internal/shader: implement a new built-in function discard

Closes #1969

internal/shader/expr.go

@@ -469,6 +469,14 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr, markLocalVariable
 			case shaderir.Texture2DF:
 				// TODO: Check arg types.
 				t = shaderir.Type{Main: shaderir.Vec4}
+			case shaderir.DiscardF:
+				if len(args) != 0 {
+					cs.addError(e.Pos(), fmt.Sprintf("number of %s's arguments must be 0 but %d", callee.BuiltinFunc, len(args)))
+				}
+				stmts = append(stmts, shaderir.Stmt{
+					Type: shaderir.Discard,
+				})
+				return nil, nil, stmts, true
 			default:
 				// TODO: Check arg types.
 				// If the argument is a non-typed constant value, treat is as a float value (#1874).

internal/shaderir/msl/msl.go

@@ -503,7 +503,7 @@ func (c *compileContext) block(p *shaderir.Program, topBlock, block *shaderir.Bl
 				lines = append(lines, fmt.Sprintf("%sreturn %s;", idt, expr(&s.Exprs[0])))
 			}
 		case shaderir.Discard:
-			lines = append(lines, idt+"discard;")
+			lines = append(lines, idt+"discard_fragment();")
 		default:
 			lines = append(lines, fmt.Sprintf("%s?(unexpected stmt: %d)", idt, s.Type))
 		}

internal/shaderir/program.go

@@ -265,6 +265,7 @@ const (
 	Dfdx        BuiltinFunc = "dfdx"
 	Dfdy        BuiltinFunc = "dfdy"
 	Fwidth      BuiltinFunc = "fwidth"
+	DiscardF    BuiltinFunc = "discard"
 )
 
 func ParseBuiltinFunc(str string) (BuiltinFunc, bool) {
@@ -317,7 +318,8 @@ func ParseBuiltinFunc(str string) (BuiltinFunc, bool) {
 		Texture2DF,
 		Dfdx,
 		Dfdy,
-		Fwidth:
+		Fwidth,
+		DiscardF:
 		return BuiltinFunc(str), true
 	}
 	return "", false

shader_test.go

@@ -1129,3 +1129,55 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #1969
+func TestShaderDiscard(t *testing.T) {
+	const w, h = 16, 16
+
+	dst := ebiten.NewImage(w, h)
+	dst.Fill(color.RGBA{0xff, 0, 0, 0xff})
+
+	src := ebiten.NewImage(w, h)
+	pix := make([]byte, 4*w*h)
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			if i >= w/2 || j >= h/2 {
+				continue
+			}
+			pix[4*(j*w+i)+3] = 0xff
+		}
+	}
+	src.WritePixels(pix)
+
+	s, err := ebiten.NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	p := imageSrc0At(texCoord)
+	if p.a == 0 {
+		discard()
+	} else {
+		return vec4(0, 1, 0, 1)
+	}
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	op := &ebiten.DrawRectShaderOptions{}
+	op.Images[0] = src
+	dst.DrawRectShader(w, h, s, op)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			want := color.RGBA{0, 0xff, 0x00, 0xff}
+			if i >= w/2 || j >= h/2 {
+				want = color.RGBA{0xff, 0, 0x00, 0xff}
+			}
+			if !sameColors(got, want, 2) {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}