OpenGL VBO VAO EBO can run without error but no graphics

Question

This is my code:

block_VAO=0
draw=False
block_EBO_buffer_len=0
def print_blocks(x:int,y:int,z:int):
    global draw,block_VAO,block_EBO_buffer_len
    if not draw:
        block_point_buffer=[]
        block_color_buffer=[]
        block_EBO_buffer=[]
        block_point_buffer+=[x-0.5,y+0.5,z-0.5,#V0
                            x+0.5,y+0.5,z-0.5,#V1
                            x+0.5,y-0.5,z-0.5,#V2
                            x-0.5,y-0.5,z-0.5,#V3
                            x-0.5,y+0.5,z+0.5,#V4
                            x+0.5,y+0.5,z+0.5,#V5
                            x+0.5,y-0.5,z+0.5,#V6
                            x-0.5,y-0.5,z+0.5]#V7
        block_EBO_buffer+=[0,1,5,4,
                          3,2,6,7,
                          0,3,7,4,
                          1,2,6,5,
                          0,1,2,3,
                          4,5,6,7]

        #ADD
        block_color_buffer+=[1.0,0.0,1.0,1.0]*24
        color_EBO_buffer+=[0]*24
        #ADD END

        block_VBO=glGenBuffers(1)
        glBindBuffer(GL_ARRAY_BUFFER,block_VBO)
        a=numpy.array(block_point_buffer,dtype='float32')
        glBufferData(GL_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)

        block_EBO=glGenBuffers(1)
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,block_EBO)
        a=numpy.array(block_EBO_buffer,dtype='uint32')
        glBufferData(GL_ELEMENT_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)
        block_EBO_buffer_len=len(a)

        #ADD
        color_VBO=glGenBuffers(1)
        glBindBuffer(GL_ARRAY_BUFFER,color_VBO)
        a=numpy.array(block_color_buffer,dtype='uint32')
        glBufferData(GL_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)

        color_EBO=glGenBuffers(1)
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,color_EBO)
        a=numpy.array(color_EBO_buffer,dtype='uint32')
        glBufferData(GL_ELEMENT_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)
        #ADD END

        block_VAO=glGenVertexArrays(1)
        glBindVertexArray(block_VAO)
        glBindBuffer(GL_ARRAY_BUFFER,block_VBO)
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,block_EBO)
        glVertexPointer(3,GL_FLOAT,0,None)
        glEnableClientState(GL_VERTEX_ARRAY)
        
        #ADD
        glBindBuffer(GL_ARRAY_BUFFER,color_VBO)
        glColorPointer(4,GL_FLOAT,0,None)
        glEnableClientState(GL_COLOR_ARRAY)
        #ADD END
        glBindVertexArray(0)
        draw=True
    glBindVertexArray(block_VAO)
    glDrawElements(GL_QUADS,block_EBO_buffer_len,GL_UNSIGNED_INT,None)
    glBindVertexArray(0)

function print_blocks is in the mainloop. If I don't bind color to VAO(I mean run without new add code), it can be drawn normally.But after I bind, no graphics will appear.How can I do to make it draw normally? I really have no thing to say now.Please!Please!Please!Please!Please!Please!

score 1 · Accepted Answer · answered Jan 07 '22 at 17:58

The Index Buffer (ELEMENT_ARRAY_BUFFER) binding is stored within the Vertex Array Object. When glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, IBO) is called the element buffer object ID is stored in the currently bound Vertex Array Object. Therefore the VAO must be bound before the element buffer with glBindVertexArray(VAO).

Compared to the Index Buffer, the Vertex Buffer binding (ARRAY_BUFFER) is a global state.
Each attribute which is stated in the VAOs state vector may refer to a different ARRAY_BUFFER. When glVertexAttribPointer is called the buffer which is currently bound to the target ARRAY_BUFFER, is associated to the specified attribute index and the ID of the object is stored in the state vector of the currently bound VAO.

