Qt 6.6.1 WebAssembly: Triangle disappears when it was clicked

8Observer8

I removed Qt 6.5.3 and installed Qt 6.7.0 beta1. I run an example with triangle above. I see a warning in the Qt Creator console:

:-1: warning: This Qt was built with Emscripten version 3.1.37. You have 3.1.25. The difference may cause issues.

I don't see a triangle in the browser (I see just a black color) but I see a message in the browser console:

WebGL: INVALID_OPERATION: drawArrays: no valid shader program in use

8Observer8

I remove the emsdk folder to install emsdk 3.1.37. As with emsdk 3.1.25 I have an error in the process of installation:

C:\>git clone https://github.com/emscripten-core/emsdk.git
Cloning into 'emsdk'...
remote: Enumerating objects: 3884, done.
remote: Counting objects: 100% (47/47), done.
remote: Compressing objects: 100% (31/31), done.
remote: Total 3884 (delta 22), reused 39 (delta 15), pack-reused 3837Receiving objects: 100% (3884/3884), 1.68 MiB | 331Receiving objects: 100% (3884/3884), 2.16 MiB | 295.00 KiB/s, done.

Resolving deltas: 100% (2552/2552), done.

C:\>cd emsdk

C:\emsdk>emsdk.bat install 3.1.37
Resolving SDK version '3.1.37' to 'sdk-releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'
Installing SDK 'sdk-releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'..
Installing tool 'node-16.20.0-64bit'..
Downloading: C:/emsdk/downloads/node-v16.20.0-win-x64.zip from https://storage.googleapis.com/webassembly/emscripten-releases-builds/deps/node-v16.20.0-win-x64.zip, 28623474 Bytes
Unpacking 'C:/emsdk/downloads/node-v16.20.0-win-x64.zip' to 'C:/emsdk/node/16.20.0_64bit'
Done installing tool 'node-16.20.0-64bit'.
Installing tool 'python-3.9.2-nuget-64bit'..
Downloading: C:/emsdk/downloads/python-3.9.2-4-amd64+pywin32.zip from https://storage.googleapis.com/webassembly/emscripten-releases-builds/deps/python-3.9.2-4-amd64+pywin32.zip, 14413267 Bytes
Unpacking 'C:/emsdk/downloads/python-3.9.2-4-amd64+pywin32.zip' to 'C:/emsdk/python/3.9.2-nuget_64bit'
Done installing tool 'python-3.9.2-nuget-64bit'.
Installing tool 'java-8.152-64bit'..
Error: Downloading URL 'https://storage.googleapis.com/webassembly/emscripten-releases-builds/deps/portable_jre_8_update_152_64bit.zip': <urlopen error [Errno 2] No such file or directory>
error: installation failed!

But when I start in the second time it was installed:

C:\emsdk>emsdk.bat install 3.1.37
Resolving SDK version '3.1.37' to 'sdk-releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'
Installing SDK 'sdk-releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'..
Skipped installing node-16.20.0-64bit, already installed.
Skipped installing python-3.9.2-nuget-64bit, already installed.
Installing tool 'java-8.152-64bit'..
Downloading: C:/emsdk/downloads/portable_jre_8_update_152_64bit.zip from https://storage.googleapis.com/webassembly/emscripten-releases-builds/deps/portable_jre_8_update_152_64bit.zip, 69241499 Bytes
Unpacking 'C:/emsdk/downloads/portable_jre_8_update_152_64bit.zip' to 'C:/emsdk/java/8.152_64bit'
Done installing tool 'java-8.152-64bit'.
Installing tool 'releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'..
Downloading: C:/emsdk/downloads/7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-wasm-binaries.zip from https://storage.googleapis.com/webassembly/emscripten-releases-builds/win/7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2/wasm-binaries.zip, 419522512 Bytes
Unpacking 'C:/emsdk/downloads/7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-wasm-binaries.zip' to 'C:/emsdk/upstream'
Done installing tool 'releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'.
Done installing SDK 'sdk-releases-7c905cfc1ca6699f6ccb288ae174902cfbdcf0a2-64bit'.

