op/paint: add nearest neighbor scaling

This adds support for nearest neighbor filtering,
which can be useful in quite a few scenarios.

  img := paint.NewImageOp(m)
  img.Filter = paint.FilterNearest
  img.Add(gtx.Ops)

Fixes: https://todo.sr.ht/~eliasnaur/gio/414
Signed-off-by: Egon Elbre <egonelbre@gmail.com>

gpu/gpu.go

@@ -186,10 +186,16 @@ type material struct {
 	uvTrans f32.Affine2D
 }
 
+const (
+	filterLinear  = 0
+	filterNearest = 1
+)
+
 // imageOpData is the shadow of paint.ImageOp.
 type imageOpData struct {
 	src    *image.RGBA
 	handle interface{}
+	filter byte
 }
 
 type linearGradientOpData struct {
@@ -207,6 +213,7 @@ func decodeImageOp(data []byte, refs []interface{}) imageOpData {
 	return imageOpData{
 		src:    refs[0].(*image.RGBA),
 		handle: handle,
+		filter: data[1],
 	}
 }
 
@@ -454,19 +461,41 @@ func (g *gpu) Profile() string {
 }
 
 func (r *renderer) texHandle(cache *resourceCache, data imageOpData) driver.Texture {
+	type cachekey struct {
+		filter byte
+		handle any
+	}
+	key := cachekey{
+		filter: data.filter,
+		handle: data.handle,
+	}
+
 	var tex *texture
-	t, exists := cache.get(data.handle)
+	t, exists := cache.get(key)
 	if !exists {
 		t = &texture{
 			src: data.src,
 		}
-		cache.put(data.handle, t)
+		cache.put(key, t)
 	}
 	tex = t.(*texture)
 	if tex.tex != nil {
 		return tex.tex
 	}
-	handle, err := r.ctx.NewTexture(driver.TextureFormatSRGBA, data.src.Bounds().Dx(), data.src.Bounds().Dy(), driver.FilterLinearMipmapLinear, driver.FilterLinear, driver.BufferBindingTexture)
+
+	var minFilter, magFilter driver.TextureFilter
+	switch data.filter {
+	case filterLinear:
+		minFilter, magFilter = driver.FilterLinearMipmapLinear, driver.FilterLinear
+	case filterNearest:
+		minFilter, magFilter = driver.FilterNearest, driver.FilterNearest
+	}
+
+	handle, err := r.ctx.NewTexture(driver.TextureFormatSRGBA,
+		data.src.Bounds().Dx(), data.src.Bounds().Dy(),
+		minFilter, magFilter,
+		driver.BufferBindingTexture,
+	)
 	if err != nil {
 		panic(err)
 	}

gpu/internal/rendertest/render_test.go

@@ -359,6 +359,73 @@ func TestImageRGBA(t *testing.T) {
 	})
 }
 
+func TestImageRGBA_ScaleLinear(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		w := newWindow(t, 128, 128)
+		defer clip.Rect{Max: image.Pt(128, 128)}.Push(o).Pop()
+		op.Affine(f32.Affine2D{}.Scale(f32.Point{}, f32.Pt(64, 64))).Add(o)
+
+		im := image.NewRGBA(image.Rect(0, 0, 2, 2))
+		im.Set(0, 0, colornames.Red)
+		im.Set(1, 0, colornames.Green)
+		im.Set(0, 1, colornames.White)
+		im.Set(1, 1, colornames.Black)
+
+		op := paint.NewImageOp(im)
+		op.Filter = paint.FilterLinear
+		op.Add(o)
+
+		paint.PaintOp{}.Add(o)
+
+		if err := w.Frame(o); err != nil {
+			t.Error(err)
+		}
+	}, func(r result) {
+		r.expect(0, 0, colornames.Red)
+		r.expect(8, 8, colornames.Red)
+
+		// TODO: this currently seems to do srgb scaling
+		// instead of linear rgb scaling,
+		r.expect(64-4, 0, color.RGBA{R: 197, G: 87, B: 0, A: 255})
+		r.expect(64+4, 0, color.RGBA{R: 175, G: 98, B: 0, A: 255})
+
+		r.expect(127, 0, colornames.Green)
+		r.expect(127-8, 8, colornames.Green)
+	})
+}
+
+func TestImageRGBA_ScaleNearest(t *testing.T) {
+	run(t, func(o *op.Ops) {
+		w := newWindow(t, 128, 128)
+		op.Affine(f32.Affine2D{}.Scale(f32.Point{}, f32.Pt(64, 64))).Add(o)
+
+		im := image.NewRGBA(image.Rect(0, 0, 2, 2))
+		im.Set(0, 0, colornames.Red)
+		im.Set(1, 0, colornames.Green)
+		im.Set(0, 1, colornames.White)
+		im.Set(1, 1, colornames.Black)
+
+		op := paint.NewImageOp(im)
+		op.Filter = paint.FilterNearest
+		op.Add(o)
+
+		paint.PaintOp{}.Add(o)
+
+		if err := w.Frame(o); err != nil {
+			t.Error(err)
+		}
+	}, func(r result) {
+		r.expect(0, 0, colornames.Red)
+		r.expect(8, 8, colornames.Red)
+
+		r.expect(64-4, 0, colornames.Red)
+		r.expect(64+4, 0, colornames.Green)
+
+		r.expect(127, 0, colornames.Green)
+		r.expect(127-8, 8, colornames.Green)
+	})
+}
+
 func TestGapsInPath(t *testing.T) {
 	ops := new(op.Ops)
 	var p clip.Path