How to set primitives to GL_LINES with moderngl in python

Question

I am using the following code to render a rectangle on screen, using moderngl and moderngl_window. This is mostly derived from their examples:

import moderngl
import moderngl_window as mglw
import glfw

import numpy as np

import OpenGL.GL
import OpenGL.GLUT
import OpenGL.GLU


class MyWin(mglw.WindowConfig):
    gl_version = (3, 3)
    title = "Hello World"
    window_size = (1280, 720)
    aspect_ratio = 16/9;
    resizable = True
    samples = 4


    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self.prog = self.ctx.program(
                vertex_shader='''
                   #version 330
                    in vec2 vert;
                    void main() {
                            gl_Position = vec4(vert.x, vert.y*1.7777, 0.0, 1.0);
                    }
                ''',
                fragment_shader='''
                    #version 330
                    out vec4 color;
                    void main() {
                            color = vec4(0.3, 0.5, 1.0, 1.0);
                    }
                ''',
            )

        self.vertices = np.array([
            -0.8, 0.8,
            -0.8, -0.8,
            0.8, -0.8,
            
            0.8, 0.8,
            -0.8, 0.8,
            0.8,-0.8
        ])



        self.vbo = self.ctx.buffer(self.vertices.astype('f4').tobytes())
        # self.texCoordBuffer = self.ctx.buffer(texCoord.astype('f4').tobytes());
        self.vao = self.ctx.simple_vertex_array(self.prog, self.vbo, 'vert');
        # self.vao.bind(1,'f',self.texCoordBuffer,'2f');
        # self.time = self.prog['time'];

        
      
    @classmethod
    def setVertices(self, vertices):
        print("vertices have been set")
        self.vertices = vertices
        

    @classmethod
    def run(cls):       
        mglw.run_window_config(cls);

    def render(self, time, frame_time):         
        # self.time.value = time
        self.ctx.clear(0.0, 0.0, 0.0)
        self.vao.render()

but I can't figure out what function I need to call with moderngl to set the rendering mode to GL_LINES instead of GL_TRIANGLES

I think I need to call glDrawArraysbut I can't find how to access it with moderngl.

Thanks for your help!

Answer 1

You can choose the Primitive type by setting the mode argument when you invoke moderngl.VertexArray.render(). The default argument is TRIANGLES. Set the mode LINES for the primitive type GL_LINES:

self.vao.render()

self.vao.render(moderngl.LINES)