If I don't active the emsdk I get this error:

:-1: warning: This Qt was built with Emscripten version 3.1.37. You have can `clang`. The difference may cause issues.
:-1: error: [Makefile:69: .\triangle-in-main-opengl3-qt6-cpp.js] Error 1

But after the activation of emsdk 3.1.37 I don't have the error above and I don't have the warning This Qt was built with Emscripten version 3.1.37. You have 3.1.25. with Qt 6.7.0 beta1. I have the black window again. But I have this error in the browser console like with emsdk 3.1.25 above:

WebGL: INVALID_OPERATION: drawArrays: no valid shader program in use

But this example doesn't show a triangle on Desktop:

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtWidgets/QApplication>

class OpenGLWidget : public QOpenGLWidget, private QOpenGLFunctions
{
public:
    OpenGLWidget()
    {
        resize(350, 350);
    }

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);

        qDebug() << "OpenGL Version:" << (const char*) glGetString(GL_VERSION);
        qDebug() << "GLSL Version:" << (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
        qDebug() << "OpenGL Vendor:" << (const char*) glGetString(GL_VENDOR);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "//out vec4 fragColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 4);
    }

private:
    QOpenGLShaderProgram m_program;
    QOpenGLBuffer m_vertPosBuffer;
};

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

pro

QT += core gui openglwidgets widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

8Observer8

The example above prints this versions of OpenGL/GLSL and the vendor to the Qt Creator console on Desktop, it is Mesa:

OpenGL Version: 3.0 Mesa 11.2.2
GLSL Version: 1.30
OpenGL Vendor: VMware, Inc.

I added the AA_UseDesktopOpenGL attribute for Desktop:

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWidget w;
    w.show();
    return app.exec();
}

Now it prints the integrated video card info:

OpenGL Version: 3.1.0 - Build 9.17.10.4459
GLSL Version: 1.40 - Intel Build 9.17.10.4459
OpenGL Vendor: Intel

But it shows a white triangle instead of a red triangle on Desktop:

When I build this example for WebAssembly it shows a black window and the message: WebGL: INVALID_OPERATION: drawArrays: no valid shader program in use

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtWidgets/QApplication>

class OpenGLWidget : public QOpenGLWidget, private QOpenGLFunctions
{
public:
    OpenGLWidget()
    {
        resize(350, 350);
        setWindowTitle("Example");
    }

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);

        qDebug() << "OpenGL Version:" << (const char*) glGetString(GL_VERSION);
        qDebug() << "GLSL Version:" << (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
        qDebug() << "OpenGL Vendor:" << (const char*) glGetString(GL_VENDOR);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "//out vec4 fragColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 4);
    }

private:
    QOpenGLShaderProgram m_program;
    QOpenGLBuffer m_vertPosBuffer;
};

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::ApplicationAttribute::AA_UseDesktopOpenGL);
    QApplication app(argc, argv);
    OpenGLWidget w;
    w.show();
    return app.exec();
}

pro

QT += core gui openglwidgets widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

8Observer8

I tried to activate the discrete video card to see the red triangle on Desktop by adding this code at the beginning of main.cpp:

#ifdef _WIN32
#include <windows.h>
extern "C" __declspec(dllexport) DWORD NvOptimusEnablement = 0x00000001;
extern "C" __declspec(dllexport) DWORD AmdPowerXpressRequestHighPerformance = 0x00000001;
#endif

The program print this info to the Qt Creater console that means that the discrete card was activated:

OpenGL Version: 4.6.0 NVIDIA 391.35
GLSL Version: 4.60 NVIDIA
OpenGL Vendor: NVIDIA Corporation

But it draws a white triangle with Qt 6.7.0 beta1.

