Support windows with sRGB-capable default framebuffers in QSurfaceFormat

Backend implementation is done for GLX for now. WGL and EGL may follow
later on.

[ChangeLog][QtGui] Added support for requesting OpenGL windows with
sRGB-capable default framebuffers. While this is implicit on some
platforms, QSurfaceFormat now has the necessary flags to request
such windows in a cross-platform manner.

Task-number: QTBUG-50987
Change-Id: I4df1f786e41e63396f46920a81afdf5ecb5eedea
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/gui/kernel/qsurfaceformat.cpp

@@ -73,6 +73,7 @@ public:
         , major(2)
         , minor(0)
         , swapInterval(1) // default to vsync
+        , colorSpace(QSurfaceFormat::DefaultColorSpace)
     {
     }
 
@@ -91,7 +92,8 @@ public:
           profile(other->profile),
           major(other->major),
           minor(other->minor),
-          swapInterval(other->swapInterval)
+          swapInterval(other->swapInterval),
+          colorSpace(other->colorSpace)
     {
     }
 
@@ -110,6 +112,7 @@ public:
     int major;
     int minor;
     int swapInterval;
+    QSurfaceFormat::ColorSpace colorSpace;
 };
 
 /*!
@@ -124,6 +127,12 @@ public:
     contains surface configuration parameters such as OpenGL profile and
     version for rendering, whether or not to enable stereo buffers, and swap
     behaviour.
+
+    \note When troubleshooting context or window format issues, it can be
+    helpful to enable the logging category \c{qt.qpa.gl}. Depending on the
+    platform, this may print useful debug information when it comes to OpenGL
+    initialization and the native visual or framebuffer configurations which
+    QSurfaceFormat gets mapped to.
 */
 
 /*!
@@ -198,6 +207,22 @@ public:
     \value CompatibilityProfile Functionality from earlier OpenGL versions is available.
 */
 
+/*!
+    \enum QSurfaceFormat::ColorSpace
+
+    This enum is used to specify the preferred color space, controlling if the
+    window's associated default framebuffer is able to do updates and blending
+    in a given encoding instead of the standard linear operations.
+
+    \value DefaultColorSpace The default, unspecified color space.
+
+    \value sRGBColorSpace When \c{GL_ARB_framebuffer_sRGB} or
+    \c{GL_EXT_framebuffer_sRGB} is supported by the platform and this value is
+    set, the window will be created with an sRGB-capable default
+    framebuffer. Note that some platforms may return windows with a sRGB-capable
+    default framebuffer even when not requested explicitly.
+ */
+
 /*!
     Constructs a default initialized QSurfaceFormat.
 
@@ -736,6 +761,47 @@ int QSurfaceFormat::swapInterval() const
     return d->swapInterval;
 }
 
+/*!
+    Sets the preferred \a colorSpace.
+
+    For example, this allows requesting windows with default framebuffers that
+    are sRGB-capable on platforms that support it.
+
+    \note When the requested color space is not supported by the platform, the
+    request is ignored. Query the QSurfaceFormat after window creation to verify
+    if the color space request could be honored or not.
+
+    \note This setting controls if the default framebuffer of the window is
+    capable of updates and blending in a given color space. It does not change
+    applications' output by itself. The applications' rendering code will still
+    have to opt in via the appropriate OpenGL calls to enable updates and
+    blending to be performed in the given color space instead of using the
+    standard linear operations.
+
+    \since 5.10
+
+    \sa colorSpace()
+ */
+void QSurfaceFormat::setColorSpace(ColorSpace colorSpace)
+{
+    if (d->colorSpace != colorSpace) {
+        detach();
+        d->colorSpace = colorSpace;
+    }
+}
+
+/*!
+    \return the color space.
+
+    \since 5.10
+
+    \sa setColorSpace()
+*/
+QSurfaceFormat::ColorSpace QSurfaceFormat::colorSpace() const
+{
+    return d->colorSpace;
+}
+
 Q_GLOBAL_STATIC(QSurfaceFormat, qt_default_surface_format)
 
