Implement palette transparency

Add Palt and PaltReset functions.

Palt sets color transparency. If true then the color will not be drawn
for next drawing operations.

Color transparency is used by Spr, SprSize and SprSizeFlip.

PaltReset sets all transparent colors to false and makes color 0 transparent.

Author: elgopher

internal/testimage/spr_1_on_top_of_2_trans_0.png

@@ -124,6 +124,7 @@ func Boot() error {
 
 	Clip(0, 0, scrWidth, scrHeight)
 	Camera(0, 0)
+	PaltReset()
 
 	return nil
 }

internal/testimage/spr_1_on_top_of_2_trans_50.png

@@ -249,16 +249,43 @@ func SprSizeFlip(n, x, y int, w, h float64, flipX, flipY bool) {
 		spriteSheetStep = -ssWidth
 	}
 
+	startingPixel := 0
+	step := 1
+
+	if flipX {
+		startingPixel = width - 1
+		step = -1
+	}
+
 	for i := 0; i < height; i++ {
 		spriteSheetLine := SpriteSheetData[spriteSheetOffset : spriteSheetOffset+width]
-		if flipX {
-			for j := 0; j < len(spriteSheetLine); j++ {
-				ScreenData[screenOffset+j] = spriteSheetLine[len(spriteSheetLine)-1-j]
+
+		for j := 0; j < len(spriteSheetLine); j++ {
+			col := spriteSheetLine[startingPixel+(step*j)]
+			if colorIsTransparent[col] {
+				continue
 			}
-		} else {
-			copy(ScreenData[screenOffset:screenOffset+width], spriteSheetLine)
+
+			ScreenData[screenOffset+j] = col
 		}
 		screenOffset += scrWidth
 		spriteSheetOffset += spriteSheetStep
 	}
 }
+
+var colorIsTransparent [256]bool
+
+// Palt sets color transparency. If true then the color will not be drawn
+// for next drawing operations.
+//
+// Color transparency is used by Spr, SprSize and SprSizeFlip.
+func Palt(color byte, transparent bool) {
+	colorIsTransparent[color] = transparent
+}
+
+var defaultTransparency = [256]bool{true}
+
+// PaltReset sets all transparent colors to false and makes color 0 transparent.
+func PaltReset() {
+	colorIsTransparent = defaultTransparency
+}

internal/testimage/spr_1_on_top_of_2_trans_50_flipx.png

@@ -80,6 +80,14 @@ func BenchmarkSprSizeFlip(b *testing.B) {
 	})
 }
 
+func BenchmarkPaltReset(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		runBenchmarks(b, func(res Resolution) {
+			pi.PaltReset()
+		})
+	}
+}
+
 type Resolution struct{ W, H int }
 
 func (s Resolution) String() string {

pi.go

@@ -454,6 +454,58 @@ func testSpr(t *testing.T, spr func(spriteNo int, x int, y int)) {
 			})
 		}
 	})
+
+	t.Run("should not draw color 0 by default", func(t *testing.T) {
+		pi.Reset()
+		pi.ScreenWidth = 8
+		pi.ScreenHeight = 8
+		pi.Resources = fstest.MapFS{
+			"sprite-sheet.png": &fstest.MapFile{Data: spriteSheet16x16},
+		}
+		pi.BootOrPanic()
+		spr(2, 0, 0)
+		// when
+		spr(1, 0, 0)
+		// then
+		expectedScreen := decodePNG(t, "internal/testimage/spr_1_on_top_of_2_trans_0.png")
+		assert.Equal(t, expectedScreen.Pixels, pi.ScreenData)
+	})
+
+	t.Run("should not draw color 0 after PaltReset", func(t *testing.T) {
+		pi.Reset()
+		pi.ScreenWidth = 8
+		pi.ScreenHeight = 8
+		pi.Resources = fstest.MapFS{
+			"sprite-sheet.png": &fstest.MapFile{Data: spriteSheet16x16},
+		}
+		pi.BootOrPanic()
+		spr(2, 0, 0)
+		pi.Palt(0, false) // make color 0 opaque
+		// when
+		pi.PaltReset() // and then make color 0 transparent again
+		spr(1, 0, 0)
+		// then
+		expectedScreen := decodePNG(t, "internal/testimage/spr_1_on_top_of_2_trans_0.png")
+		assert.Equal(t, expectedScreen.Pixels, pi.ScreenData)
+	})
+
+	t.Run("should not draw transparent colors", func(t *testing.T) {
+		pi.Reset()
+		pi.ScreenWidth = 8
+		pi.ScreenHeight = 8
+		pi.Resources = fstest.MapFS{
+			"sprite-sheet.png": &fstest.MapFile{Data: spriteSheet16x16},
+		}
+		pi.BootOrPanic()
+		spr(2, 0, 0)
+		// when
+		pi.Palt(0, false)
+		pi.Palt(50, true)
+		spr(1, 0, 0)
+		// then
+		expectedScreen := decodePNG(t, "internal/testimage/spr_1_on_top_of_2_trans_50.png")
+		assert.Equal(t, expectedScreen.Pixels, pi.ScreenData)
+	})
 }
 
 func TestSprSize(t *testing.T) {
@@ -565,6 +617,24 @@ func TestSprSizeFlip(t *testing.T) {
 			})
 		}
 	})
+
+	t.Run("should not draw transparent colors", func(t *testing.T) {
+		pi.Reset()
+		pi.ScreenWidth = 8
+		pi.ScreenHeight = 8
+		pi.Resources = fstest.MapFS{
+			"sprite-sheet.png": &fstest.MapFile{Data: spriteSheet16x16},
+		}
+		pi.BootOrPanic()
+		pi.SprSizeFlip(2, 0, 0, 1.0, 1.0, true, false)
+		// when
+		pi.Palt(0, false)
+		pi.Palt(50, true)
+		pi.SprSizeFlip(1, 0, 0, 1.0, 1.0, true, false)
+		// then
+		expectedScreen := decodePNG(t, "internal/testimage/spr_1_on_top_of_2_trans_50_flipx.png")
+		assert.Equal(t, expectedScreen.Pixels, pi.ScreenData)
+	})
 }
 
 func clone(s []byte) []byte {