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/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")
 	}
 }