Render to texture with transparent background

Hey there,
I’m trying to render to a texture that has it’s background color set to (0, 0, 0, 0).
My code looks like this:

    RenderTexture = MakeShared<Texture2D>(parent.GetContext());
    RenderTexture->SetSize(500, 500, Graphics::GetRGBAFormat(), TEXTURE_RENDERTARGET);

    RenderMaterial = MakeShared<Material>(parent.GetContext());
    RenderMaterial->SetTechnique(0, cache->GetResource<Technique>("Techniques/DiffUnlitAlpha.xml"));
    RenderMaterial->SetTexture(TU_DIFFUSE, RenderTexture);
    OrthoScene = MakeShared<Scene>(parent.GetContext());
    //setup a scene with an orthographic camera here

    auto *surface = RenderTexture->GetRenderSurface();
    auto vp = MakeShared<Viewport>(parent.GetContext(), OrthoScene, camera);
    surface->SetViewport(0, vp);

    auto *rp = vp->GetRenderPath();
    for(unsigned i = 0; i < rp->GetNumCommands(); i++)
        RenderPathCommand *cmd = rp->GetCommand(i);
        if(cmd->type_ == RenderCommandType::CMD_CLEAR)
            cmd->useFogColor_ = false;
            cmd->clearColor_ = Color(0, 0, 0, 0);

    auto *quad = SceneNode->CreateComponent<CustomGeometry>();
    //setup quad here

I’ve been able to set the background color to a custom value by modifying clearColor_, but setting the alpha value has no effect.

Am I missing something?

I see no faults in your code.
What happens when you change the color to (1, 1, 1, 0); is the resulting background black or white?

…and welcome to the forums! :confetti_ball: :slightly_smiling_face:

…and welcome to the forums!


What happens when you change the color to (1, 1, 1, 0) ; is the resulting background black or white?

The resulting background is white. Setting the RGB actually gives the desired color, but setting the alpha has no effect.

I figured out that D3D by default doesn’t write alpha values and must be enabled with SetRenderState ( D3DRS_COLORWRITEENABLE, D3DCOLORWRITEENABLE_RED | D3DCOLORWRITEENABLE_GREEN | D3DCOLORWRITEENABLE_BLUE | D3DCOLORWRITEENABLE_ALPHA );. Urho3D apparently doesn’t do this. Maybe that’s worth to implement properly.

Afaik OpenGL does write alpha values, so I’ll switch to it.


I’m glad you figured it out. It’s been years since I touched a Windows machine.