gpu: don't automatically clear screen before rendering

Gio UI may be overlaid on top of custom graphics such as in the glfw example.
That will only work if Gio doesn't clear the screen (to white).

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless.go

@@ -6,6 +6,7 @@ package headless
 
 import (
 	"image"
+	"image/color"
 	"runtime"
 
 	"gioui.org/gpu"
@@ -108,6 +109,7 @@ func (w *Window) Release() {
 // operation list.
 func (w *Window) Frame(frame *op.Ops) error {
 	return contextDo(w.ctx, func() error {
+		w.gpu.Clear(color.NRGBA{A: 0xff, R: 0xff, G: 0xff, B: 0xff})
 		w.gpu.Collect(w.size, frame)
 		return w.gpu.Frame()
 	})

app/loop.go

@@ -4,6 +4,7 @@ package app
 
 import (
 	"image"
+	"image/color"
 	"runtime"
 
 	"gioui.org/app/internal/window"
@@ -85,6 +86,7 @@ func (l *renderLoop) renderLoop(ctx window.Context) error {
 				l.refreshErr <- ctx.MakeCurrent()
 			case frame := <-l.frames:
 				ctx.Lock()
+				g.Clear(color.NRGBA{A: 0xff, R: 0xff, G: 0xff, B: 0xff})
 				g.Collect(frame.viewport, frame.ops)
 				// Signal that we're done with the frame ops.
 				l.ack <- struct{}{}

gpu/compute.go

@@ -230,6 +230,11 @@ func (g *compute) Collect(viewport image.Point, ops *op.Ops) {
 	}
 }
 
+func (g *compute) Clear(col color.NRGBA) {
+	g.drawOps.clear = true
+	g.drawOps.clearColor = f32color.LinearFromSRGB(col)
+}
+
 func (g *compute) Frame() error {
 	viewport := g.drawOps.viewport
 	tileDims := image.Point{
@@ -285,8 +290,11 @@ func (g *compute) encode(viewport image.Point) {
 	// Flip Y-axis.
 	flipY := f32.Affine2D{}.Scale(f32.Pt(0, 0), f32.Pt(1, -1)).Offset(f32.Pt(0, float32(viewport.Y)))
 	g.enc.transform(flipY)
-	g.enc.rect(f32.Rectangle{Max: layout.FPt(viewport)}, false)
-	g.enc.fill(f32color.NRGBAToRGBA(g.drawOps.clearColor.SRGB()))
+	if g.drawOps.clear {
+		g.drawOps.clear = false
+		g.enc.rect(f32.Rectangle{Max: layout.FPt(viewport)}, false)
+		g.enc.fill(f32color.NRGBAToRGBA(g.drawOps.clearColor.SRGB()))
+	}
 	g.encodeOps(flipY, viewport, g.drawOps.allImageOps)
 }
 

gpu/gpu.go

@@ -31,9 +31,17 @@ import (
 )
 
 type GPU interface {
+	// Release non-Go resources. The GPU is no longer valid after Release.
 	Release()
-	Collect(viewport image.Point, frameOps *op.Ops)
+	// Clear sets the clear color for the next Frame.
+	Clear(color color.NRGBA)
+	// Collect the graphics operations from frame, given the viewport.
+	Collect(viewport image.Point, frame *op.Ops)
+	// Frame clears the color buffer and draws the collected operations.
 	Frame() error
+	// Profile returns the last available profiling information. Profiling
+	// information is requested when Collect sees a ProfileOp, and the result
+	// is available through Profile at some later time.
 	Profile() string
 }
 
@@ -65,6 +73,7 @@ type drawOps struct {
 	cache      *resourceCache
 	vertCache  []byte
 	viewport   image.Point
+	clear      bool
 	clearColor f32color.RGBA
 	// allImageOps is the combined list of imageOps and
 	// zimageOps, in drawing order.
@@ -404,6 +413,11 @@ func (g *gpu) init(ctx backend.Device) error {
 	return nil
 }
 
+func (g *gpu) Clear(col color.NRGBA) {
+	g.drawOps.clear = true
+	g.drawOps.clearColor = f32color.LinearFromSRGB(col)
+}
+
 func (g *gpu) Release() {
 	g.renderer.release()
 	g.drawOps.pathCache.release()
@@ -442,7 +456,10 @@ func (g *gpu) Frame() error {
 	g.ctx.DepthFunc(backend.DepthFuncGreater)
 	// Note that Clear must be before ClearDepth if nothing else is rendered
 	// (len(zimageOps) == 0). If not, the Fairphone 2 will corrupt the depth buffer.
-	g.ctx.Clear(g.drawOps.clearColor.Float32())
+	if g.drawOps.clear {
+		g.drawOps.clear = false
+		g.ctx.Clear(g.drawOps.clearColor.Float32())
+	}
 	g.ctx.ClearDepth(0.0)
 	g.ctx.Viewport(0, 0, viewport.X, viewport.Y)
 	g.renderer.drawZOps(g.cache, g.drawOps.zimageOps)
@@ -799,7 +816,6 @@ func floor(v float32) int {
 
 func (d *drawOps) reset(cache *resourceCache, viewport image.Point) {
 	d.profile = false
-	d.clearColor = f32color.RGBA{R: 1.0, G: 1.0, B: 1.0, A: 1.0}
 	d.cache = cache
 	d.viewport = viewport
 	d.imageOps = d.imageOps[:0]
@@ -1006,6 +1022,7 @@ loop:
 				d.imageOps = d.imageOps[:0]
 				z = 0
 				d.clearColor = mat.color.Opaque()
+				d.clear = true
 				continue
 			}
 			z++

internal/rendertest/util_test.go

@@ -161,7 +161,7 @@ func verifyRef(t *testing.T, img *image.RGBA, frame int) (ok bool) {
 	}
 	ref, ok := r.(*image.RGBA)
 	if !ok {
-		t.Error("ref image note RGBA")
+		t.Errorf("image is a %T, expected *image.RGBA", r)
 		return
 	}
 	if len(ref.Pix) != len(img.Pix) {