Re: How to draw a cube using indices ?

You've already bound your index buffer so you don't need to give it as argument.
In your display function:

gl.glDrawElements(gl.GL_TRIANGLES, len(index), gl.GL_UNSIGNED_INT, index)

becomes

gl.glDrawElements(gl.GL_TRIANGLES, len(index), gl.GL_UNSIGNED_INT, None)



Nicolas


> On 27 Nov 2015, at 14:57, Florian NICOLAS <[email protected]> wrote:
> 
> 
> Hi everybody!
> 
> I recently started to learn OpenGL through Python thanks to several tutorial (especially the Nicolas P. Rougier one: http://www.labri.fr/perso/nrougier/teaching/opengl/). 
> 
> I am now switching to 3D and I am trying to draw a cube.
> 
> Thus, I manage to get some triangles which do not render a cube (this seems to be normal as I do not duplicate my vertices and I use the glDrawArrays function). 
> 
> However, after, I build an index "vector" to further use the glDrawElements function to render my cube. As a result, I do not get any error but nothing appears on screen.
> 
> I hope you could be of some help!
> 
> Thanks.
> 
> Here is my code:
> 
> #! /usr/bin/env python
> # -*- coding: utf-8 -*-
> 
> import sys
> import ctypes
> import numpy as np
> import OpenGL.GL as gl
> import OpenGL.GLUT as glut
> 
> vertex_code = """
> 
>     uniform float scale;
>     uniform mat4 matCam;
>     attribute vec4 color;
>     attribute vec3 position;
>     varying vec4 v_color;
>     void main()
>     {
>         gl_Position = matCam*vec4(scale*position, 1.0);
>         v_color = color;
>     } """
> 
> fragment_code = """
>     varying vec4 v_color;
>     void main()
>     {
>         gl_FragColor = v_color;
>     } """
> 
> def display():
>     gl.glClear(gl.GL_COLOR_BUFFER_BIT)
>     #gl.glDrawArrays(gl.GL_TRIANGLES, 0, 12)
>     gl.glDrawElements(gl.GL_TRIANGLES, len(index), gl.GL_UNSIGNED_INT, index) # render nothing (i.e. only the background color)
>     glut.glutSwapBuffers()
> 
> def reshape(width,height):
>     gl.glViewport(0, 0, width, height)
> 
> def keyboard( key, x, y ):
>     if key == '\033':
>         sys.exit( )
>         
> def timer(fps):
>     global clock
>     clock += 0.0005*1000.0/fps
>     print(clock)
> #    eye = np.array([0,0,1])
> #    center = np.array([0,clock,0])
> #    up = np.array([0,1,0])
> #    mat = computeLookAtMatrix(eye, center, up)
>     theta = clock;
>     mat = np.array([[np.cos(theta), 0, np.sin(theta), 0],
>                 [0, 1, 0, 0],
>                 [-np.sin(theta), 0, np.cos(theta), 0],
>                 [0, 0, 0, 1]])
>     loc = gl.glGetUniformLocation(program, "matCam")
>     gl.glUniformMatrix4fv(loc, 1, False, mat)
>                  
>     
>     
>     glut.glutTimerFunc(1000/fps, timer, fps)
>     glut.glutPostRedisplay()
> 
> 
> # GLUT init
> # --------------------------------------
> glut.glutInit()
> glut.glutInitDisplayMode(glut.GLUT_DOUBLE | glut.GLUT_RGBA)
> glut.glutCreateWindow('Hello world!')
> glut.glutReshapeWindow(512,512)
> glut.glutReshapeFunc(reshape)
> glut.glutDisplayFunc(display)
> glut.glutKeyboardFunc(keyboard)
> glut.glutTimerFunc(1000/60, timer, 60)
> 
> # Build data
> # --------------------------------------
> data = np.zeros(8, [("position", np.float32, 3),
>                     ("color",    np.float32, 4)])
>                     