 /*!
@@ -841,6 +907,7 @@ QDebug operator<<(QDebug dbg, const QSurfaceFormat &f)
                   << ", samples " << d->numSamples
                   << ", swapBehavior " << d->swapBehavior
                   << ", swapInterval " << d->swapInterval
+                  << ", colorSpace " << d->colorSpace
                   << ", profile  " << d->profile
                   << ')';
 

src/gui/kernel/qsurfaceformat.h

@@ -85,6 +85,12 @@ public:
     };
     Q_ENUM(OpenGLContextProfile)
 
+    enum ColorSpace {
+        DefaultColorSpace,
+        sRGBColorSpace
+    };
+    Q_ENUM(ColorSpace)
+
     QSurfaceFormat();
     /*implicit*/ QSurfaceFormat(FormatOptions options);
     QSurfaceFormat(const QSurfaceFormat &other);
@@ -145,6 +151,9 @@ public:
     int swapInterval() const;
     void setSwapInterval(int interval);
 
+    ColorSpace colorSpace() const;
+    void setColorSpace(ColorSpace colorSpace);
+
     static void setDefaultFormat(const QSurfaceFormat &format);
     static QSurfaceFormat defaultFormat();
 

src/platformsupport/glxconvenience/qglxconvenience.cpp

@@ -76,7 +76,11 @@ enum {
 #undef FontChange
 #endif
 
-QVector<int> qglx_buildSpec(const QSurfaceFormat &format, int drawableBit)
+#ifndef GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB
+#define GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB 0x20B2
+#endif
+
+QVector<int> qglx_buildSpec(const QSurfaceFormat &format, int drawableBit, int flags)
 {
     QVector<int> spec;
 
@@ -120,6 +124,10 @@ QVector<int> qglx_buildSpec(const QSurfaceFormat &format, int drawableBit)
              << GLX_SAMPLES_ARB
              << format.samples();
 
+    if ((flags & QGLX_SUPPORTS_SRGB) && format.colorSpace() == QSurfaceFormat::sRGBColorSpace)
+        spec << GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB
+             << True;
+
     spec << GLX_DRAWABLE_TYPE
          << drawableBit
 
@@ -179,14 +187,14 @@ template <class T>
 using QXlibArrayPointer = QScopedArrayPointer<T, QXlibScopedPointerDeleter<T>>;
 }
 
-GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format, bool highestPixelFormat, int drawableBit)
+GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format, bool highestPixelFormat, int drawableBit, int flags)
 {
     QXcbSoftwareOpenGLEnforcer softwareOpenGLEnforcer;
 
     GLXFBConfig config = 0;
 
     do {
-        const QVector<int> spec = qglx_buildSpec(format, drawableBit);
+        const QVector<int> spec = qglx_buildSpec(format, drawableBit, flags);
 
         int confcount = 0;
         QXlibArrayPointer<GLXFBConfig> configs(glXChooseFBConfig(display, screen, spec.constData(), &confcount));
@@ -205,6 +213,13 @@ GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format
         for (int i = 0; i < confcount; i++) {
             GLXFBConfig candidate = configs[i];
 
+            if ((flags & QGLX_SUPPORTS_SRGB) && format.colorSpace() == QSurfaceFormat::sRGBColorSpace) {
+                int srgbCapable = 0;
+                glXGetFBConfigAttrib(display, candidate, GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB, &srgbCapable);
+                if (!srgbCapable)
+                    continue;
+            }
+
             QXlibPointer<XVisualInfo> visual(glXGetVisualFromFBConfig(display, candidate));
 
             const int actualRed = qPopulationCount(visual->red_mask);
@@ -228,28 +243,28 @@ GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format
     return config;
 }
 
-XVisualInfo *qglx_findVisualInfo(Display *display, int screen, QSurfaceFormat *format, int drawableBit)
+XVisualInfo *qglx_findVisualInfo(Display *display, int screen, QSurfaceFormat *format, int drawableBit, int flags)
 {
     Q_ASSERT(format);
 
     XVisualInfo *visualInfo = 0;
 
