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;
}