gpu,gpu/shaders: [compute] decode sRGB texels in shader when EXT_sRGB is missing

This change avoids the hard dependency on GPU support for sRGB encoded
textures in the compute renderer.

With this change and the previously added CPU fallback, Gio no longer
rely on any GPU functionality outside the OpenGL ES 2.0 level.

Fixes gio#49
Fixes gio#154
Fixes gio#97
Fixes gio#36
Fixes gio#172

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

gpu/compute.go

@@ -42,6 +42,7 @@ type compute struct {
 	texOps        []textureOp
 	viewport      image.Point
 	maxTextureDim int
+	srgb          bool
 
 	programs struct {
 		elements   computeProgram
@@ -98,8 +99,14 @@ type compute struct {
 
 		buffer sizedBuffer
 
-		uniforms *materialUniforms
+		vert struct {
-		uniBuf   driver.Buffer
+			uniforms *materialVertUniforms
+			buf      driver.Buffer
+		}
+
+		frag struct {
+			buf driver.Buffer
+		}
 
 		// CPU fields
 		cpuTex cpu.ImageDescriptor
@@ -154,11 +161,16 @@ type copyUniforms struct {
 	_       [8]byte // Pad to 16 bytes.
 }
 
-type materialUniforms struct {
+type materialVertUniforms struct {
 	scale [2]float32
 	pos   [2]float32
 }
 
+type materialFragUniforms struct {
+	emulateSRGB float32
+	_           [12]byte // Pad to 16 bytes
+}
+
 type collector struct {
 	hasher     maphash.Hash
 	profile    bool
@@ -372,6 +384,7 @@ func newCompute(ctx driver.Device) (*compute, error) {
 	g := &compute{
 		ctx:           ctx,
 		maxTextureDim: maxDim,
+		srgb:          caps.Features.Has(driver.FeatureSRGB),
 		conf:          new(config),
 		memHeader:     new(memoryHeader),
 	}
@@ -447,15 +460,27 @@ func newCompute(ctx driver.Device) (*compute, error) {
 		return nil, err
 	}
 	g.materials.layout = progLayout
-	g.materials.uniforms = new(materialUniforms)
+	g.materials.vert.uniforms = new(materialVertUniforms)
 
-	buf, err = ctx.NewBuffer(driver.BufferBindingUniforms, int(unsafe.Sizeof(*g.materials.uniforms)))
+	buf, err = ctx.NewBuffer(driver.BufferBindingUniforms, int(unsafe.Sizeof(*g.materials.vert.uniforms)))
 	if err != nil {
 		g.Release()
 		return nil, err
 	}
-	g.materials.uniBuf = buf
+	g.materials.vert.buf = buf
 	g.materials.prog.SetVertexUniforms(buf)
+	var emulateSRGB materialFragUniforms
+	if !g.srgb {
+		emulateSRGB.emulateSRGB = 1.0
+	}
+	buf, err = ctx.NewBuffer(driver.BufferBindingUniforms, int(unsafe.Sizeof(emulateSRGB)))
+	if err != nil {
+		g.Release()
+		return nil, err
+	}
+	buf.Upload(byteslice.Struct(&emulateSRGB))
+	g.materials.frag.buf = buf
+	g.materials.prog.SetFragmentUniforms(buf)
 
 	for _, shader := range shaders {
 		if !g.useCPU {
@@ -875,9 +900,9 @@ restart:
 		}
 	}
 	// Transform to clip space: [-1, -1] - [1, 1].
-	g.materials.uniforms.scale = [2]float32{2 / float32(texSize), 2 / float32(texSize)}
+	g.materials.vert.uniforms.scale = [2]float32{2 / float32(texSize), 2 / float32(texSize)}
-	g.materials.uniforms.pos = [2]float32{-1, -1}
+	g.materials.vert.uniforms.pos = [2]float32{-1, -1}
-	g.materials.uniBuf.Upload(byteslice.Struct(g.materials.uniforms))
+	g.materials.vert.buf.Upload(byteslice.Struct(g.materials.vert.uniforms))
 	vertexData := byteslice.Slice(m.quads)
 	n := pow2Ceil(len(vertexData))
 	m.buffer.ensureCapacity(false, g.ctx, driver.BufferBindingVertices, n)
@@ -952,7 +977,11 @@ restart:
 			a.tex = nil
 		}
 		sz := a.packer.maxDim
-		handle, err := g.ctx.NewTexture(driver.TextureFormatSRGBA, sz, sz, driver.FilterLinear, driver.FilterLinear, driver.BufferBindingTexture)
+		format := driver.TextureFormatSRGBA
+		if !g.srgb {
+			format = driver.TextureFormatRGBA8
+		}
+		handle, err := g.ctx.NewTexture(format, sz, sz, driver.FilterLinear, driver.FilterLinear, driver.BufferBindingTexture)
 		if err != nil {
 			return fmt.Errorf("compute: failed to create image atlas: %v", err)
 		}
@@ -1316,7 +1345,8 @@ func (g *compute) Release() {
 		g.materials.fbo,
 		g.materials.tex,
 		&g.materials.buffer,
-		g.materials.uniBuf,
+		g.materials.vert.buf,
+		g.materials.frag.buf,
 		g.timers.t,
 	}
 	g.materials.cpuTex.Free()

gpu/gpu.go

@@ -9,7 +9,6 @@ package gpu
 
 import (
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"image"
 	"image/color"
@@ -361,11 +360,8 @@ func New(api API) (GPU, error) {
 	defer d.EndFrame()
 	forceCompute := os.Getenv("GIORENDERER") == "forcecompute"
 	feats := d.Caps().Features
-	if !feats.Has(driver.FeatureSRGB) {
-		return nil, errors.New("gpu: no sRGB texture formats found")
-	}
 	switch {
-	case !forceCompute && feats.Has(driver.FeatureFloatRenderTargets):
+	case !forceCompute && feats.Has(driver.FeatureFloatRenderTargets) && feats.Has(driver.FeatureSRGB):
 		return newGPU(d)
 	}
 	return newCompute(d)

gpu/shaders/material.frag

@@ -10,6 +10,12 @@ layout(location = 0) in vec2 vUV;
 
 layout(location = 0) out vec4 fragColor;
 
+layout(binding=0) uniform Color {
+	// If emulateSRGB is set (!= 0), the input texels are sRGB encoded. We save the
+	// conversion step below, at the cost of texture filtering in sRGB space.
+	float emulateSRGB;
+};
+
 vec3 RGBtosRGB(vec3 rgb) {
 	bvec3 cutoff = greaterThanEqual(rgb, vec3(0.0031308));
 	vec3 below = vec3(12.92)*rgb;
@@ -19,6 +25,8 @@ vec3 RGBtosRGB(vec3 rgb) {
 
 void main() {
 	vec4 texel = texture(tex, vUV);
-	texel.rgb = RGBtosRGB(texel.rgb);
+	if (emulateSRGB == 0.0) {
+		texel.rgb = RGBtosRGB(texel.rgb);
+	}
 	fragColor = texel;
 }