Basic OpenGL (python)

Question

I'm looking into opengl and python and have created a basic program to draw a rectangle using two triangles.

def draw(self, shader):
    shader.bind()
    #glDrawArrays(GL_TRIANGLES, 1, 3)
    glDrawElements(GL_TRIANGLES,
                   len(self.indices),
                   GL_UNSIGNED_INT,
                   0)
    glBindVertexArray(0)
    shader.unbind()

def _setupMesh(self):
    # VAO
    glGenVertexArrays(1, self._VAO)
    glBindVertexArray(self._VAO)

    #VBO
    glGenBuffers(1, self._VBO)
    glBindBuffer(GL_ARRAY_BUFFER, self._VBO)
    self.vertices_size = (GLfloat * len(self.vertices))(*self.vertices)
    glBufferData(GL_ARRAY_BUFFER,
                 len(self.vertices)*sizeof(GLfloat),
                 self.vertices_size,
                 GL_STATIC_DRAW)


    #EBO
    glGenBuffers(1, self._EBO)
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, self._EBO)
    self.indices_size = (GLfloat * len(self.indices))(*self.indices)
    glBufferData(GL_ELEMENT_ARRAY_BUFFER,
                 len(self.indices)*sizeof(GLfloat),
                 self.indices_size,
                 GL_STATIC_DRAW)

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), c_void_p(0))
    glEnableVertexAttribArray(0)

I send in data as follows:

vertices: [  0.5,  0.5,  0.0,
         0.5, -0.5,  0.0,
        -0.5, -0.5,  0.0,
        -0.5,  0.5,  0.0] 
indices: [0,1,3,1,2,3]

The glDrawElements call does nothing, glDrawArrays paints a nice triangle in the window.

Somthing obvious I suspect why the glDrawElements doesn't work?

Answer 1

The data type of the indices has to be GL_UNSIGNED_BYTE, GL_UNSIGNED_SHORT, or GL_UNSIGNED_INT. (see glDrawElements)

To create a proper array of values with data type unsigned int you can either use array:

from array import array

indAr = array("I", self.indices)
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indAr.tostring(), GL_STATIC_DRAW)

Or you can use numpy.array:

import numpy

numIndAr = numpy.array(self.indices, dtype='uint')
glBufferData(GL_ELEMENT_ARRAY_BUFFER, numIndAr, GL_STATIC_DRAW)

If a vertex array object is used, then the 4th parameter of glDrawElements has to be None and not 0:

glDrawElements(GL_TRIANGLES, len(self.indices), GL_UNSIGNED_INT, None)