internal/restorable: resolve a stale state when possible

image.go

@@ -267,7 +267,31 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) {
 		})
 	}
 
-	i.image.DrawTriangles(srcs, vs, is, blend, i.adjustedBounds(), [graphics.ShaderSrcImageCount]image.Rectangle{img.adjustedBounds()}, shader.shader, i.tmpUniforms, graphicsdriver.FillRuleFillAll, canSkipMipmap(geoM, filter), false, restorable.HintNone)
+	dr := i.adjustedBounds()
+	hint := restorable.HintNone
+	if overwritesDstRegion(options.Blend, dr, geoM, sx0, sy0, sx1, sy1) {
+		hint = restorable.HintOverwriteDstRegion
+	}
+
+	i.image.DrawTriangles(srcs, vs, is, blend, dr, [graphics.ShaderSrcImageCount]image.Rectangle{img.adjustedBounds()}, shader.shader, i.tmpUniforms, graphicsdriver.FillRuleFillAll, canSkipMipmap(geoM, filter), false, hint)
+}
+
+// overwritesDstRegion reports whether the given parameters overwrite the destination region completely.
+func overwritesDstRegion(blend Blend, dstRegion image.Rectangle, geoM GeoM, sx0, sy0, sx1, sy1 int) bool {
+	if blend != BlendCopy {
+		return false
+	}
+	// Check the result vertices is not a rotated rectangle.
+	if geoM.b != 0 || geoM.c != 0 {
+		return false
+	}
+	// Check the result vertices completely covers dstRegion.
+	x0, y0 := geoM.Apply(float64(sx0), float64(sy0))
+	x1, y1 := geoM.Apply(float64(sx1), float64(sy1))
+	if float64(dstRegion.Min.X) < x0 || float64(dstRegion.Min.Y) < y0 || float64(dstRegion.Max.X) > x1 || float64(dstRegion.Max.Y) > y1 {
+		return false
+	}
+	return true
 }
 
 // Vertex represents a vertex passed to DrawTriangles.
@@ -856,7 +880,14 @@ func (i *Image) DrawRectShader(width, height int, shader *Shader, options *DrawR
 	i.tmpUniforms = i.tmpUniforms[:0]
 	i.tmpUniforms = shader.appendUniforms(i.tmpUniforms, options.Uniforms)
 
-	i.image.DrawTriangles(imgs, vs, is, blend, i.adjustedBounds(), srcRegions, shader.shader, i.tmpUniforms, graphicsdriver.FillRuleFillAll, true, false, restorable.HintNone)
+	dr := i.adjustedBounds()
+	hint := restorable.HintNone
+	// Do not use srcRegions[0].Dx() and srcRegions[0].Dy() as these might be empty.
+	if overwritesDstRegion(options.Blend, dr, geoM, srcRegions[0].Min.X, srcRegions[0].Min.Y, srcRegions[0].Min.X+width, srcRegions[0].Min.Y+height) {
+		hint = restorable.HintOverwriteDstRegion
+	}
+
+	i.image.DrawTriangles(imgs, vs, is, blend, i.adjustedBounds(), srcRegions, shader.shader, i.tmpUniforms, graphicsdriver.FillRuleFillAll, true, false, hint)
 }
 
 // SubImage returns an image representing the portion of the image p visible through r.

internal/restorable/image.go

@@ -347,10 +347,25 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 	}
 
 	// Even if the image is already stale, call makeStale to extend the stale region.
-	if srcstale || !needsRestoration() || !i.needsRestoration() || i.stale {
+	if srcstale || !needsRestoration() || !i.needsRestoration() {
 		i.makeStale(dstRegion)
+	} else if i.stale {
+		var overwrite bool
+		if hint == HintOverwriteDstRegion {
+			overwrite = i.areStaleRegionsIncludedIn(dstRegion)
+		}
+		if overwrite {
+			i.basePixels.Clear(dstRegion)
+			i.clearDrawTrianglesHistory()
+			i.stale = false
+			i.staleRegions = i.staleRegions[:0]
 		} else {
-		// TODO: Consider overwritten.
+			// Even if the image is already stale, call makeStale to extend the stale region.
+			i.makeStale(dstRegion)
+		}
+	}
+
+	if !i.stale {
 		i.appendDrawTrianglesHistory(srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule, hint)
 	}
 
@@ -364,6 +379,18 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 	i.image.DrawTriangles(imgs, vertices, indices, blend, dstRegion, srcRegions, shader.shader, uniforms, fillRule)
 }
 
+func (i *Image) areStaleRegionsIncludedIn(r image.Rectangle) bool {
+	if !i.stale {
+		return false
+	}
+	for _, sr := range i.staleRegions {
+		if !sr.In(r) {
+			return false
+		}
+	}
+	return true
+}
+
 // appendDrawTrianglesHistory appends a draw-image history item to the image.
 func (i *Image) appendDrawTrianglesHistory(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, hint Hint) {
 	if i.stale || !i.needsRestoration() {