ui/app/internal/gpu: replace look up texture with calculation

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

ui/app/internal/gpu/arealut.go

@@ -1,76 +0,0 @@
-// 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
-		}
-	}
-}

ui/app/internal/gpu/path.go

@@ -48,7 +48,6 @@ type stenciler struct {
 	uIntersectUVOffset gl.Uniform
 	uIntersectUVScale  gl.Uniform
 	indexBuf           gl.Buffer
-	areaLUT            gl.Texture
 }
 
 type fboSet struct {
@@ -122,13 +121,7 @@ func newStenciler(ctx *context) *stenciler {
 	if err != nil {
 		panic(err)
 	}
-	uAreaLUT := gl.GetUniformLocation(ctx.Functions, prog, "areaLUT")
 	ctx.UseProgram(prog)
-	ctx.Uniform1i(uAreaLUT, 0)
-	areaLUT, err := loadLUT(ctx, genAreaLUT(256, 256))
-	if err != nil {
-		panic(err)
-	}
 	iprog, err := gl.CreateProgram(ctx.Functions, intersectVSrc, intersectFSrc, intersectAttribs)
 	if err != nil {
 		panic(err)
@@ -141,7 +134,6 @@ func newStenciler(ctx *context) *stenciler {
 		defFBO:             defFBO,
 		prog:               prog,
 		iprog:              iprog,
-		areaLUT:            areaLUT,
 		uScale:             gl.GetUniformLocation(ctx.Functions, prog, "scale"),
 		uOffset:            gl.GetUniformLocation(ctx.Functions, prog, "offset"),
 		uPathOffset:        gl.GetUniformLocation(ctx.Functions, prog, "pathOffset"),
@@ -205,7 +197,6 @@ func (s *fboSet) delete(ctx *context, idx int) {
 
 func (s *stenciler) release() {
 	s.fbos.delete(s.ctx, 0)
-	s.ctx.DeleteTexture(s.areaLUT)
 	s.ctx.DeleteProgram(s.prog)
 	s.ctx.DeleteBuffer(s.indexBuf)
 }
@@ -281,7 +272,6 @@ func (s *stenciler) begin(sizes []image.Point) {
 	s.ctx.BlendFunc(gl.ONE, gl.ONE)
 	s.fbos.resize(s.ctx, sizes)
 	s.ctx.ClearColor(0.0, 0.0, 0.0, 0.0)
-	s.ctx.BindTexture(gl.TEXTURE_2D, s.areaLUT)
 	s.ctx.UseProgram(s.prog)
 	s.ctx.EnableVertexAttribArray(attribPathCorner)
 	s.ctx.EnableVertexAttribArray(attribPathMaxY)
@@ -365,23 +355,6 @@ func (c *coverer) cover(z float32, mat materialType, col [4]float32, scale, off,
 	c.ctx.DrawArrays(gl.TRIANGLE_STRIP, 0, 4)
 }
 
-func loadLUT(ctx *context, lut *image.Gray) (gl.Texture, error) {
-	tex := ctx.CreateTexture()
-	ctx.BindTexture(gl.TEXTURE_2D, tex)
-	ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE)
-	ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE)
-	ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR)
-	ctx.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR)
-	ctx.PixelStorei(gl.UNPACK_ALIGNMENT, 1)
-	if lut.Stride != lut.Bounds().Dx() {
-		panic("unsupported LUT stride")
-	}
-	tt := ctx.caps.alphaTriple
-	ctx.TexImage2D(gl.TEXTURE_2D, 0, tt.internalFormat, lut.Bounds().Dx(), lut.Bounds().Dy(), tt.format, tt.typ, lut.Pix)
-	ctx.PixelStorei(gl.UNPACK_ALIGNMENT, 4)
-	return tex, nil
-}
-
 const stencilVSrc = `
 #version 100
 
@@ -494,19 +467,20 @@ void main() {
 	// And the slope.
 	vec2 d_half = mix(p1, v, t);
 	float dy = d_half.y/d_half.x;
-	// Together, y and dy form a line approximation. The areaLUT table
-	// maps the line to a pixel coverage.
+	// Together, y and dy form a line approximation.
+
+	// Compute the fragment area above the line.
+	// The area is symmetric around dy = 0. Scale slope with extent width.
 	float width = extent.y - extent.x;
-	// The first axis maps y in [-8;+8] to [0;1].
-	float areau = y/16.0 + 0.5;
-	// The second axis maps slopes in [0;16] to [0;1]. The area is symmetric
-	// around dy = 0. Scale slope with extent width.
-	float areav = abs(dy*width)/16.0;
-	// Look up coverage from y and slope and scale to extent.
-	float cover = texture2D(areaLUT, vec2(areau, areav)).r*width;
-	if (width == 0.0)
-		cover = 0.0; // Needed on the iOS simulator.
-	gl_FragColor.r = cover;
+	dy = abs(dy*width);
+
+	vec4 sides = vec4(dy*+0.5 + y, dy*-0.5 + y, (+0.5-y)/dy, (-0.5-y)/dy);
+	sides = clamp(sides+0.5, 0.0, 1.0);
+
+	float area = 0.5*(sides.z - sides.z*sides.y + 1.0 - sides.x+sides.x*sides.w);
+	area *= width;
+
+	gl_FragColor.r = area;
 }
 `