From f5fa968038cc43f42c8359896144df2d1a2d7f5b Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Sat, 4 May 2019 14:29:37 +0200
Subject: [PATCH] ui/app/internal/gpu: implement OpenGL ES 2 float fbo
 fallbacks

Also applicable to WebGL.

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 ui/app/internal/gl/gl.go       |  1 +
 ui/app/internal/gpu/context.go | 46 +++++++++++++++++++++++++++++-----
 ui/app/internal/gpu/path.go    |  9 ++++---
 3 files changed, 46 insertions(+), 10 deletions(-)

diff --git a/ui/app/internal/gl/gl.go b/ui/app/internal/gl/gl.go
index 72b37af2..25d3213a 100644
--- a/ui/app/internal/gl/gl.go
+++ b/ui/app/internal/gl/gl.go
@@ -35,6 +35,7 @@ const (
 	FRAMEBUFFER_BINDING                   = 0x8ca6
 	FRAMEBUFFER_COMPLETE                  = 0x8cd5
 	HALF_FLOAT                            = 0x140b
+	HALF_FLOAT_OES                        = 0x8d61
 	GREATER                               = 0x204
 	LINEAR                                = 0x2601
 	LINK_STATUS                           = 0x8b82
diff --git a/ui/app/internal/gpu/context.go b/ui/app/internal/gpu/context.go
index 91cad11f..e3cf0cb9 100644
--- a/ui/app/internal/gpu/context.go
+++ b/ui/app/internal/gpu/context.go
@@ -1,6 +1,7 @@
 package gpu
 
 import (
+	"errors"
 	"strings"
 
 	"gioui.org/ui/app/internal/gl"
@@ -14,6 +15,17 @@ type context struct {
 type caps struct {
 	EXT_disjoint_timer_query bool
 	srgbMode                 srgbMode
+	// floatTriple holds the settings for floating point
+	// textures.
+	floatTriple textureTriple
+}
+
+// textureTriple holds the type settings for
+// a TexImage2D call.
+type textureTriple struct {
+	internalFormat int
+	format         gl.Enum
+	typ            gl.Enum
 }
 
 type srgbMode uint8
@@ -33,20 +45,42 @@ func newContext(glctx gl.Context) (*context, error) {
 	if err != nil {
 		return nil, err
 	}
+	srgbMode, err := srgbModeFor(ver, exts)
+	if err != nil {
+		return nil, err
+	}
+	floatTriple, err := floatTripleFor(ver, exts)
+	if err != nil {
+		return nil, err
+	}
 	ctx.caps = caps{
 		EXT_disjoint_timer_query: strings.Contains(exts, "GL_EXT_disjoint_timer_query"),
-		srgbMode:                 srgbModeFor(ver, exts),
+		srgbMode:                 srgbMode,
+		floatTriple:              floatTriple,
 	}
 	return ctx, nil
 }
 
-func srgbModeFor(ver [2]int, exts string) srgbMode {
+func floatTripleFor(ver [2]int, exts string) (textureTriple, error) {
 	switch {
 	case ver[0] >= 3:
-		return srgbES3
-	case strings.Contains(exts, "EXT_sRGB"):
-		return srgbEXT
+		return textureTriple{gl.R16F, gl.Enum(gl.RED), gl.Enum(gl.HALF_FLOAT)}, nil
+	case strings.Contains(exts, "GL_OES_texture_half_float"):
+		return textureTriple{gl.RGBA, gl.Enum(gl.RGBA), gl.Enum(gl.HALF_FLOAT_OES)}, nil
+	case strings.Contains(exts, "GL_OES_texture_float"):
+		return textureTriple{gl.RGBA, gl.Enum(gl.RGBA), gl.Enum(gl.FLOAT)}, nil
 	default:
-		panic("neither OpenGL ES 3 nor EXT_sRGB is supported")
+		return textureTriple{}, errors.New("floating point texture not supported")
+	}
+}
+
+func srgbModeFor(ver [2]int, exts string) (srgbMode, error) {
+	switch {
+	case ver[0] >= 3:
+		return srgbES3, nil
+	case strings.Contains(exts, "EXT_sRGB"):
+		return srgbEXT, nil
+	default:
+		return 0, errors.New("neither OpenGL ES 3 nor EXT_sRGB is supported")
 	}
 }
diff --git a/ui/app/internal/gpu/path.go b/ui/app/internal/gpu/path.go
index f3030855..3450cece 100644
--- a/ui/app/internal/gpu/path.go
+++ b/ui/app/internal/gpu/path.go
@@ -151,7 +151,7 @@ func newStenciler(ctx *context) *stenciler {
 	}
 }
 
-func (s *fboSet) resize(ctx *context, sizes []image.Point, internalFormat int, format, ty gl.Enum) {
+func (s *fboSet) resize(ctx *context, sizes []image.Point) {
 	// Add fbos.
 	for i := len(s.fbos); i < len(sizes); i++ {
 		tex := ctx.CreateTexture()
@@ -177,7 +177,8 @@ func (s *fboSet) resize(ctx *context, sizes []image.Point, internalFormat int, f
 		if resize {
 			f.size = sz
 			ctx.BindTexture(gl.TEXTURE_2D, f.tex)
-			ctx.TexImage2D(gl.TEXTURE_2D, 0, internalFormat, sz.X, sz.Y, format, ty, nil)
+			tt := ctx.caps.floatTriple
+			ctx.TexImage2D(gl.TEXTURE_2D, 0, tt.internalFormat, sz.X, sz.Y, tt.format, tt.typ, nil)
 			ctx.BindFramebuffer(gl.FRAMEBUFFER, f.fbo)
 			ctx.FramebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, f.tex, 0)
 		}
@@ -254,7 +255,7 @@ func (s *stenciler) beginIntersect(sizes []image.Point) {
 	// 8 bit coverage is enough, but OpenGL ES only supports single channel
 	// floating point formats. Replace with GL_RGB+GL_UNSIGNED_BYTE if
 	// no floating point support is available.
-	s.intersections.resize(s.ctx, sizes, gl.R16F, gl.RED, gl.HALF_FLOAT)
+	s.intersections.resize(s.ctx, sizes)
 	s.ctx.ClearColor(1.0, 0.0, 0.0, 0.0)
 	s.ctx.UseProgram(s.iprog)
 }
@@ -278,7 +279,7 @@ func (s *stenciler) begin(sizes []image.Point) {
 	s.ctx.BindTexture(gl.TEXTURE_2D, gl.Texture{})
 	s.ctx.ActiveTexture(gl.TEXTURE0)
 	s.ctx.BlendFunc(gl.ONE, gl.ONE)
-	s.fbos.resize(s.ctx, sizes, gl.R16F, gl.RED, gl.HALF_FLOAT)
+	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)