如何在PyOpenGL中使用顶点数组对象和glDrawElements进行绘制

Z时代
2024-01-10
分类：问答

我有以下代码，应该在屏幕上简单地绘制一个绿色三角形。它使用顶点数组对象和索引缓冲区进行绘制，并且具有我可以制作的最简单的着色器。

最初，我没有使用索引缓冲区，只是简单地进行了绘画调用，glDrawArrays但工作正常，但是当我将其更改为使用时，glDrawElements则屏幕上没有任何内容绘制出来（它完全是黑色的）。

from OpenGL.GL import shaders
from OpenGL.arrays import vbo
from OpenGL.GL import *
from OpenGL.raw.GL.ARB.vertex_array_object import glGenVertexArrays, \
                                                  glBindVertexArray
import pygame
import numpy as np
def run():
    pygame.init()
    screen = pygame.display.set_mode((800,600), pygame.OPENGL)
    #Create the Vertex Array Object
    vertexArrayObject = GLuint(0)
    glGenVertexArrays(1, vertexArrayObject)
    glBindVertexArray(vertexArrayObject)
    #Create the VBO
    vertices = np.array([[0,1,0],[-1,-1,0],[1,-1,0]], dtype='f')
    vertexPositions = vbo.VBO(vertices)
    #Create the index buffer object
    indices = np.array([0,1,2], dtype='uint16')
    indexPositions = vbo.VBO(indices, target=GL_ELEMENT_ARRAY_BUFFER)
    indexPositions.bind()
    vertexPositions.bind()
    glEnableVertexAttribArray(0) # from 'location = 0' in shader
    glVertexAttribPointer(0, 3, GL_FLOAT, False, 0, None)
    glBindVertexArray(0)
    vertexPositions.unbind()
    indexPositions.unbind()
    #Now create the shaders
    VERTEX_SHADER = shaders.compileShader("""
    #version 330
    layout(location = 0) in vec4 position;
    void main()
    {
        gl_Position = position;
    }
    """, GL_VERTEX_SHADER)
    FRAGMENT_SHADER = shaders.compileShader("""
    #version 330
    out vec4 outputColor;
    void main()
    {
        outputColor = vec4(0.0f, 1.0f, 0.0f, 1.0f);
    }
    """, GL_FRAGMENT_SHADER)
    shader = shaders.compileProgram(VERTEX_SHADER, FRAGMENT_SHADER)
    #The draw loop
    while True:
        glUseProgram(shader)
        glBindVertexArray(vertexArrayObject)
        #glDrawArrays(GL_TRIANGLES, 0, 3) #This line works
        glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, 0) #This line does not
        glBindVertexArray(0)
        glUseProgram(0)
        # Show the screen
        pygame.display.flip()
run()

如果我只是glDrawElements注释掉该glDrawArrays行并取消注释该行，则它可以正常工作，因此至少可以正确输入顶点VBO。

我在这里做错了什么？我能够找到的所有OpenGL文档都表明我正确地执行了此操作，因此我显然对OpenGL本身或PyOpenGL包装器有误解。

更改VAO设置以使用更直接的OpenGL函数，而不是VBO包装器，例如：

vertices = np.array([[0,1,0],[-1,-1,0],[1,-1,0]], dtype='f')
vertexPositions = glGenBuffers(1)
glBindBuffer(GL_ARRAY_BUFFER, vertexPositions)
glBufferData(GL_ARRAY_BUFFER, vertices, GL_STATIC_DRAW)
#Create the index buffer object
indices = np.array([0,1,2], dtype='uint16')
indexPositions = glGenBuffers(1)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexPositions)
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices, GL_STATIC_DRAW)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexPositions)
glBindBuffer(GL_ARRAY_BUFFER, vertexPositions)
glEnableVertexAttribArray(0) # from 'location = 0' in shader
glVertexAttribPointer(0, 3, GL_FLOAT, False, 0, None)
glBindVertexArray(0)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0)
glBindBuffer(GL_ARRAY_BUFFER, 0)

没有区别。glDrawArrays仍然有效，glDrawElements但仍然没有。

回答：

谢谢@NicolBolas。他激励我真正采用此代码并使之生效。而不是理论化：）我删除了vertexArrayObject（它是多余的，因为我们已经有用于顶点和索引的VBO）。因此，您只需在调用glDraw

*之前绑定索引和顶点缓冲区（以及属性）即可。当然，将None（空指针）传递给glDrawElements索引而不是0！非常重要。

from OpenGL.GL import shaders
from OpenGL.arrays import vbo
from OpenGL.GL import *
from OpenGL.raw.GL.ARB.vertex_array_object import glGenVertexArrays, \
                                                  glBindVertexArray
import pygame
import numpy as np
def run():
    pygame.init()
    screen = pygame.display.set_mode((800,600), pygame.OPENGL|pygame.DOUBLEBUF)
    #Create the VBO
    vertices = np.array([[0,1,0],[-1,-1,0],[1,-1,0]], dtype='f')
    vertexPositions = vbo.VBO(vertices)
    #Create the index buffer object
    indices = np.array([[0,1,2]], dtype=np.int32)
    indexPositions = vbo.VBO(indices, target=GL_ELEMENT_ARRAY_BUFFER)
    #Now create the shaders
    VERTEX_SHADER = shaders.compileShader("""
    #version 330
    layout(location = 0) in vec4 position;
    void main()
    {
        gl_Position = position;
    }
    """, GL_VERTEX_SHADER)
    FRAGMENT_SHADER = shaders.compileShader("""
    #version 330
    out vec4 outputColor;
    void main()
    {
        outputColor = vec4(0.0f, 1.0f, 0.0f, 1.0f);
    }
    """, GL_FRAGMENT_SHADER)
    shader = shaders.compileProgram(VERTEX_SHADER, FRAGMENT_SHADER)
    #The draw loop
    while True:
        glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)
        glUseProgram(shader)
        indexPositions.bind()
        vertexPositions.bind()
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 3, GL_FLOAT, False, 0, None)
        #glDrawArrays(GL_TRIANGLES, 0, 3) #This line still works
        glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, None) #This line does work too!
        # Show the screen
        pygame.display.flip()
run()