ui/app/internal/gl: add workaround for broken sRGB FBOs

Safari on macOS don't treat sRGB correctly.

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

ui/app/internal/gl/functions.go

@@ -409,6 +409,14 @@ func (f *Functions) Scissor(x, y, width, height int32) {
 	C.glScissor(C.GLint(x), C.GLint(y), C.GLsizei(width), C.GLsizei(height))
 }
 
+func (f *Functions) ReadPixels(x, y, width, height int, format, ty Enum, data []byte) {
+	var p unsafe.Pointer
+	if len(data) > 0 {
+		p = unsafe.Pointer(&data[0])
+	}
+	C.glReadPixels(C.GLint(x), C.GLint(y), C.GLsizei(width), C.GLsizei(height), C.GLenum(format), C.GLenum(ty), p)
+}
+
 func (f *Functions) RenderbufferStorage(target, internalformat Enum, width, height int) {
 	C.glRenderbufferStorage(C.GLenum(target), C.GLenum(internalformat), C.GLsizei(width), C.GLsizei(height))
 }

ui/app/internal/gl/gl_windows.go

@@ -69,6 +69,7 @@ var (
 	_glInvalidateFramebuffer              = LibGLESv2.NewProc("glInvalidateFramebuffer")
 	_glLinkProgram                        = LibGLESv2.NewProc("glLinkProgram")
 	_glPixelStorei                        = LibGLESv2.NewProc("glPixelStorei")
+	_glReadPixels                         = LibGLESv2.NewProc("glReadPixels")
 	_glRenderbufferStorage                = LibGLESv2.NewProc("glRenderbufferStorage")
 	_glScissor                            = LibGLESv2.NewProc("glScissor")
 	_glShaderSource                       = LibGLESv2.NewProc("glShaderSource")
@@ -319,6 +320,9 @@ func (c *Functions) LinkProgram(p Program) {
 func (c *Functions) PixelStorei(pname Enum, param int32) {
 	syscall.Syscall(_glPixelStorei.Addr(), 2, uintptr(pname), uintptr(param), 0)
 }
+func (f *Functions) ReadPixels(x, y, width, height int, format, ty Enum, data []byte) {
+	syscall.Syscall6(_glReadPixels.Addr(), uintptr(x), uintptr(y), uintptr(width), uintptr(height), uintptr(format), uintptr(ty), uintptr(unsafe.Pointer(&data[0])))
+}
 func (c *Functions) RenderbufferStorage(target, internalformat Enum, width, height int) {
 	syscall.Syscall6(_glRenderbufferStorage.Addr(), 4, uintptr(target), uintptr(internalformat), uintptr(width), uintptr(height), 0, 0)
 }

ui/app/internal/gl/srgb.go

@@ -4,6 +4,7 @@ package gl
 
 import (
 	"fmt"
+	"runtime"
 	"strings"
 )
 
@@ -116,6 +117,23 @@ func (s *SRGBFBO) Refresh(w, h int) error {
 	if st := s.c.CheckFramebufferStatus(FRAMEBUFFER); st != FRAMEBUFFER_COMPLETE {
 		return fmt.Errorf("sRGB framebuffer incomplete (%dx%d), status: %#x error: %x", s.width, s.height, st, s.c.GetError())
 	}
+
+	if runtime.GOOS == "js" {
+		// With macOS Safari, rendering to and then reading from a SRGB8_ALPHA8
+		// texture result in twice gamma corrected colors. Using a plain RGBA
+		// texture seems to work.
+		s.c.ClearColor(.5, .5, .5, 1.0)
+		s.c.Clear(COLOR_BUFFER_BIT)
+		var pixel [4]byte
+		s.c.ReadPixels(0, 0, 1, 1, RGBA, UNSIGNED_BYTE, pixel[:])
+		if pixel[0] == 128 { // Correct sRGB color value is ~188
+			s.c.TexImage2D(TEXTURE_2D, 0, RGBA, w, h, RGBA, UNSIGNED_BYTE, nil)
+			if st := s.c.CheckFramebufferStatus(FRAMEBUFFER); st != FRAMEBUFFER_COMPLETE {
+				return fmt.Errorf("fallback RGBA framebuffer incomplete (%dx%d), status: %#x error: %x", s.width, s.height, st, s.c.GetError())
+			}
+		}
+	}
+
 	return nil
 }
 
@@ -149,13 +167,17 @@ precision mediump float;
 uniform sampler2D tex;
 varying vec2 vUV;
 
-void main() {
-    vec4 col = texture2D(tex, vUV);
-	vec3 rgb = col.rgb;
+vec3 gamma(vec3 rgb) {
 	vec3 exp = vec3(1.055)*pow(rgb, vec3(0.41666)) - vec3(0.055);
 	vec3 lin = rgb * vec3(12.92);
 	bvec3 cut = lessThan(rgb, vec3(0.0031308));
-	rgb = vec3(cut.r ? lin.r : exp.r, cut.g ? lin.g : exp.g, cut.b ? lin.b : exp.b);
+	return vec3(cut.r ? lin.r : exp.r, cut.g ? lin.g : exp.g, cut.b ? lin.b : exp.b);
+}
+
+void main() {
+    vec4 col = texture2D(tex, vUV);
+	vec3 rgb = col.rgb;
+	rgb = gamma(rgb);
 	gl_FragColor = vec4(rgb, col.a);
 }
 `