graphics: Refactoring: Flip Y by geometry matrix instead of projection matrix

graphicscontext.go

@@ -34,6 +34,8 @@ type graphicsContext struct {
 	screen      *Image
 	initialized bool
 	invalidated bool // browser only
+	offsetX     float64
+	offsetY     float64
 }
 
 func (c *graphicsContext) Invalidate() {
@@ -60,6 +62,9 @@ func (c *graphicsContext) SetSize(screenWidth, screenHeight int, screenScale flo
 	_ = c.screen.Clear()
 
 	c.offscreen = offscreen
+
+	c.offsetX = px0
+	c.offsetY = py0
 }
 
 func (c *graphicsContext) initializeIfNeeded() error {
@@ -93,7 +98,21 @@ func (c *graphicsContext) Update(afterFrameUpdate func()) error {
 		afterFrameUpdate()
 	}
 	if 0 < updateCount {
-		drawWithFittingScale(c.screen, c.offscreen, nil, filterScreen)
+		dw, dh := c.screen.Size()
+		sw, _ := c.offscreen.Size()
+		scale := float64(dw) / float64(sw)
+
+		op := &DrawImageOptions{}
+
+		// c.screen is special: its Y axis is down to up,
+		// and the origin point is lower left.
+		op.GeoM.Scale(scale, -scale)
+		op.GeoM.Translate(0, float64(dh))
+		op.GeoM.Translate(c.offsetX, c.offsetY)
+
+		op.CompositeMode = CompositeModeCopy
+		op.Filter = filterScreen
+		_ = c.screen.DrawImage(c.offscreen, op)
 	}
 
 	if err := restorable.ResolveStaleImages(); err != nil {

internal/graphics/framebuffer.go

@@ -109,12 +109,6 @@ func (f *framebuffer) projectionMatrix(height int) []float32 {
 	}
 	w, h := f.viewportSize()
 	m := orthoProjectionMatrix(0, w, 0, h)
-	if f.flipY {
-		m[4*1+1] *= -1
-		m[4*3+1] += float32(height) / float32(h) * 2
-	}
-	m[4*3+0] += float32(f.offsetX) / float32(w) * 2
-	m[4*3+1] += float32(f.offsetY) / float32(h) * 2
 	f.proMatrix = m
 	return f.proMatrix
 }

internal/restorable/image.go

@@ -171,10 +171,10 @@ func (i *Image) clearIfVolatile() {
 	}
 
 	w, h := i.image.Size()
-	x0 := -float32(i.paddingX0)
-	y0 := -float32(i.paddingY0)
-	x1 := float32(w) + float32(i.paddingX1)
-	y1 := float32(h) + float32(i.paddingY1)
+	x0 := float32(0)
+	y0 := float32(0)
+	x1 := float32(w) + float32(i.paddingX0+i.paddingX1)
+	y1 := float32(h) + float32(i.paddingY0+i.paddingY1)
 	// For the rule of values, see vertices.go.
 	clearVertices := []float32{
 		x0, y0, 0, 0, 1, 1,