-    GLXFBConfig config = qglx_findConfig(display, screen, *format, false, drawableBit);
+    GLXFBConfig config = qglx_findConfig(display, screen, *format, false, drawableBit, flags);
     if (config)
         visualInfo = glXGetVisualFromFBConfig(display, config);
 
     if (visualInfo) {
-        qglx_surfaceFormatFromGLXFBConfig(format, display, config);
+        qglx_surfaceFormatFromGLXFBConfig(format, display, config, flags);
         return visualInfo;
     }
 
     // attempt to fall back to glXChooseVisual
     do {
-        QVector<int> attribs = qglx_buildSpec(*format, drawableBit);
+        QVector<int> attribs = qglx_buildSpec(*format, drawableBit, flags);
         visualInfo = glXChooseVisual(display, screen, attribs.data());
 
         if (visualInfo) {
-            qglx_surfaceFormatFromVisualInfo(format, display, visualInfo);
+            qglx_surfaceFormatFromVisualInfo(format, display, visualInfo, flags);
             return visualInfo;
         }
     } while (qglx_reduceFormat(format));
@@ -257,7 +272,7 @@ XVisualInfo *qglx_findVisualInfo(Display *display, int screen, QSurfaceFormat *f
     return visualInfo;
 }
 
-void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display, GLXFBConfig config)
+void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display, GLXFBConfig config, int flags)
 {
     int redSize     = 0;
     int greenSize   = 0;
@@ -268,6 +283,7 @@ void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display,
     int sampleBuffers = 0;
     int sampleCount = 0;
     int stereo      = 0;
+    int srgbCapable = 0;
 
     glXGetFBConfigAttrib(display, config, GLX_RED_SIZE,     &redSize);
     glXGetFBConfigAttrib(display, config, GLX_GREEN_SIZE,   &greenSize);
@@ -277,6 +293,8 @@ void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display,
     glXGetFBConfigAttrib(display, config, GLX_STENCIL_SIZE, &stencilSize);
     glXGetFBConfigAttrib(display, config, GLX_SAMPLES_ARB,  &sampleBuffers);
     glXGetFBConfigAttrib(display, config, GLX_STEREO,       &stereo);
+    if (flags & QGLX_SUPPORTS_SRGB)
+        glXGetFBConfigAttrib(display, config, GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB, &srgbCapable);
 
     format->setRedBufferSize(redSize);
     format->setGreenBufferSize(greenSize);
@@ -288,11 +306,12 @@ void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display,
         glXGetFBConfigAttrib(display, config, GLX_SAMPLES_ARB, &sampleCount);
         format->setSamples(sampleCount);
     }
+    format->setColorSpace(srgbCapable ? QSurfaceFormat::sRGBColorSpace : QSurfaceFormat::DefaultColorSpace);
 
     format->setStereo(stereo);
 }
 
-void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display, XVisualInfo *visualInfo)
+void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display, XVisualInfo *visualInfo, int flags)
 {
     int redSize     = 0;
     int greenSize   = 0;
@@ -303,6 +322,7 @@ void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display,
     int sampleBuffers = 0;
     int sampleCount = 0;
     int stereo      = 0;
+    int srgbCapable = 0;
 
     glXGetConfig(display, visualInfo, GLX_RED_SIZE,     &redSize);
     glXGetConfig(display, visualInfo, GLX_GREEN_SIZE,   &greenSize);
@@ -312,6 +332,8 @@ void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display,
     glXGetConfig(display, visualInfo, GLX_STENCIL_SIZE, &stencilSize);
     glXGetConfig(display, visualInfo, GLX_SAMPLES_ARB,  &sampleBuffers);
     glXGetConfig(display, visualInfo, GLX_STEREO,       &stereo);
+    if (flags & QGLX_SUPPORTS_SRGB)
+        glXGetConfig(display, visualInfo, GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB, &srgbCapable);
 
     format->setRedBufferSize(redSize);
     format->setGreenBufferSize(greenSize);
@@ -323,6 +345,7 @@ void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display,
         glXGetConfig(display, visualInfo, GLX_SAMPLES_ARB, &sampleCount);
         format->setSamples(sampleCount);
     }
+    format->setColorSpace(srgbCapable ? QSurfaceFormat::sRGBColorSpace : QSurfaceFormat::DefaultColorSpace);
 
     format->setStereo(stereo);
 }
@@ -391,5 +414,10 @@ bool qglx_reduceFormat(QSurfaceFormat *format)
         return true;
     }
 
