Update a QImage using a shader in QOpenGLWidget

Question

I have a simple vertex shader

static const char *vertexShader=
        "attribute vec4 vPosition;  \n"

        "void main(){\n"
            "gl_Position = vPosition;\n"
        "}";

Also I have a shader which creates a "Billboard" effect on an image.

static const char *fragmentShader=
            "uniform float grid;\n"
            "uniform float dividerValue;\n"
            "uniform float step_x;\n"
            "uniform float step_y;\n"
            "uniform sampler2D source;\n"
            "uniform lowp float qt_Opacity;\n"
            "uniform vec2 qt_TexCoord0;\n"

            "void main(){\n"
                "vec2 uv = qt_TexCoord0.xy;\n"
                "float offx = floor(uv.x  / (grid * step_x));\n"
                "float offy = floor(uv.y  / (grid * step_y));\n"
                "vec3 res = texture2D(source, vec2(offx * grid * step_x , offy * grid * step_y)).rgb;\n"
                "vec2 prc = fract(uv / vec2(grid * step_x, grid * step_y));\n"
                "vec2 pw = pow(abs(prc - 0.5), vec2(2.0));\n"
                "float  rs = pow(0.45, 2.0);\n"
                "float gr = smoothstep(rs - 0.1, rs + 0.1, pw.x + pw.y);\n"
                "float y = (res.r + res.g + res.b) / 3.0;\n"
                "vec3 ra = res / y;\n"
                "float ls = 0.3;\n"
                "float lb = ceil(y / ls);\n"
                "float lf = ls * lb + 0.3;\n"
                "res = lf * res;\n"
                "vec3 col = mix(res, vec3(0.1, 0.1, 0.1), gr);\n"
                "if (uv.x < dividerValue)\n"
                    "gl_FragColor = qt_Opacity * vec4(col, 1.0);\n"
                "else\n"
                    "gl_FragColor = qt_Opacity * texture2D(source, uv);\n"
            "}";

What I'd like to do is to use this shader to apply this effect on an image in QtOpenGlWidget. But I dont get how to set my image as a texture and pass it to a shader and then to return it modified with a shader effect. What is I want to achieve is: https://i.stack.imgur.com/Y1KCm.jpg But my shader doesn't affect image https://i.stack.imgur.com/PEYGi.jpg . My GLWidget class:

GLWidget::GLWidget(Helper *helper, QWidget *parent)
: QOpenGLWidget(parent), helper(helper)
{
    QImage img("E:\\pictures\\0151.jpg");
    image = img;
    image = image.convertToFormat(QImage::Format_RGBA8888);

    setFixedSize(512, 512);
    setAutoFillBackground(false);

    targetWidth = width();
    targetHeight = height();

    qDebug() << "targetWidth="<<targetWidth;
    qDebug() << "targetHeight ="<<targetHeight ;
//this values i am trying to pass to my fragment shader
    grid = 5.0;//grid on image

    dividerValue = 0.5;

    step_x = 0.0015625;
    step_y = height() ? (2.5 * step_x * targetWidth / targetHeight) : 0.0;
}

void GLWidget::initializeGL()
{
    initializeOpenGLFunctions();

    m_program = new QOpenGLShaderProgram;
    m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShader);
    m_program->addShaderFromSourceCode(QOpenGLShader::Fragment,fragmentShader);//?

    m_program->link();
    m_program->bind();
    m_program->release();
}
//we can use paintEvent to display our image with opengl
void GLWidget::paintEvent(QPaintEvent *event)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    m_program->bind();

    QPainter painter;
    painter.begin(this);
    painter.drawImage(0,0,image);

    QOpenGLTexture texture(image); //I dont know how to setUniformValue(m_program->uniformLocation("source"),texture) to my shader


    GLuint m_texture;
    glGenTextures(1, &m_texture);
    glBindTexture(GL_TEXTURE_2D, m_texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, image.width(), image.height(), 0, GL_BGRA, GL_UNSIGNED_BYTE, image.bits());
    glGenerateMipmap(GL_TEXTURE_2D);
    glEnable(GL_TEXTURE_2D);
  //open an image
   m_program->setUniformValue("grid", grid);
   m_program->setUniformValue("dividerValue",dividerValue);
   m_program->setUniformValue("step_x", step_x);
   m_program->setUniformValue("step_y", step_y);
   m_program->setUniformValue(m_program->uniformLocation("source"),m_texture);      
   painter.end();
   m_program->release();
}

Answer 1

When you bind a texture, it is bound to the currently active texture image unit (See Binding textures to samplers).

