gl: Add support for sRGB in default fb at least

...so that setting QRhiSwapChain::sRGB behaves identically between
all 4 backends.

examples/rhi/shared/examplefw.h

@@ -483,6 +483,8 @@ int main(int argc, char **argv)
         fmt.setSamples(sampleCount);
     if (scFlags.testFlag(QRhiSwapChain::NoVSync))
         fmt.setSwapInterval(0);
+    if (scFlags.testFlag(QRhiSwapChain::sRGB))
+        fmt.setColorSpace(QSurfaceFormat::sRGBColorSpace);
     QSurfaceFormat::setDefaultFormat(fmt);
 
     // Vulkan setup.

src/rhi/qrhi.cpp

@@ -1887,7 +1887,7 @@ QRhiRenderBuffer::QRhiRenderBuffer(QRhiImplementation *rhi, Type type_, const QS
      QRhiResourceUpdateBatch::generateMips(). Multisample textures cannot have
      mipmaps.
 
-    \value sRGB Use an sRGB format
+    \value sRGB Use an sRGB format.
 
     \value UsedAsTransferSource The texture is used as the source of a texture
     copy or readback, meaning the texture is given as the source in
@@ -2955,7 +2955,12 @@ QRhiGraphicsPipeline::QRhiGraphicsPipeline(QRhiImplementation *rhi)
     \value SurfaceHasNonPreMulAlpha Indicates the target surface has
     transparencyt with non-premultiplied alpha.
 
-    \value sRGB Requests to pick an sRGB format.
+    \value sRGB Requests to pick an sRGB format for the swapchain and/or its
+    render target views, where applicable. Note that this implies that sRGB
+    framebuffer update and blending will get enabled for all content targeting
+    this swapchain, and opting out is not possible. For OpenGL, set
+    \l{QSurfaceFormat::sRGBColorSpace}{sRGBColorSpace} on the QSurfaceFormat of
+    the QWindow in addition.
 
     \value UsedAsTransferSource Indicates the the swapchain will be used as the
     source of a readback in QRhiResourceUpdateBatch::readBackTexture().

src/rhi/qrhigles2.cpp

@@ -177,6 +177,14 @@ QT_BEGIN_NAMESPACE
 #define GL_PRIMITIVE_RESTART_FIXED_INDEX  0x8D69
 #endif
 
+#ifndef GL_FRAMEBUFFER_SRGB
+#define GL_FRAMEBUFFER_SRGB 0x8DB9
+#endif
+
+#ifndef GL_FRAMEBUFFER_SRGB_CAPABLE
+#define GL_FRAMEBUFFER_SRGB_CAPABLE 0x8DBA
+#endif
+
 static QSurfaceFormat qrhigles2_effectiveFormat()
 {
     QSurfaceFormat fmt = QSurfaceFormat::defaultFormat();
@@ -329,6 +337,14 @@ bool QRhiGles2::create(QRhi::Flags flags)
     caps.r8Format = f->hasOpenGLFeature(QOpenGLFunctions::TextureRGFormats);
     caps.r16Format = f->hasOpenGLExtension(QOpenGLExtensions::Sized16Formats);
 
+    caps.srgbCapableDefaultFramebuffer = false;
+    if (ctx->hasExtension(QByteArrayLiteral("GL_ARB_framebuffer_sRGB"))) {
+        GLint srgbCapable = 0;
+        f->glGetIntegerv(GL_FRAMEBUFFER_SRGB_CAPABLE, &srgbCapable);
+        if (srgbCapable)
+            caps.srgbCapableDefaultFramebuffer = true;
+    }
+
     nativeHandlesStruct.context = ctx;
 
