Problem to render with VAO

OpenGL OpenGL: Basic Coding
1

Hi
I have a problem to render with a VAO. I want to set up a VAO and render with calls of glBindVertexArray() and glDrawArrays(), but I have to call glEnableVertexAttribArray() and glVertexAttribPointer() everytime I bind the VAO.
I wanted to set up a VAO in a class named grid and bind a VBO in a function, where I calculate vertex-data, like seen below.


glGenVertexArrays(1, &VAO);
glGenBuffers(1, &VBO);
glBindVertexArray(VAO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, FLOATS_PER_VERTEX*numVertices*sizeof(GLfloat), vertices, GL_STATIC_DRAW);
glEnableVertexAttribArray(gridShader::loc_pos);
glEnableVertexAttribArray(gridShader::loc_col);
glVertexAttribPointer(gridShader::loc_pos, 3, GL_FLOAT, GL_FALSE, FLOATS_PER_VERTEX*sizeof(GLfloat), (GLvoid*)0);
glVertexAttribPointer(gridShader::loc_col, 3, GL_FLOAT, GL_FALSE, FLOATS_PER_VERTEX*sizeof(GLfloat), (GLvoid*)(3*sizeof(GLfloat)));
glBindVertexArray(0);

In paintGL() I just wan’t to call glBindVertexArray() but nothing is rendered.


viewpointMatrix.setColumn(3, -viewpoint+QVector3D(0,0,zoom));
glBindVertexArray(grid->VAO);
gridShader::shaderProg.bind();
gridShader::shaderProg.setUniformValue(gridShader::loc_projMatrix, projectionMatrix*viewpointMatrix);
glLineWidth(1);
glDrawArrays(GL_LINES, 0, grid->numVertices);
glBindVertexArray(0);

If all functions are called again in paintGL() everything works fine, but I think thats not the purpose of a VAO.


viewpointMatrix.setColumn(3, -viewpoint+QVector3D(0,0,zoom));
glBindVertexArray(grid->VAO);
gridShader::shaderProg.bind();
gridShader::shaderProg.setUniformValue(gridShader::loc_projMatrix, projectionMatrix*viewpointMatrix);
glBindBuffer(GL_ARRAY_BUFFER, grid->VBO);
glEnableVertexAttribArray(gridShader::loc_pos);
glEnableVertexAttribArray(gridShader::loc_col);
glVertexAttribPointer(gridShader::loc_pos, 3, GL_FLOAT, GL_FALSE, FLOATS_PER_VERTEX*sizeof(GLfloat), (GLvoid*)0);
glVertexAttribPointer(gridShader::loc_col, 3, GL_FLOAT, GL_FALSE, FLOATS_PER_VERTEX*sizeof(GLfloat), (GLvoid*)(3*sizeof(GLfloat)));
glLineWidth(1);
glDrawArrays(GL_LINES, 0, grid->numVertices);
glBindVertexArray(0);

Hope that somebody can tell me what I’m doing wrong.
Thanks

2

There’s nothing wrong with the code which you posted, so the problem lies elsewhere.

Check for OpenGL errors, check whether the object is being re-initialised, check that the context which is bound when you create the VAO is the same one which is bound when you try to use it.

3

The problem was the current context, but I still have a problem when trying to run the program on linux.
On windows I can call makeCurrent() in the function where I set up the VAO. The context now is the same like in initializeGL() and the program is working.


void GLwidget::createGrid(GLuint size, GLfloat *vertices)
{
  makeCurrent();
  qDebug()<< QOpenGLContext::currentContext();
  glBindVertexArray(gridVAO[0]);
  glBindBuffer(GL_ARRAY_BUFFER, gridVBO[0]);
  glBufferData(GL_ARRAY_BUFFER, size*sizeof(GLfloat), vertices, GL_STATIC_DRAW);
  glEnableVertexAttribArray(gridShader::loc_pos);
  glEnableVertexAttribArray(gridShader::loc_col);
  glVertexAttribPointer(gridShader::loc_pos, 3, GL_FLOAT, GL_FALSE, FLOATS_PER_GRID_VERTEX*sizeof(GLfloat), (GLvoid*)0);
  glVertexAttribPointer(gridShader::loc_col, 3, GL_FLOAT, GL_FALSE, FLOATS_PER_GRID_VERTEX*sizeof(GLfloat), (GLvoid*)(3*sizeof(GLfloat)));
  glLineWidth(1);
  glBindBuffer(GL_ARRAY_BUFFER, 0);
  glBindVertexArray(0);
}

When I try to run it on Linux, makeCurrent() doesn’t set the context to GLwidget. Do you know if I have to use another function?

4

[QUOTE=Dartagnan;1285137]
When I try to run it on Linux, makeCurrent() doesn’t set the context to GLwidget. Do you know if I have to use another function?[/QUOTE]
makeCurrent() should have been called by Qt prior to calling initializeGL() or paintGL().

If you aren’t using those methods, note that you can’t call makeCurrent() before the widget has been realized.