The active texture unit can be selected by glActiveTexture. The default texture unit is GL_TEXTURE0.

The value which you have to provide to the texture sampler uniform is not the name of a texture, it is the texture unit (number), where the texture is bound to:

int texture_unit = 0;                                 // <-----  e.g. texture unit 0
glActiveTexture( GL_TEXTURE0 + texture_unit );
glBindTexture( GL_TEXTURE_2D, m_texture );

.....

m_program->bind(); 
m_program->setUniformValue( "source", texture_unit ); // <----- texture unit

For a QOpenGLTexture object the texture unit can be selected by QOpenGLTexture::bind:

int texture_unit = 1;                                 // <-----  e.g. texture unit 1

QOpenGLTexture texture(image);
texture.bind( texture_unit );

m_program->bind(); 
m_program->setUniformValue( "source", texture_unit ); // <----- texture unit

Note, since OpenGL 4.2 the texture unit can be initialized within the shader, by a Binding point:

layout(binding = 0) uniform sampler2D source; // binding = 0 -> texture unit 0

Extension to the answer:

The following code will draw the a image to the entire widget with processing it by your shader. Finally the rendered image is read back from the GPU:

class GLWidget : public QOpenGLWidget
{
    .....

    QOpenGLShaderProgram * m_program = nullptr;
    QOpenGLTexture       * m_texture = nullptr;
};

void GLWidget::initializeGL()
{
    initializeOpenGLFunctions();

    QImage img("E:\\pictures\\0151.jpg");
    m_texture = new QOpenGLTexture( img );

    m_program = new QOpenGLShaderProgram;
    m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShader);
    m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShader);
    m_program->bindAttributeLocation("vPosition", 0);
    m_program->link();
}

void GLWidget::paintEvent(QPaintEvent *event)
{
    // celar the framebuffer
    glClearColor(0, 0, 0, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // bind the texture
    uint texture_unit = 1;
    m_texture->bind( texture_unit );

    // activate the shader
    m_program->bind();
    m_program->setUniformValue( "source", texture_unit );
    m_program->setUniformValue( "grid", grid );
    m_program->setUniformValue( "dividerValue", dividerValue );
    m_program->setUniformValue( "step_x", step_x );
    m_program->setUniformValue( "step_y", step_y );

    // draw a quad over the entire widget
    GLfloat vertices[]{ -1.0f, -1.0f, 1.0f, -1.0f, -1.0f,  1.0f, 1.0f, 1.0f };
    m_program->enableAttributeArray(0);
    m_program->setAttributeArray(0, GL_FLOAT, vertices, 2);
    glDrawArrays( GL_TRIANGLE_STRIP, 0, 4 );
    m_program->disableAttributeArray(0);

    // release the shader
    m_program->release();

    // read the rendered image
    int width  = ....;
    int height = ....;
    unsigned char *pixels = new unsigned char[width * height * 4];
    glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

    QImage *img = new QImage( pixels, width, height, QImage::Format_RGBA8888 );

    .....
}

Further you have to do some changes to the vertex shader and fragment shader. In the vertex shader you have to pass the vertex position to the fragment shader:

attribute vec2 vPosition;
varying vec2 v_pos;

void main()
{
    v_pos = vPosition.xy;
    gl_Position = vec4(vPosition.xy, 0.0, 1.0);
}

In the fragment shader you have to calcualte the texute coordinate fromt the vertex position:

varying vec2 v_pos;

void main()
{
    vec2 uv = v_pos.xy * 0.5 + 0.5;

    ....
}

See also glwidget.cpp Example File.