     if (rsh) {
@@ -1474,6 +1490,12 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                 f->glBindFramebuffer(GL_FRAMEBUFFER, cmd.args.bindFramebuffer.fbo);
             else
                 f->glBindFramebuffer(GL_FRAMEBUFFER, ctx->defaultFramebufferObject());
+            if (caps.srgbCapableDefaultFramebuffer) {
+                if (cmd.args.bindFramebuffer.srgb)
+                    f->glEnable(GL_FRAMEBUFFER_SRGB);
+                else
+                    f->glDisable(GL_FRAMEBUFFER_SRGB);
+            }
             break;
         case QGles2CommandBuffer::Command::Clear:
             f->glDisable(GL_SCISSOR_TEST);
@@ -1812,6 +1834,7 @@ void QRhiGles2::beginPass(QRhiCommandBuffer *cb,
         Q_UNREACHABLE();
         break;
     }
+    fbCmd.args.bindFramebuffer.srgb = rtD->srgbUpdateAndBlend;
     cbD->commands.append(fbCmd);
 
     cbD->currentTarget = rt;
@@ -2656,6 +2679,7 @@ bool QGles2SwapChain::buildOrResize()
     rt.d.pixelSize = pixelSize;
     rt.d.dpr = m_window->devicePixelRatio();
     rt.d.attCount = m_depthStencil ? 2 : 1;
+    rt.d.srgbUpdateAndBlend = m_flags.testFlag(QRhiSwapChain::sRGB);
 
     frameCount = 0;
 

src/rhi/qrhigles2_p.h

@@ -137,6 +137,7 @@ struct QGles2RenderTargetData
     QSize pixelSize;
     float dpr = 1;
     int attCount = 0;
+    bool srgbUpdateAndBlend = false;
 };
 
 struct QGles2ReferenceRenderTarget : public QRhiRenderTarget
@@ -313,6 +314,7 @@ struct QGles2CommandBuffer : public QRhiCommandBuffer
             } clear;
             struct {
                 GLuint fbo;
+                bool srgb;
             } bindFramebuffer;
             struct {
                 GLenum target;
@@ -559,7 +561,8 @@ public:
               bgraExternalFormat(false),
               bgraInternalFormat(false),
               r8Format(false),
-              r16Format(false)
+              r16Format(false),
+              srgbCapableDefaultFramebuffer(false)
         { }
         int maxTextureSize;
         // Multisample fb and blit are supported (GLES 3.0 or OpenGL 3.x). Not
@@ -572,6 +575,7 @@ public:
         uint bgraInternalFormat : 1;
         uint r8Format : 1;
         uint r16Format : 1;
+        uint srgbCapableDefaultFramebuffer : 1;
     } caps;
     bool inFrame = false;
     bool inPass = false;

todo.txt

+gl: srgb textures/rt
+vk: image in wrong layout when beginFrame-endFrame without a pass in it
+reduce viewport/scissor/other state changes in a pass (?)
+anisotropic
 res.sh.: example to show read/write same texture?
 event/fence for res.sh.?
 advanced blend modes
-gl: tex formats (texture, readback)
-gl: srgb? (glEnable and co.)
 should istexformatsupported should check srgb combinations
-vk: image in wrong layout when beginFrame-endFrame without a pass in it
 cbuffer alignment rules - some things fail to translate (to hlsl e.g. with structs), which is fine but how to mitigate
 what does image copy do for compressed formats?
-reduce viewport/scissor/other state changes in a pass
 vk: test FrameOpDeviceLost somehow
 d3d device loss?
-more what-if-resource-rebuilt cases
-anisotropic
 gl: more ubuf types
 more QImage->tex formats
 if tex adjust its size (e.g. npot on gl), should QImage get scaled automatically?
@@ -43,6 +41,9 @@ dxc for d3d as an alternative to fxc?
 hlsl -> dxc -> spirv -> spirv-cross hmmm...
 
 +++ done
+gl: tex formats (texture)
+gl: srgb? (glEnable and co.)
+more what-if-resource-rebuilt cases
 res.sh.: mtl
 res.sh.: gl
 res.sh.: exercise it in multiwindow_threaded too