all: initial import

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/internal/gpu/arealut.go

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package gpu
+
+import (
+	"image"
+)
+
+// genAreaLUT generates the lookup table conpatible with the stencilFSrc
+// fragment shaders. The table contains the area of a pixel square above
+// a line. The square has area 1 and is centered in (0, 0).
+// The y-axis intersection of the line in [-8;+8] is specified by the
+// first coordinate.
+// The slope of the line [0;16] is specified by the second coordinate.
+func genAreaLUT(width, height int) *image.Gray {
+	lut := image.NewGray(image.Rectangle{Max: image.Point{X: width, Y: height}})
+	for v := 0; v < height; v++ {
+		a := float32(v) * 16 / float32(height)
+		for u := 0; u < width; u++ {
+			var area float32
+			switch u {
+			case 0:
+				area = 1.0
+			case width - 1:
+				area = 0.0
+			default:
+				b := (float32(u) - float32(width)/2) / 16
+				// f(x) = ax+b.
+				area = computeLineArea(a, b)
+			}
+			lut.Pix[v*height+u] = uint8(area*255 + 0.5)
+		}
+	}
+	return lut
+}
+
+func computeLineArea(a, b float32) float32 {
+	// Compute intersections with the square edges.
+	// Right and left.
+	ry := a*+0.5 + b
+	ly := a*-0.5 + b
+	// Top and bottom.
+	tx := (+0.5 - b) / a
+	bx := (-0.5 - b) / a
+	// The line will intersect zero or two edges.
+	if ry <= -0.5 {
+		// Line is below the square.
+		return 1.0
+	}
+	if ly >= 0.5 {
+		// Line is above the square.
+		return 0.0
+	}
+	// The slope is positive, so there are only 4 possible
+	// pairs of edges: (bottom, right), (left, right),
+	// (bottom, top), (left, top).
+	if ry <= 0.5 {
+		// Intersection with right edge.
+		if ly <= -0.5 {
+			// (bottom, right).
+			return 1.0 - (0.5-bx)*(ry-(-0.5))/2
+		} else {
+			// (left, right).
+			return 1.0*(0.5-ry) + 1.0*(ry-ly)/2
+		}
+	} else {
+		// Intersection with top edge.
+		if ly <= -0.5 {
+			// (bottom, top).
+			return (bx-(-0.5))*1.0 + (tx-bx)*1.0/2
+		} else {
+			// (left, top).
+			return (tx - (-0.5)) * (0.5 - ly) / 2
+		}
+	}
+}