> data['color']    = [ (1,0,0,1), (0,1,0,1), (0,0,1,1), (1,1,0,1),
>                     (1,0,0,1), (0,1,0,1), (0,0,1,1), (1,1,0,1) ]
>                     
> data['position'] = [ (-1,-1,1),
>                      (1,-1,1),
>                         (1,1,1),   
>                         (-1,1,1),
>                         (-1,-1,-1),
>                         (1,-1,-1),
>                         (1,1,-1),
>                         (-1,1,-1)]
>                         
> index = np.array([0,1,2,
>                 2,3,0,
>                 1,5,6,
>                 6,2,1,
>                 7,6,5,
>                 5,4,7,
>                 4,0,3,
>                 3,7,4,
>                 4,5,1,
>                 1,0,4,
>                 3,2,6,
>                 6,7,3])
> 
> # Build & activate program
> # --------------------------------------
> 
> # Request a program and shader slots from GPU
> program  = gl.glCreateProgram()
> vertex   = gl.glCreateShader(gl.GL_VERTEX_SHADER)
> fragment = gl.glCreateShader(gl.GL_FRAGMENT_SHADER)
> 
> # Set shaders source
> gl.glShaderSource(vertex, vertex_code)
> gl.glShaderSource(fragment, fragment_code)
> 
> # Compile shaders
> gl.glCompileShader(vertex)
> gl.glCompileShader(fragment)
> 
> # Attach shader objects to the program
> gl.glAttachShader(program, vertex)
> gl.glAttachShader(program, fragment)
> 
> # Build program
> gl.glLinkProgram(program)
> 
> # Get rid of shaders (no more needed)
> gl.glDetachShader(program, vertex)
> gl.glDetachShader(program, fragment)
> 
> # Make program the default program
> gl.glUseProgram(program)
> 
> 
> # Build buffer
> # --------------------------------------
> 
> # Request a buffer slot from GPU
> buffer = gl.glGenBuffers(1)
> 
> # Make this buffer the default one
> gl.glBindBuffer(gl.GL_ARRAY_BUFFER, buffer)
> 
> # Upload data
> gl.glBufferData(gl.GL_ARRAY_BUFFER, data.nbytes, data, gl.GL_DYNAMIC_DRAW)
> 
> # same for index buffer
> buffer_index= gl.glGenBuffers(1)
> gl.glBindBuffer(gl.GL_ELEMENT_ARRAY_BUFFER, buffer_index)
> gl.glBufferData(gl.GL_ELEMENT_ARRAY_BUFFER, index.nbytes, index, gl.GL_STATIC_DRAW)
> 
> 
> # Bind attributes
> # --------------------------------------
> stride = data.strides[0]
> offset = ctypes.c_void_p(0)
> loc = gl.glGetAttribLocation(program, "position")
> gl.glEnableVertexAttribArray(loc)
> gl.glBindBuffer(gl.GL_ARRAY_BUFFER, buffer)
> gl.glVertexAttribPointer(loc, 3, gl.GL_FLOAT, False, stride, offset)
> 
> offset = ctypes.c_void_p(data.dtype["position"].itemsize)
> loc = gl.glGetAttribLocation(program, "color")
> gl.glEnableVertexAttribArray(loc)
> gl.glBindBuffer(gl.GL_ARRAY_BUFFER, buffer)
> gl.glVertexAttribPointer(loc, 4, gl.GL_FLOAT, False, stride, offset)
> 
> gl.glBindBuffer(gl.GL_ELEMENT_ARRAY_BUFFER, buffer_index)
> 
> # Bind uniforms
> # --------------------------------------
> loc = gl.glGetUniformLocation(program, "scale")
> gl.glUniform1f(loc, 0.5)
> clock = 0
> 
> loc = gl.glGetUniformLocation(program, "matCam")
> print(loc)
> gl.glUniformMatrix4fv(loc, 1, False, np.eye(4))
> 
> # Enter mainloop
> # --------------------------------------
> glut.glutMainLoop()
> ------------------------------------------------------------------------------
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