity input of the PreviewSurface shader. The rgb output of the UvTexture node outputs just rgb, not a and so without connecting alpha in addition, it won’t consider texture transparency of course.

1 Like