Bug: rounded rectangle does not render correctly

[amortaza]

Below is the minimum code that demonstrates the problem.

When the canvas_width is 205, it renders correctly (the rectangle is correctly cutoff as it exceeds canvas height).

When the canvas_width is 204, it renders incorrectly.

require (
	github.com/gogpu/gg v0.28.6
	github.com/gogpu/gogpu v0.19.6
)

package main

import (
	"github.com/gogpu/gg"
	_ "github.com/gogpu/gg/gpu" // Register GPU accelerator (SDF + MSAA 4x)
	"github.com/gogpu/gg/integration/ggcanvas"
	"github.com/gogpu/gogpu"
)

func main() {

	const width, height = 800, 800
	const canvas_width, canvas_height = 204, 100

	app := gogpu.NewApp(gogpu.DefaultConfig().
		WithTitle("GoGPU + gg: Three-Tier GPU Rendering").
		WithSize(width, height).
		WithContinuousRender(false)) // Event-driven: 0% CPU when paused

	var canvas *ggcanvas.Canvas

	app.OnDraw(func(dc *gogpu.Context) {
		if canvas == nil {
			provider := app.GPUContextProvider()
			canvas, _ = ggcanvas.New(provider, canvas_width, canvas_height)
		}

		canvas.Draw(func(cc *gg.Context) {
			renderFrame(cc, canvas_width, canvas_height)
		})

		canvas.RenderTo(dc.AsTextureDrawer())
	})

	app.OnClose(func() {
		gg.CloseAccelerator()
		if canvas != nil {
			_ = canvas.Close()
			canvas = nil
		}
	})

	app.Run()
}

func renderFrame(cc *gg.Context, width, height int) {
	cc.SetRGBA(0.2, 0.6, 1.0, 0.7)
	cc.DrawRoundedRectangle(10, 10, 100, 100, 12)
	cc.Fill()
}

[kolkov]

Thank you for the detailed bug report with a minimal reproduction case, @amortaza!

I investigated the rendering pipeline and found several areas where canvas dimensions interact with GPU rendering:

What we found:

• The SDF render pipeline (used for rounded rectangles) creates GPU textures matching the canvas size, renders the shape, then reads back pixels
• On DX12, row pitch must be aligned to 256 bytes — both 204×4=816 and 205×4=820 round up to 1024
• The readback path in render_session.go properly handles this alignment (strips padding per row)
• The rounded rect at (10, 10, 100, 100) is well within both 204 and 205 pixel bounds, so tile boundary calculations shouldn't be the issue

Recommendation:
Since v0.28.6, there have been significant rendering fixes in v0.29.0 and v0.29.1:

• Unified render session with proper DX12 row pitch alignment
• GPU stroke pipeline improvements
• Multiple readback and compositing fixes

Could you try upgrading to the latest versions?

require (
    github.com/gogpu/gg v0.29.1
    github.com/gogpu/gogpu v0.20.1
)

If the issue persists after upgrade, please share:

1. Your OS and GPU model
2. Which backend is being used (Vulkan/DX12) — you can check log output at startup
3. Whether adding CGO_ENABLED=0 to your build affects the result

This will help us narrow down whether it's a driver-specific issue or something in the rendering pipeline.

[intelafshin]

Issue persists.

require (
	github.com/gogpu/gg v0.30.1
	github.com/gogpu/gogpu v0.20.9
)

Windows 11 Pro
NVIDIA RTX 3500 Ada Generation Laptop GPU (driver 32.0.15.7371)
Backend: "Pure Go (gogpu/wgpu/vulkan)"
add "CGO_ENABLED=0" had no effect

[kolkov]

@intelafshin Thanks for confirming and providing your environment details!

We've identified the root cause. The bug is in the Vulkan backend's buffer-to-image copy conversion (wgpu/hal/vulkan/command.go). When BytesPerRow is padded to 256-byte alignment, the code incorrectly infers bytesPerTexel via integer division BytesPerRow / Width instead of using the texture format's known block size.

For width=204: 1024 / 204 = 5 (wrong, should be 4) → Vulkan gets wrong row stride → pixel corruption.
For width=205: 1024 / 205 = 4 (lucky) → works by accident.

The fix aligns with how the Rust wgpu reference implementation handles this — using format.block_copy_size() (a static lookup) rather than inferring from dimensions.

We're pushing the fix now. Will update here when released.

[kolkov]

The fix has been released across the ecosystem:

• wgpu v0.18.1 — the actual fix (blockCopySize() static lookup)
• gogpu v0.22.1 — updated dependency
• gg v0.31.1 — updated dependency

To pick up the fix:

go get -u github.com/gogpu/gg@v0.31.1

Would appreciate confirmation that the rounded rectangle renders correctly on your setup!

[intelafshin]

The issue has been resolved!
Thank you

[kolkov]

Thank you very much for testing and your patience!