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Thread: Qt OpenGL (QOpenGLWidget) - Simple Triangle

  1. #1
    Join Date
    Jul 2016
    Posts
    1
    Qt products
    Qt5
    Platforms
    Unix/X11 Windows

    Default Qt OpenGL (QOpenGLWidget) - Simple Triangle

    As a newbee to QT+OpenGL using QOpenGLWidget, I am unable to color my triangle. Please find my code here, using QMainWindow for GUI ...

    Qt Code:
    1. // main.cpp
    2. #include "mainwindow.h"
    3. #include <QApplication>
    4.  
    5. int main(int argc, char *argv[])
    6. {
    7. QApplication a(argc, argv);
    8. MainWindow w;
    9. w.show();
    10. return a.exec();
    11. }
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    Here the GUI - window

    Qt Code:
    1. // MainWindow.h
    2. #ifndef MAINWINDOW_H
    3. #define MAINWINDOW_H
    4.  
    5. #include <QMainWindow>
    6.  
    7. namespace Ui {
    8. class MainWindow;
    9. }
    10.  
    11. class MainWindow : public QMainWindow
    12. {
    13. Q_OBJECT
    14.  
    15. public:
    16. explicit MainWindow(QWidget *parent = 0);
    17. ~MainWindow();
    18.  
    19. private:
    20. Ui::MainWindow *ui;
    21. };
    22.  
    23. #endif // MAINWINDOW_H
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    Here implementation - file ...

    Qt Code:
    1. #include "mainwindow.h"
    2. #include "ui_mainwindow.h"
    3.  
    4. MainWindow::MainWindow(QWidget *parent) :
    5. QMainWindow(parent),
    6. ui(new Ui::MainWindow)
    7. {
    8. ui->setupUi(this);
    9. }
    10.  
    11. MainWindow::~MainWindow()
    12. {
    13. delete ui;
    14. }
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    Here is the widget rendering Opengl-Context.

    Qt Code:
    1. #ifndef OPENGLWIDGET_H
    2. #define OPENGLWIDGET_H
    3.  
    4. #include <QWidget>
    5. #include <QOpenGLWidget>
    6. #include <QOpenGLFunctions>
    7. #include <QOpenGLContext>
    8. #include <QOpenGLShaderProgram>
    9. #include <QOpenGLBuffer>
    10. #include <QOpenGLVertexArrayObject>
    11. #include <QMatrix4x4>
    12.  
    13. class OpenglWidget : public QOpenGLWidget, public QOpenGLFunctions
    14. {
    15. public:
    16. OpenglWidget(QWidget *parent = 0);
    17. ~OpenglWidget();
    18.  
    19. protected:
    20. void initializeGL();
    21. void resizeGL(int width, int height);
    22. void paintGL();
    23. GLuint m_posAttr;
    24. GLuint m_colAttr;
    25. GLuint m_matrixUniform;
    26. QOpenGLShaderProgram *m_program;
    27. };
    28. #endif // OPENGLWIDGET_H
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    Here is the implementation file ...

    Qt Code:
    1. #include "openglwidget.h"
    2. OpenglWidget::OpenglWidget(QWidget *parent) :
    3. QOpenGLWidget(parent)
    4. {
    5. setFormat(QSurfaceFormat::defaultFormat());
    6. }
    7.  
    8. OpenglWidget::~OpenglWidget()
    9. {
    10. }
    11.  
    12. static const char *vertexShaderSource =
    13. "attribute highp vec4 posAttr;\n"
    14. "attribute lowp vec4 colAttr;\n"
    15. "varying lowp vec4 col;\n"
    16. "uniform highp mat4 matrix;\n"
    17. "void main() {\n"
    18. " col = vec4(1, 0, 0, 1);\n"
    19. " gl_Position = matrix * posAttr;\n"
    20. "}\n";
    21.  
    22. static const char *fragmentShaderSource =
    23. "varying lowp vec4 col;\n"
    24. "void main() {\n"
    25. " gl_FragColor = col;\n"
    26. "}\n";
    27.  
    28. void OpenglWidget::initializeGL()
    29. {
    30. makeCurrent();
    31. initializeOpenGLFunctions();
    32. glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
    33.  
    34. // Create Shader (Do not release until VAO is created)
    35. m_program = new QOpenGLShaderProgram(this);
    36. m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
    37. m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
    38. m_program->link();
    39. m_posAttr = m_program->attributeLocation("posAttr");
    40. m_colAttr = m_program->attributeLocation("colAttr");
    41. m_matrixUniform = m_program->attributeLocation("matrix");
    42.  
    43. m_program->release();
    44. }
    45.  
    46. void OpenglWidget::paintGL()
    47. {
    48. glClear(GL_COLOR_BUFFER_BIT);
    49. makeCurrent();
    50.  
    51. //m_program->bind();
    52.  
    53. QMatrix4x4 matrix;
    54. matrix.perspective(60.0f, 4.0f/3.0f, 0.1f, 100.0f);
    55. matrix.translate(0, 0, -2);
    56.  
    57. m_program->setUniformValue(m_matrixUniform, matrix);
    58.  
    59. GLfloat vertices[] = {
    60. 0.0f, 0.707f, 0.0f,
    61. -0.5f, -0.5f, 0.0f,
    62. 0.5f, -0.5f, 0.0f
    63. };
    64.  
    65. GLfloat colors[] = {
    66. 1.0f, 0.0f, 0.0f,
    67. 0.0f, 1.0f, 0.0f,
    68. 0.0f, 0.0f, 1.0f
    69. };
    70.  
    71. glVertexAttribPointer(m_posAttr, 3, GL_FLOAT, GL_FALSE, 0, vertices);
    72. glVertexAttribPointer(m_colAttr, 3, GL_FLOAT, GL_FALSE, 0, colors);
    73.  
    74. glEnableVertexAttribArray(0);
    75. glEnableVertexAttribArray(1);
    76.  
    77. glDrawArrays(GL_TRIANGLES, 0, 3);
    78.  
    79. glDisableVertexAttribArray(1);
    80. glDisableVertexAttribArray(0);
    81.  
    82. m_program->release();
    83. }
    84.  
    85. void OpenglWidget::resizeGL(int width, int height)
    86. {
    87. glViewport(0, 0, width, height);
    88. }
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    Here the rendered triangle is just white. I am unable to understand whether the shader is compiled and attributes are linked but still, I am unable to color the triangle.

    If I can get any guidance... ?

    test_triangle.jpg
    Last edited by saket; 31st July 2016 at 17:53. Reason: updated contents

  2. #2
    Join Date
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    Default Re: Qt OpenGL (QOpenGLWidget) - Simple Triangle

    In line 43 of the "implementation file", you release() the shader program. Why? And you've commented out the call the bind(). Why? In line 83, you again release() the shader program (although that call does nothing since you never bind the shader to the rendering pipeline in the first place.

    I think you should study the Cube example program.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

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