8Observer8

I created a bug report about the problem above on Desktop: Drawing a triangle doesn't work on laptop Asus K53SV with Mesa using Qt 6.7.0 beta1: https://bugreports.qt.io/browse/QTBUG-120949

8Observer8

I created a separated topic for the problem above on Desktop: Drawing a triangle doesn't work on laptop Asus K53SV with Mesa using Qt 6.7.0 beta1

8Observer8

@Christian-Ehrlicher solved the problem above for Desktop. I forgot to activate (bind) the program in paintGL():

void paintGL() override
{
    m_program.bind();
    glClear(GL_COLOR_BUFFER_BIT);
    glDrawArrays(GL_TRIANGLES, 0, 4);
}

This program works on Desktop but it still doesn't work on WebAssembly. I use Qt 6.7.0 beta1. But now it doesn't print this message WebGL: INVALID_OPERATION: drawArrays: no valid shader program in use to the browser console. The window is black on browser.

pro

QT += core gui openglwidgets widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtWidgets/QApplication>

class OpenGLWidget : public QOpenGLWidget, private QOpenGLFunctions
{
public:
    OpenGLWidget()
    {
        resize(350, 350);
        setWindowTitle("OpenGL Triangle");
    }

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);

        qDebug() << "OpenGL Version:" << (const char*) glGetString(GL_VERSION);
        qDebug() << "GLSL Version:" << (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
        qDebug() << "OpenGL Vendor:" << (const char*) glGetString(GL_VENDOR);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "//out vec4 fragColor;\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
    }

    void paintGL() override
    {
        m_program.bind();
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 4);
    }

private:
    QOpenGLShaderProgram m_program;
    QOpenGLBuffer m_vertPosBuffer;
};

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

8Observer8

I made a bug report for drawing a triangle without clicking. It draws a black screen. I attacked a zip here: https://bugreports.qt.io/browse/QTBUG-120954

8Observer8

The solution for drawing a triangle with Qt 6.7.0 beta1 is to add bind() for the vertex buffer:

    void paintGL() override
    {
        m_program.bind();
        m_vertPosBuffer.bind();
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);
    }

But why a background is black?

I set it to blue. If I switch to Desktop I see the blue background:

pro

QT += core gui openglwidgets widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtWidgets/QApplication>

class OpenGLWidget : public QOpenGLWidget, private QOpenGLFunctions
{
public:
    OpenGLWidget()
    {
        resize(350, 350);
        setWindowTitle("OpenGL Triangle");
    }

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);

        qDebug() << "OpenGL Version:" << (const char*) glGetString(GL_VERSION);
        qDebug() << "GLSL Version:" << (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
        qDebug() << "OpenGL Vendor:" << (const char*) glGetString(GL_VENDOR);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
    }

    void paintGL() override
    {
        m_program.bind();
        m_vertPosBuffer.bind();
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);
    }

private:
    QOpenGLShaderProgram m_program;
    QOpenGLBuffer m_vertPosBuffer;
};

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

8Observer8

So when I run an example above on WebAssembly it draws a background in black color. I created a bug report: https://bugreports.qt.io/browse/QTBUG-120956

8Observer8

I found a way how to draw a custom background color. In the example above I set a background color in the initializeGL() method:

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);

But now I moved the setting of background color to the paintGL() method:

    void paintGL() override
    {
        // ...
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);
    }

pro

QT += core gui openglwidgets widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtGui/QSurfaceFormat>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtWidgets/QApplication>

class OpenGLWidget : public QOpenGLWidget, private QOpenGLFunctions
{
public:
    OpenGLWidget()
    {
        resize(350, 350);
        setWindowTitle("OpenGL Triangle");
    }

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();

        qDebug() << "OpenGL Version:" << (const char*) glGetString(GL_VERSION);
        qDebug() << "GLSL Version:" << (const char*) glGetString(GL_SHADING_LANGUAGE_VERSION);
        qDebug() << "OpenGL Vendor:" << (const char*) glGetString(GL_VENDOR);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
    }

    void paintGL() override
    {
        m_program.bind();
        m_vertPosBuffer.bind();
        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
        glClearColor(0.4f, 0.8f, 0.9f, 1.f);
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);
    }

private:
    QOpenGLShaderProgram m_program;
    QOpenGLBuffer m_vertPosBuffer;
};

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

8Observer8

This is a solution of the topic with picking color of a triangle. I didn't test it in Qt 6.6.1. It works in Qt 6.7.0 beta1.

