Can't Get Qt OpenGL to Draw

[Andaharoo]

I've been trying to use OpenGL in Qt for the past few hours. I can't seem to figure out why the heck I can't get anything to draw after modifying one of the examples. Here's the shaders for starters. Very simple. Note, I've started trying stupid things...

in vec3 pos;
void main()
{
    gl_Position = vec4(pos, 1.0);
}

out vec4 fColor;
void main()
{
    fColor = vec4(1.0, 0.0, 0.0, 1.0);
}

I can confirm the shaders are setup fine. No errors. Here's where I create a vertex buffer,

arrayBuf = new QOpenGLBuffer(QOpenGLBuffer::VertexBuffer);
arrayBuf->setUsagePattern(QOpenGLBuffer::StaticDraw);
arrayBuf->create();

GLfloat vertices[6][3] =
{
    { -0.90f, -0.90f, -1.0f }, // Triangle 1
    { 0.85f, -0.90f, -1.0f },
    { -0.90f, 0.85f, -1.0f },

    { 0.90f, -0.85f, -1.0f }, // Triangle 2
    { 0.90f, 0.90f, -1.0f },
    { -0.85f, 0.90f, -1.0f }
};

arrayBuf->bind();
arrayBuf->allocate(vertices, sizeof(vertices));

Then to draw

glClearColor(0.4f, 0.58f, 0.9f, 1.0f);

program.bind();
arrayBuf->bind();

int vertexLocation = program->attributeLocation("pos");
program->setAttributeBuffer(vertexLocation, GL_FLOAT, 0, 3, sizeof(GLfloat) * 3);
program->enableAttributeArray(vertexLocation);

glDrawArrays(GL_TRIANGLES, 0, 6);

program->disableAttributeArray(vertexLocation);
arrayBuf->release();
program.release();

I got a GL_INVALID_OPERATION setAttributeBuffer. I'd do a more in depth debugging to try and see what's going on but for some reason neither vs graphics debugging or Nividia nsight is working with Qt OpenGL.

[SGaist]

Hi,

Which example did you modify ?

Did it run successfully before you started fiddling with it ?

Which version of Qt ?
On which platform ?

[Andaharoo]

Thanks, you motivated me to go back and draw more comparisons from the example. Just didn't want to start over and was so sure I was doing something wrong with the buffers but it turned out to be a problem with the OpenGL version I was asking it to use. On a side note do you have any idea why I can't use a std vector or qlist instead of the array shown above? If I replace GLfloat vertices[6][3]... and arrayBuf->allocate... with

std::vector<VertexData> data;
data.push_back(VertexData(-0.9f, -0.9f, -5.0f));
data.push_back(VertexData(-0.85f, -0.9f, -5.0f));
data.push_back(VertexData(-0.9f, -0.85f, -5.0f));

data.push_back(VertexData(-0.9f, -0.85f, -5.0f));
data.push_back(VertexData(-0.9f, -0.9f, -5.0f));
data.push_back(VertexData(-0.85f, -0.9f, -5.0f));
...
arrayBuf->allocate(&data[0], sizeof(VertexData) * data.size());

Then it doesn't work anymore. I've been able to do this with OpenGL in the past.

[jiancaiyang]

Hi have you tried Qt examples before? There are many Qt examples available that can help you manage rendering triangles correctly.

[SGaist]

Shouldn't you be using data.data for that ? See std::vector::data ?

[MartinChan 0]

2017.01.18 Update:
I draw a rectangle with texture correctly under QtOpenGL framework correctly,the source code u can get here,maybe u can enroll an account to get the code.

---

2018.01.17 Update:
I solve the question by using glDrawArrays rather than glDrawElements, I believe it must have some problems when I tried glDrawElements;

---

Origin:
@jiancaiyang I have encountered the same question when I try to draw a simple white angle,here is my GLWidget Code,my qt version is 5.6msvc.I intimate code from OpenGL OpenGLTutrioal

glwidget.h:

#ifndef GLWIDGET_H
#define GLWIDGET_H
#include <QOpenGLWidget>
#include <QOpenGLShaderProgram>
#include <QVector3D>
#include <QOpenGLFunctions>

class GLWidget : public QOpenGLWidget ,public QOpenGLFunctions
{
    Q_OBJECT

public:
    GLWidget(QWidget *parent = 0);
    ~GLWidget();

    QSize sizeHint() const;

protected:
    void initializeGL() Q_DECL_OVERRIDE;
    void resizeGL(int width,int height) Q_DECL_OVERRIDE;
    void paintGL();

private:
    QMatrix4x4 pMatrix;
    QOpenGLShaderProgram shaderProgram;
    QVector<QVector3D> vertices;

};

#endif // GLWIDGET_H

glwidget.cpp:

#include "glwidget.h"

GLWidget::GLWidget(QWidget *parent)
{

}

GLWidget::~GLWidget()
{

}

QSize GLWidget::sizeHint() const
{
    return QSize(640,480);
}

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

    glEnable(GL_DEPTH_TEST);
    glEnable(GL_CULL_FACE);

    glClearColor(0.0f,0.0f,0.0f,1.0f);

    bool ok;
    ok = shaderProgram.addShaderFromSourceFile(QOpenGLShader::Vertex,":/vertexShader.vsh");
    ok = shaderProgram.addShaderFromSourceFile(QOpenGLShader::Fragment,":/fragmentShader.fsh");
    ok = shaderProgram.link();


    vertices << QVector3D(1,0,-2) << QVector3D(0,1,-2) << QVector3D(-1,0,2);

}

void GLWidget::resizeGL(int width, int height)
{
    if ( height == 0 )
    {
        height = 1;
    }

    pMatrix.setToIdentity();
    pMatrix.perspective(60.0,(float)width / (float)height,0.001,1000);

    glViewport(0,0,width,height);

}

void GLWidget::paintGL()
{
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    QMatrix4x4 mMatrix;
    QMatrix4x4 vMatrix;

    bool ok;
    ok = shaderProgram.bind();

    shaderProgram.setUniformValue("mvpMatrix", pMatrix * vMatrix * mMatrix);

    shaderProgram.setUniformValue("color",QColor(Qt::white));

    shaderProgram.setAttributeArray("vertex", vertices.constData());
    shaderProgram.enableAttributeArray("vertex");

    glDrawArrays(GL_TRIANGLES, 0, vertices.size());

    shaderProgram.disableAttributeArray("vertex");
    shaderProgram.release();
}

vertexShader.vsh:

uniform mat4 mvpMatrix;

in vec4 vertex;

void main(void)
{
    gl_Position = mvpMatrix * vertex;
}

fragmentShader.fsh:

uniform vec4 color;

out vec4 fragColor;

void main(void)
{
    fragColor = color;
}