QOpenGLWidget with RGBA 10-10-10-2 isn't created correctly

[patb]

When requesting a QSurfaceFormat with 10R-10G-10B-2A, OpenGL succeeds but QSurfaceFormat reports 8-8-8-2 instead of 10-10-10-2.

The bug exists since at least Qt 6.5.3 probably much longer.

This is critical for us (Autodesk) as one of our applications recently migrated from Qt5/QGLWidget to Qt6/QOpenGLWidget an 10-bit color workflow isn't working anymore.

I'd like to attach a file but (I can't) so here's a sample main.cpp that will show the bug.

---

#include <QApplication>
#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QSurfaceFormat>
#include <QVBoxLayout>
#include <QWidget>
#include <iostream>

void explain()
{
    std::cout << "EXPLANATION OF THE BUG\n" << std::endl;
    std::cout << "Location:" << std::endl;
    std::cout << "   File: qtbase/src/plugins/platforms/cocoa/qcocoaglcontext.mm" << std::endl;
    std::cout << "   Line: 267-271\n" << std::endl;
    std::cout << "The Bug:" << std::endl;
    std::cout << "   Qt divides NSOpenGLPFAColorSize by 4, assuming it includes alpha." << std::endl;
    std::cout << "   However, NSOpenGLPFAColorSize returns ONLY R+G+B bits (no alpha).\n" << std::endl;
    std::cout << "Buggy Code:" << std::endl;
    std::cout << "   int colorSize = pixelFormatAttribute(NSOpenGLPFAColorSize);" << std::endl;
    std::cout << "   colorSize /= 4;  // WRONG!" << std::endl;
    std::cout << "   m_format.setRedBufferSize(colorSize);" << std::endl;
    std::cout << "   m_format.setGreenBufferSize(colorSize);" << std::endl;
    std::cout << "   m_format.setBlueBufferSize(colorSize);\n" << std::endl;
    std::cout << "What Happens with 10-10-10-2:" << std::endl;
    std::cout << "   1. You request: 10-10-10-2 (30 bits RGB + 2 bits alpha)" << std::endl;
    std::cout << "   2. macOS gives closest match: 32 bits RGB total" << std::endl;
    std::cout << "   3. NSOpenGLPFAColorSize returns: 32 (R+G+B only)" << std::endl;
    std::cout << "   4. Qt divides by 4: 32 / 4 = 8" << std::endl;
    std::cout << "   5. Qt reports: 8-8-8-2 WRONG!" << std::endl;
    std::cout << "   6. Actual format: 10-10-10-2 (OpenGL confirms this)\n" << std::endl;
    std::cout << "Solution:" << std::endl;
    std::cout << "   Query OpenGL directly using glGetIntegerv():" << std::endl;
    std::cout << "   glGetIntegerv(GL_RED_BITS/GL_GREEN_BITS/GL_BLUE_BITS)" << std::endl;
    std::cout << "   This bypasses Qt's incorrect reporting.\n" << std::endl;
    std::cout << "Platform Status:" << std::endl;
    std::cout << "   macOS:   BUG (present in Qt 6.10.1)" << std::endl;
    std::cout << "   Windows: OK  (queries WGL_RED_BITS_ARB separately)" << std::endl;
    std::cout << "   Linux:   OK  (queries GLX_RED_SIZE separately)" << std::endl;
}

class TestGLWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
public:
    TestGLWidget(QWidget* parent = nullptr)
        : QOpenGLWidget(parent)
    {
        // Request 10-10-10-2 format
        QSurfaceFormat fmt;
        fmt.setRedBufferSize(10);
        fmt.setGreenBufferSize(10);
        fmt.setBlueBufferSize(10);
        fmt.setAlphaBufferSize(2);
        fmt.setDepthBufferSize(24);
        fmt.setStencilBufferSize(8);
        fmt.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
        
        std::cout << "REQUESTED FORMAT:" << std::endl;
        std::cout << "  R-G-B-A: " << fmt.redBufferSize() << "-"
                  << fmt.greenBufferSize() << "-"
                  << fmt.blueBufferSize() << "-"
                  << fmt.alphaBufferSize() << std::endl;
        
        setFormat(fmt);
    }

protected:
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        
        std::cout << "AFTER initializeGL():" << std::endl;
        
        // What Qt reports via format()
        QSurfaceFormat qtFormat = format();
        std::cout << "Qt format() reports:" << std::endl;
        std::cout << "  R-G-B-A: " << qtFormat.redBufferSize() << "-"
                  << qtFormat.greenBufferSize() << "-"
                  << qtFormat.blueBufferSize() << "-"
                  << qtFormat.alphaBufferSize() << std::endl;
        
        // What OpenGL actually has
        GLint redBits = 0, greenBits = 0, blueBits = 0, alphaBits = 0;
        glGetIntegerv(GL_RED_BITS, &redBits);
        glGetIntegerv(GL_GREEN_BITS, &greenBits);
        glGetIntegerv(GL_BLUE_BITS, &blueBits);
        glGetIntegerv(GL_ALPHA_BITS, &alphaBits);
        
        std::cout << "OpenGL glGetIntegerv() reports:" << std::endl;
        std::cout << "  R-G-B-A: " << redBits << "-"
                  << greenBits << "-"
                  << blueBits << "-"
                  << alphaBits << std::endl;
        
        if (qtFormat.redBufferSize() != redBits ||
            qtFormat.greenBufferSize() != greenBits ||
            qtFormat.blueBufferSize() != blueBits) {
            std::cout << "BUG DETECTED!" << std::endl;
            std::cout << "   Qt reports different values than OpenGL!" << std::endl;
            std::cout << "   This is the Qt macOS bug in qcocoaglcontext.mm:268\n" << std::endl;
            
            // Provide detailed explanation of the bug
            explain();
        } else {
            std::cout << "Qt correctly reports the OpenGL format\n" << std::endl;
        }
    }
    
    void paintGL() override
    {
        glClearColor(0.2f, 0.3f, 0.4f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);
    }
};

class MainWindow : public QWidget
{
public:
    MainWindow()
    {
        setWindowTitle("Qt 10-bit Format Bug Test");
        resize(640, 480);
        
        QVBoxLayout* layout = new QVBoxLayout(this);
        TestGLWidget* glWidget = new TestGLWidget();
        layout->addWidget(glWidget);
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    
    MainWindow window;
    window.show();
    
    return app.exec();
}

[SGaist]

Hi and welcome to devnet,

Do you see this on all platforms ?