The shader program must be bind before drawing
The vertex buffer must be bind before drawing
glClearColor must be called before glClear in paintGL for WebAssembly

This example prints a color in the point of mouse click using the glReadPixles method:

    void paintGL() override
    {
        glClearColor(0.2f, 0.2f, 0.2f, 1.f);
        glClear(GL_COLOR_BUFFER_BIT);
        m_program.bind();
        m_vertPosBuffer.bind();
        m_program.setAttributeBuffer(m_aPositionLocation, GL_FLOAT, 0, 2);
        m_program.enableAttributeArray(m_aPositionLocation);
        glDrawArrays(GL_TRIANGLES, 0, 3);
        // qDebug() << glGetError() << "\n";

        if (m_mouseClicked)
        {
            // Read the pixel
            GLubyte pixel[4];
            glReadPixels(m_mouseX, m_mouseY, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);
            // qDebug() << glGetError() << "\n";
            qDebug() << pixel[0] / 255.f << pixel[1] / 255.f << pixel[2] / 255.f;
            m_mouseClicked = false;
        }
    }

pro

QT += core gui openglwidgets widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

TARGET = app

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtGui/QMouseEvent>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGLWidgets/QOpenGLWidget>
#include <QtWidgets/QApplication>
#include <QtWidgets/QLabel>
#include <QtWidgets/QVBoxLayout>
#include <QtWidgets/QWidget>

class OpenGLWidget : public QOpenGLWidget, private QOpenGLFunctions
{
    Q_OBJECT

private:
    void initializeGL() override
    {
        initializeOpenGLFunctions();

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));
        m_aPositionLocation = m_program.attributeLocation("aPosition");
    }

    void paintGL() override
    {
        glClearColor(0.2f, 0.2f, 0.2f, 1.f);
        glClear(GL_COLOR_BUFFER_BIT);
        m_program.bind();
        m_vertPosBuffer.bind();
        m_program.setAttributeBuffer(m_aPositionLocation, GL_FLOAT, 0, 2);
        m_program.enableAttributeArray(m_aPositionLocation);
        glDrawArrays(GL_TRIANGLES, 0, 3);
        // qDebug() << glGetError() << "\n";

        if (m_mouseClicked)
        {
            // Read the pixel
            GLubyte pixel[4];
            glReadPixels(m_mouseX, m_mouseY, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);
            // qDebug() << glGetError() << "\n";
            qDebug() << pixel[0] / 255.f << pixel[1] / 255.f << pixel[2] / 255.f;
            m_mouseClicked = false;
        }
    }

    void mousePressEvent(QMouseEvent *event) override
    {
        m_mouseX = event->pos().x();
        m_mouseY = height() - event->pos().y() - 1;
        m_mouseClicked = true;
        update();
    }

private:
    int m_mouseX;
    int m_mouseY;
    bool m_mouseClicked = false;
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;
    int m_aPositionLocation;
};

class MainWindow : public QWidget
{
    Q_OBJECT

public:
    MainWindow()
    {
        setWindowTitle("Pick a red or green triangle");
        resize(350, 350);

        m_nameLabel = new QLabel("Click on a triangle or background");
        m_nameLabel->setSizePolicy(QSizePolicy::Policy::Fixed, QSizePolicy::Policy::Fixed);
        OpenGLWidget *openGLWidget = new OpenGLWidget();

        QVBoxLayout *layout = new QVBoxLayout();
        layout->addWidget(m_nameLabel);
        layout->addWidget(openGLWidget);
        setLayout(layout);
    }

private:
    QLabel *m_nameLabel;
};

#include "main.moc"

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

8Observer8

Piotr Wierciński wrote here: https://bugreports.qt.io/browse/QTBUG-120956 that it is better to use QOpenGLWindow instead of QOpenGLWidget with WebAssembly:

If separate context is needed, consider using QOpenGLWindow , which will have it's own canvas in DOM and it's own WebGL context, instead of QOpenGLWidget

