op/paint: add support for efficient ImageOp subimages

The new field ImageOp.Rect is initialized to cover the entire source
image, but can be modified to draw only a section of it.

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

gpu/gpu.go

@@ -116,6 +116,7 @@ type clipOp struct {
 
 // imageOpData is the shadow of paint.ImageOp.
 type imageOpData struct {
+	rect   image.Rectangle
 	src    *image.RGBA
 	handle interface{}
 }
@@ -148,7 +149,18 @@ func decodeImageOp(data []byte, refs []interface{}) imageOpData {
 	if handle == nil {
 		return imageOpData{}
 	}
+	bo := binary.LittleEndian
 	return imageOpData{
+		rect: image.Rectangle{
+			Min: image.Point{
+				X: int(bo.Uint32(data[1:])),
+				Y: int(bo.Uint32(data[5:])),
+			},
+			Max: image.Point{
+				X: int(bo.Uint32(data[9:])),
+				Y: int(bo.Uint32(data[13:])),
+			},
+		},
 		src:    refs[0].(*image.RGBA),
 		handle: handle,
 	}
@@ -736,12 +748,7 @@ func (d *drawState) materialFor(cache *resourceCache, rect f32.Rectangle, off f3
 		m.material = materialTexture
 		dr := boundRectF(rect.Add(off))
 		sz := d.image.src.Bounds().Size()
-		sr := f32.Rectangle{
-			Max: f32.Point{
-				X: float32(sz.X),
-				Y: float32(sz.Y),
-			},
-		}
+		sr := toRectF(d.image.rect)
 		if dx := float32(dr.Dx()); dx != 0 {
 			// Don't clip 1 px width sources.
 			if sdx := sr.Dx(); sdx > 1 {

internal/opconst/ops.go

@@ -35,7 +35,7 @@ const (
 	TypeTransformLen    = 1 + 4*2
 	TypeLayerLen        = 1
 	TypeRedrawLen       = 1 + 8
-	TypeImageLen        = 1
+	TypeImageLen        = 1 + 4*4
 	TypePaintLen        = 1 + 4*4
 	TypeColorLen        = 1 + 4
 	TypeAreaLen         = 1 + 1 + 4*4

op/paint/paint.go

@@ -16,6 +16,7 @@ import (
 
 // ImageOp sets the material to an image.
 type ImageOp struct {
+	Rect    image.Rectangle
 	uniform bool
 	color   color.RGBA
 	src     *image.RGBA
@@ -48,6 +49,7 @@ func NewImageOp(src image.Image) ImageOp {
 		bounds := src.Bounds()
 		if bounds.Min == (image.Point{}) && src.Stride == bounds.Dx()*4 {
 			return ImageOp{
+				Rect:   src.Bounds(),
 				src:    src,
 				handle: new(int),
 			}
@@ -82,6 +84,11 @@ func (i ImageOp) Add(o *op.Ops) {
 	}
 	data := o.Write(opconst.TypeImageLen, i.src, i.handle)
 	data[0] = byte(opconst.TypeImage)
+	bo := binary.LittleEndian
+	bo.PutUint32(data[1:], uint32(i.Rect.Min.X))
+	bo.PutUint32(data[5:], uint32(i.Rect.Min.Y))
+	bo.PutUint32(data[9:], uint32(i.Rect.Max.X))
+	bo.PutUint32(data[13:], uint32(i.Rect.Max.Y))
 }
 
 func (c ColorOp) Add(o *op.Ops) {

widget/material/image.go

@@ -30,7 +30,7 @@ func (t *Theme) Image(img paint.ImageOp) Image {
 }
 
 func (im Image) Layout(gtx *layout.Context) {
-	size := im.Src.Size()
+	size := im.Src.Rect.Size()
 	wf, hf := float32(size.X), float32(size.Y)
 	w, h := gtx.Px(unit.Dp(wf*im.Scale)), gtx.Px(unit.Dp(hf*im.Scale))
 	cs := gtx.Constraints