interna/affine: Avoid UnsafeElements at At

Updates #1796

colorm.go

@@ -99,7 +99,7 @@ func (c *ColorM) ChangeHSV(hueTheta float64, saturationScale float64, valueScale
 
 // Element returns a value of a matrix at (i, j).
 func (c *ColorM) Element(i, j int) float64 {
-	return float64(affine.ColorMElement(c.affineColorM(), i, j))
+	return float64(c.affineColorM().At(i, j))
 }
 
 // SetElement sets an element at (i, j).

internal/affine/colorm.go

@@ -44,6 +44,7 @@ var (
 type ColorM interface {
 	IsIdentity() bool
 	ScaleOnly() bool
+	At(i, j int) float32
 	UnsafeElements(body *[16]float32, translate *[4]float32)
 	Apply(clr color.Color) color.Color
 
@@ -454,15 +455,25 @@ func (c *colorMImplBodyTranslate) Invert() ColorM {
 	return m
 }
 
-// ColorMElement returns a value of a matrix at (i, j).
-func ColorMElement(c ColorM, i, j int) float32 {
-	var b [16]float32
-	var t [4]float32
-	c.UnsafeElements(&b, &t)
-	if j < ColorMDim-1 {
-		return b[i+j*(ColorMDim-1)]
+func (c ColorMIdentity) At(i, j int) float32 {
+	if i == j {
+		return 1
 	}
-	return t[i]
+	return 0
+}
+
+func (c colorMImplScale) At(i, j int) float32 {
+	if i == j {
+		return c.scale[i]
+	}
+	return 0
+}
+
+func (c *colorMImplBodyTranslate) At(i, j int) float32 {
+	if j < ColorMDim-1 {
+		return c.body[i+j*(ColorMDim-1)]
+	}
+	return c.translate[i]
 }
 
 // ColorMSetElement sets an element at (i, j).

internal/affine/colorm_test.go

@@ -169,8 +169,8 @@ func abs(x float32) float32 {
 func equalWithDelta(a, b ColorM, delta float32) bool {
 	for j := 0; j < 5; j++ {
 		for i := 0; i < 4; i++ {
-			ea := ColorMElement(a, i, j)
-			eb := ColorMElement(b, i, j)
+			ea := a.At(i, j)
+			eb := b.At(i, j)
 			if abs(ea-eb) > delta {
 				return false
 			}

internal/driver/graphics.go

@@ -35,6 +35,7 @@ const (
 
 type ColorM interface {
 	IsIdentity() bool
+	At(i, j int) float32
 	UnsafeElements(body *[16]float32, translate *[4]float32)
 }
 

internal/mipmap/mipmap.go

@@ -124,13 +124,10 @@ func (m *Mipmap) DrawTriangles(srcs [graphics.ShaderImageNum]*Mipmap, vertices [
 	}
 
 	if colorm.ScaleOnly() {
-		var body [16]float32
-		var translate [4]float32
-		colorm.UnsafeElements(&body, &translate)
-		cr := body[0]
-		cg := body[5]
-		cb := body[10]
-		ca := body[15]
+		cr := colorm.At(0, 0)
+		cg := colorm.At(1, 1)
+		cb := colorm.At(2, 2)
+		ca := colorm.At(3, 3)
 		colorm = affine.ColorMIdentity{}
 		const n = graphics.VertexFloatNum
 		for i := 0; i < len(vertices)/n; i++ {