+    if (format->colorSpace() == QSurfaceFormat::sRGBColorSpace) {
+        format->setColorSpace(QSurfaceFormat::DefaultColorSpace);
+        return true;
+    }
+
     return false;
 }

src/platformsupport/glxconvenience/qglxconvenience_p.h

@@ -57,11 +57,16 @@
 #include <X11/Xlib.h>
 #include <GL/glx.h>
 
-QVector<int> qglx_buildSpec(const QSurfaceFormat &format, int drawableBit = GLX_WINDOW_BIT);
+enum QGlxFlags
-XVisualInfo *qglx_findVisualInfo(Display *display, int screen, QSurfaceFormat *format, int drawableBit = GLX_WINDOW_BIT);
+{
-GLXFBConfig qglx_findConfig(Display *display, int screen, QSurfaceFormat format, bool highestPixelFormat = false, int drawableBit = GLX_WINDOW_BIT);
+    QGLX_SUPPORTS_SRGB = 0x01
-void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display, GLXFBConfig config);
+};
-void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display, XVisualInfo *visualInfo);
+
+QVector<int> qglx_buildSpec(const QSurfaceFormat &format, int drawableBit = GLX_WINDOW_BIT, int flags = 0);
+XVisualInfo *qglx_findVisualInfo(Display *display, int screen, QSurfaceFormat *format, int drawableBit = GLX_WINDOW_BIT, int flags = 0);
+GLXFBConfig qglx_findConfig(Display *display, int screen, QSurfaceFormat format, bool highestPixelFormat = false, int drawableBit = GLX_WINDOW_BIT, int flags = 0);
+void qglx_surfaceFormatFromGLXFBConfig(QSurfaceFormat *format, Display *display, GLXFBConfig config, int flags = 0);
+void qglx_surfaceFormatFromVisualInfo(QSurfaceFormat *format, Display *display, XVisualInfo *visualInfo, int flags = 0);
 bool qglx_reduceFormat(QSurfaceFormat *format);
 
 #endif // QGLXCONVENIENCE_H

src/plugins/platforms/xcb/gl_integrations/xcb_glx/qxcbglxwindow.cpp

@@ -41,6 +41,7 @@
 
 #include "qxcbscreen.h"
 #include <QtGlxSupport/private/qglxconvenience_p.h>
+#include <QDebug>
 
 QT_BEGIN_NAMESPACE
 
@@ -58,13 +59,27 @@ const xcb_visualtype_t *QXcbGlxWindow::createVisual()
     QXcbScreen *scr = xcbScreen();
     if (!scr)
         return Q_NULLPTR;
-    XVisualInfo *visualInfo = qglx_findVisualInfo(DISPLAY_FROM_XCB(scr), scr->screenNumber(), &m_format);
+
+    qDebug(lcQpaGl) << "Requested format before FBConfig/Visual selection:" << m_format;
+
+    const char *glxExts = glXQueryExtensionsString(DISPLAY_FROM_XCB(scr), scr->screenNumber());
+    int flags = 0;
+    if (glxExts) {
+        qCDebug(lcQpaGl, "Available GLX extensions: %s", glxExts);
+        if (strstr(glxExts, "GLX_EXT_framebuffer_sRGB") || strstr(glxExts, "GLX_ARB_framebuffer_sRGB"))
+            flags |= QGLX_SUPPORTS_SRGB;
+    }
+
+    XVisualInfo *visualInfo = qglx_findVisualInfo(DISPLAY_FROM_XCB(scr), scr->screenNumber(), &m_format, GLX_WINDOW_BIT, flags);
     if (!visualInfo) {
         qWarning() << "No XVisualInfo for format" << m_format;
         return Q_NULLPTR;
     }
     const xcb_visualtype_t *xcb_visualtype = scr->visualForId(visualInfo->visualid);
     XFree(visualInfo);
+
+    qDebug(lcQpaGl) << "Got format:" << m_format;
+
     return xcb_visualtype;
 }