CODEWITHSUNDEEP
openglqt5.4
user3724853
user3724853

Reputation: 567

glViewport didn't work on Qt5.4 QOpenGLWidget::resizeGL

I put glViewport in QOpenGLWidget::resizeGL overwritted virtual function, which did get called and set viewport only use part of widget space. But it didn't have any effect, content still draw to full-size of widget. Did I missing anything?

Here is my code,

mywidget.h:

class MyWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
    Q_OBJECT

public:
    explicit MyWidget(QWidget* parent = NULL);

protected:
    virtual void initializeGL();
    virtual void paintGL();
    virtual void resizeGL(int w, int h);

private:
    QOpenGLShaderProgram program;
};

and mywidget.cpp:

MyWidget::MyWidget(QWidget* parent) : QOpenGLWidget(parent)
{
}

static const GLfloat squareVertices[] = {
    -1.0f, -1.0f,
     1.0f, -1.0f,
    -1.0f,  1.0f
};

void MyWidget::initializeGL()
{
    initializeOpenGLFunctions();
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

    program.addShaderFromSourceCode(QOpenGLShader::Vertex,
        "attribute highp vec4 vec;\n"
        "void main(void)\n"
        "{\n"
        "    gl_Position = vec;\n"
        "}");
    program.addShaderFromSourceCode(QOpenGLShader::Fragment,
        "void main(void)\n"
        "{\n"
        "    gl_FragColor = vec4(0.5, 0.5, 0.5, 1.0);\n"
        "}");
    program.link();
    program.bind();

    program.enableAttributeArray(0);
    program.setAttributeArray(0, squareVertices, 2);
}

void MyWidget::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);

    //glViewport(0, 0, width()/2, height()/2);
    // if I put glViewport here, everything work as intended,
    // but we shouldn't need to set viewport everytime render a frame

    program.bind();
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);
}

void MyWidget::resizeGL(int w, int h)
{
    glViewport(0, 0, w/2, h/2);
    // this line didn't have any effect, opengl still draw using full widget space
}

Upvotes: 8

Views: 1746

Answers (2)

datenwolf
datenwolf

Reputation: 162184

glViewport should be called in the drawing code, i.e. the paintGL handler, not somewhere else. Having the glViewport call in the window reshape handler is a bad anti-pattern that's just too far spread in OpenGL tutorials; it doesn't belong there.

Upvotes: 3

Milan Dhameliya
Milan Dhameliya

Reputation: 129

Try following,

void MyWidget::resizeGL(int w, int h)
{
    glViewport(0, 0, w, h);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    GLdouble dWt;
    GLdouble dHt;

    if(w > h) {
        dHt = 1;
        dWt = ((double)w) / h;
    } else {
        dHt = ((double)h) / w;
        dWt = 1;
    }

    if(bPerspective) {
        glFrustum(-dWt, dWt, -dHt, dHt, 5, 100);
    } else {
        glOrtho(-dWt, dWt, -dHt, dHt, 5.0, 100.0);
    }

    gluLookAt(5.0, 2.0, -10.0,
              0.0, 0.0, 0.0,
              0.0, 1.0, 0.0);

    glMatrixMode(GL_MODELVIEW);
}

Upvotes: 0

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