gpu: [compute] clip image materials to their rendered size

Before this change, transformed images would take up as much atlas
texture space to fit them. However, we can easily run out of space for
large images or images with large scaling applied.

This change limits transformed images to their rendered bounds which is
the window size in the worst case.

Updates gio#219 (fixes the chat kitchen example)

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

gpu/compute.go

@@ -285,6 +285,7 @@ type materialVertex struct {
 type textureKey struct {
 	handle    interface{}
 	transform f32.Affine2D
+	bounds    image.Rectangle
 }
 
 // textureOp represents an paintOp that requires texture space.
@@ -936,7 +937,11 @@ func (g *compute) renderMaterials() error {
 				break
 			}
 			imgAtlas = op.imgAlloc.atlas
-			quad, bounds := g.materialQuad(imgAtlas.size, op.key.transform, op.img, op.imgAlloc.rect.Min)
+			quad := g.materialQuad(imgAtlas.size, op.key.transform, op.img, op.imgAlloc.rect.Min)
+			boundsf := quadBounds(quad)
+			bounds := boundRectF(boundsf)
+			bounds = bounds.Intersect(op.key.bounds)
+
 			size := bounds.Size()
 			alloc, fits := g.atlasAlloc(allocQuery{
 				atlas:    atlas,
@@ -1102,7 +1107,7 @@ func pow2Ceil(v int) int {
 
 // materialQuad constructs a quad that represents the transformed image. It returns the quad
 // and its bounds.
-func (g *compute) materialQuad(imgAtlasSize image.Point, M f32.Affine2D, img imageOpData, uvPos image.Point) ([4]materialVertex, image.Rectangle) {
+func (g *compute) materialQuad(imgAtlasSize image.Point, M f32.Affine2D, img imageOpData, uvPos image.Point) [4]materialVertex {
 	imgSize := layout.FPt(img.src.Bounds().Size())
 	sx, hx, ox, hy, sy, oy := M.Elems()
 	transOff := f32.Pt(ox, oy)
@@ -1121,12 +1126,6 @@ func (g *compute) materialQuad(imgAtlasSize image.Point, M f32.Affine2D, img ima
 	q2 = q2.Add(transOff)
 	q3 = q3.Add(transOff)
 
-	boundsf := f32.Rectangle{
-		Min: min(min(q0, q1), min(q2, q3)),
-		Max: max(max(q0, q1), max(q2, q3)),
-	}
-
-	bounds := boundRectF(boundsf)
 	uvPosf := layout.FPt(uvPos)
 	atlasScale := f32.Pt(1/float32(imgAtlasSize.X), 1/float32(imgAtlasSize.Y))
 	uvBounds := f32.Rectangle{
@@ -1143,7 +1142,18 @@ func (g *compute) materialQuad(imgAtlasSize image.Point, M f32.Affine2D, img ima
 		{posX: q2.X, posY: q2.Y, u: uvBounds.Max.X, v: uvBounds.Max.Y},
 		{posX: q3.X, posY: q3.Y, u: uvBounds.Max.X, v: uvBounds.Min.Y},
 	}
-	return quad, bounds
+	return quad
+}
+
+func quadBounds(q [4]materialVertex) f32.Rectangle {
+	q0 := f32.Pt(q[0].posX, q[0].posY)
+	q1 := f32.Pt(q[1].posX, q[1].posY)
+	q2 := f32.Pt(q[2].posX, q[2].posY)
+	q3 := f32.Pt(q[3].posX, q[3].posY)
+	return f32.Rectangle{
+		Min: min(min(q0, q1), min(q2, q3)),
+		Max: max(max(q0, q1), max(q2, q3)),
+	}
 }
 
 func max(p1, p2 f32.Point) f32.Point {
@@ -1855,10 +1865,12 @@ func (c *collector) collect(root *op.Ops, viewport image.Point, texOps *[]textur
 			// except for their integer offsets can share a transformed image.
 			t := op.state.t.Offset(layout.FPt(op.offset))
 			t, off := separateTransform(t)
+			bounds := boundRectF(op.intersect).Sub(off)
 			*texOps = append(*texOps, textureOp{
 				img: op.state.image,
 				off: off,
 				key: textureKey{
+					bounds:    bounds,
 					transform: t,
 					handle:    op.state.image.handle,
 				},