Gogpu wgpu with gg canvas transparency

[z46-dev]

Hello!

I have this pipeline like

var hud *ggcanvas.Canvas

// ...

gpuApp.OnDraw(func(dc *gogpu.Context) {

// ...
               if hud == nil {
			var err error

			if hud, err = ggcanvas.New(gpuApp.GPUContextProvider(), hudWidth, hudHeight); err != nil {
				log.Fatalf("create HUD canvas: %v", err)
			}
		}

// ... draw some stuff with wgpu 

                if err = hud.Draw(func(ctx *gg.Context) {
			ctx.Clear()

			ctx.SetRGB(1, 1, 1)
			ctx.SetFont(face)
			ctx.DrawString(fmt.Sprintf("FPS: %d", fps), 24, 36)
			ctx.DrawString("GPU: "+gpuApp.GPUContextProvider().AdapterInfo().Name, 24, 60)
			ctx.DrawString(fmt.Sprintf("Cam: %.1f %.1f %.1f", cam.Position.X, cam.Position.Y, cam.Position.Z), 24, 84)
		}); err != nil {
			log.Errorf("draw HUD: %v", err)
		}

		hud.RenderTo(dc.AsTextureDrawer())

//...

})

My issue is that no matter what I've tried, the hud canvas always has a black background, and I've been unable to get it go be transparent so what renders under it can be seen. What could I do?

[kolkov]

Hi! Good question.

ctx.Clear() already fills with transparent (RGBA 0,0,0,0), and the GPU blend state uses premultiplied alpha (One / OneMinusSrcAlpha), so transparency should work out of the box.

A couple of things to check:

1. Don't use ClearWithColor with an opaque color:

// ❌ This makes the entire HUD opaque black:
ctx.ClearWithColor(gg.Black)

// ✅ This is transparent (Clear() does this by default):
ctx.Clear()

2. Check that DrawString color has correct alpha:

// ✅ White text on transparent background:
ctx.SetRGBA(1, 1, 1, 1)  // or ctx.SetRGB(1, 1, 1)

3. Surface format: If your swapchain uses a format without alpha (e.g., BGRA8UnormSrgb on some platforms), the final composite to the screen may ignore alpha. The HUD texture itself is RGBA with alpha, but the swapchain may not preserve it.

4. Render order: Make sure hud.RenderTo() is called after your 3D scene rendering within the same OnDraw callback, so the HUD composites on top.

If Clear() + SetRGB(1,1,1) + DrawString still gives a black background, could you share the full OnDraw callback? The issue might be in the 3D rendering setup clearing the surface after the HUD is composited.

[z46-dev]

Hi! My solution ended up being not using the canvas, but just a context as a composite.

composite.BeginGPUFrame()
		composite.Clear()
		composite.DrawGPUTextureBase(scene.ggView, 0, 0, width, height)

		composite.SetRGBA(0, 0, 0, 0.38)
		composite.DrawRoundedRectangle(hudX, hudY, hudWidth, hudHeight, 8)
		if err = composite.Fill(); err != nil {
			log.Errorf("fill HUD panel: %v", err)
		}

		composite.SetRGBA(1, 1, 1, 0.14)
		composite.SetLineWidth(2)
		composite.DrawRoundedRectangle(hudX, hudY, hudWidth, hudHeight, 8)
		if err = composite.Stroke(); err != nil {
			log.Errorf("stroke HUD panel: %v", err)
		}

		composite.SetFont(face)

		// Main text.
		composite.SetRGB(1, 1, 1)
		composite.DrawString(fmt.Sprintf("FPS: %d", fps), hudX+12, hudY+28)
		composite.DrawString("GPU: "+gpuName, hudX+12, hudY+52)
		composite.DrawString(fmt.Sprintf("Cam: %.1f %.1f %.1f", cam.Position.X, cam.Position.Y, cam.Position.Z), hudX+12, hudY+76)
		composite.DrawString(fmt.Sprintf("Time: %.2fs", elapsed), hudX+12, hudY+100)

		if err = composite.FlushGPUWithView(finalSurface, uint32(width), uint32(height)); err != nil {
			log.Errorf("flush composite frame: %v", err)
		}

Thank you for your help!

[kolkov]

Great solution! Your pattern (DrawGPUTextureBase + FlushGPUWithView) is actually the recommended approach for 3D + 2D HUD composition — it's the zero-readback compositor path (blit-only, no MSAA overhead).

We also found and fixed the root cause of the black background with RenderTo: the default render pass clear color was hardcoded to opaque black {0,0,0,1} instead of transparent {0,0,0,0}. Fixed in gogpu v0.40.1.

So both approaches now work:

• Your approach (recommended for 3D+HUD): DrawGPUTextureBase + direct GPU composition
• Canvas.RenderTo (now fixed): transparent background by default

go get github.com/gogpu/gogpu@v0.40.1