I tried it and he is right. With QOpenGLWindow I can call glClearColor in initializeGL:

#include <QtGui/QOpenGLFunctions>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.2f, 0.5f, 0.3f, 1.f);
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
    }
};

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

background-color-qopenglwindow-webassembly-qt6-cpp.pro

QT += core gui opengl widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

TARGET = app

8Observer8

I even must not bind a shader program (m_program.bind()) and a vertex buffer (m_vertPosBuffer.bind()) inside of paintGLwith QOpenGLWindow:

main.cpp

#include <QtGui/QOpenGLFunctions>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.2f, 0.5f, 0.3f, 1.f);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));

        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);
    }
};

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

simple-triangle-qopenglwindow-qt6-cpp.pro

QT += core gui opengl widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

TARGET = app

8Observer8

Color picking with glReadPixels works without problems on WebAssembly with QOpenGLWindow and QOpenGLWidget:

pick-color-of-simple-triangle-qopenglwindow-qt6-cpp-webassembly.gif

main.cpp

#include <QtGui/QMouseEvent>
#include <QtGui/QOpenGLFunctions>
#include <QtOpenGL/QOpenGLBuffer>
#include <QtOpenGL/QOpenGLShaderProgram>
#include <QtOpenGL/QOpenGLWindow>
#include <QtWidgets/QApplication>

class OpenGLWindow : public QOpenGLWindow, private QOpenGLFunctions
{
    int m_mouseX;
    int m_mouseY;
    bool m_mouseClicked = false;
    QOpenGLBuffer m_vertPosBuffer;
    QOpenGLShaderProgram m_program;

    void initializeGL() override
    {
        initializeOpenGLFunctions();
        glClearColor(0.2f, 0.2f, 0.2f, 1.f);

        QString vertexShaderSource =
            "attribute vec2 aPosition;\n"
            "void main()\n"
            "{\n"
            "    gl_Position = vec4(aPosition, 0.0, 1.0);\n"
            "}\n";

        QString fragmentShaderSource =
            "#ifdef GL_ES\n"
            "precision mediump float;\n"
            "#endif\n"
            "void main()\n"
            "{\n"
            "    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
            "}\n";

        m_program.create();
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Vertex,
                                          vertexShaderSource);
        m_program.addShaderFromSourceCode(QOpenGLShader::ShaderTypeBit::Fragment,
                                          fragmentShaderSource);
        m_program.link();
        m_program.bind();

        float vertPositions[] = {
            -0.5f, -0.5f,
            0.5f, -0.5f,
            0.f, 0.5f
        };
        m_vertPosBuffer.create();
        m_vertPosBuffer.bind();
        m_vertPosBuffer.allocate(vertPositions, sizeof(vertPositions));

        m_program.setAttributeBuffer("aPosition", GL_FLOAT, 0, 2);
        m_program.enableAttributeArray("aPosition");
    }

    void paintGL() override
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 3);

        if (m_mouseClicked)
        {
            // Read the pixel
            GLubyte pixel[4];
            glReadPixels(m_mouseX, m_mouseY, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);
            // qDebug() << glGetError() << "\n";
            qDebug() << pixel[0] / 255.f << pixel[1] / 255.f << pixel[2] / 255.f;
            m_mouseClicked = false;
        }
    }

    void mousePressEvent(QMouseEvent *event) override
    {
        m_mouseX = event->pos().x();
        m_mouseY = height() - event->pos().y() - 1;
        m_mouseClicked = true;
        update();
    }
};

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

pick-color-of-simple-triangle-qopenglwindow-qt6-cpp.pro

QT += core gui opengl widgets

win32: LIBS += -lopengl32

CONFIG += c++17

SOURCES += \
    main.cpp

TARGET = app

8Observer8

@8Observer8 said in Qt 6.6.1 WebAssebly: Triangle disappears when it was clicked:

Color picking with glReadPixels works without problems on WebAssembly with QOpenGLWindow and QOpenGLWidget

I had a problem with color picking on Android but it was solved here: https://forum.qt.io/post/787931