Therefor the VAO needs to be bound before the element buffer is bound and created.

Furthermore the type of the color attribute needs to be floating point:

a=numpy.array(block_color_buffer,dtype='uint32')

a=numpy.array(block_color_buffer,dtype='float32')

Besides that you cannot specify one mesh with multiple index buffers. See Why does OpenGL not support multiple index buffering?. You can just specify 1 array of indices.

Minimal example based on your original code:

from OpenGL.GLUT import *
from OpenGL.GLU import *
from OpenGL.GL import *
import numpy

rotate = [33, 40, 20]

block_VAO=0
draw=False
block_EBO_buffer_len=0
def create_blocks(x:int, y:int, z:int):
    global draw, block_VAO, block_EBO_buffer_len
    if draw:
        return
    draw = True    
    block_point_buffer=[]
    block_color_buffer=[]
    block_EBO_buffer=[]
    block_point_buffer+=[x-0.5,y+0.5,z-0.5,#V0
                        x+0.5,y+0.5,z-0.5,#V1
                        x+0.5,y-0.5,z-0.5,#V2
                        x-0.5,y-0.5,z-0.5,#V3
                        x-0.5,y+0.5,z+0.5,#V4
                        x+0.5,y+0.5,z+0.5,#V5
                        x+0.5,y-0.5,z+0.5,#V6
                        x-0.5,y-0.5,z+0.5]#V7
    block_EBO_buffer+=[0,1,5,4,
                        3,2,6,7,
                        0,3,7,4,
                        1,2,6,5,
                        0,1,2,3,
                        4,5,6,7]

    block_color_buffer+=[1.0,0.0,1.0,1.0]*8

    block_VBO=glGenBuffers(1)
    glBindBuffer(GL_ARRAY_BUFFER,block_VBO)
    a=numpy.array(block_point_buffer,dtype='float32')
    glBufferData(GL_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)

    color_VBO=glGenBuffers(1)
    glBindBuffer(GL_ARRAY_BUFFER,color_VBO)
    a=numpy.array(block_color_buffer,dtype='float32')
    glBufferData(GL_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)

    block_VAO=glGenVertexArrays(1)
    glBindVertexArray(block_VAO)

    block_EBO=glGenBuffers(1)
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,block_EBO)
    a=numpy.array(block_EBO_buffer,dtype='uint32')
    glBufferData(GL_ELEMENT_ARRAY_BUFFER,sys.getsizeof(a),a,GL_STATIC_DRAW)
    block_EBO_buffer_len=len(a)

    glBindBuffer(GL_ARRAY_BUFFER,block_VBO)
    glVertexPointer(3,GL_FLOAT,0,None)
    glEnableClientState(GL_VERTEX_ARRAY)

    glBindBuffer(GL_ARRAY_BUFFER,color_VBO)
    glColorPointer(4,GL_FLOAT,0,None)
    glEnableClientState(GL_COLOR_ARRAY)

    glBindVertexArray(0)

def display():
    glMatrixMode(GL_MODELVIEW)
    glClear(GL_COLOR_BUFFER_BIT)
    glLoadIdentity()
    glTranslatef(0, 0, -4.5)
    glRotatef(rotate[0], 1, 0.0, 0)
    glRotatef(rotate[1], 0, 1, 0)
    glRotatef(rotate[2], 0, 0, 1)
    glScalef(1, 1, 1)

    glBindVertexArray(block_VAO)
    glDrawElements(GL_QUADS,block_EBO_buffer_len,GL_UNSIGNED_INT,None)
    glBindVertexArray(0)
    rotate[1] += 0.1
    
    glutSwapBuffers()
    glutPostRedisplay()

def reshape(width, height):
    glViewport(0, 0, width, height)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    gluPerspective(40.0, width / height, 0.5, 20.0)
    glMatrixMode(GL_MODELVIEW)

glutInit(sys.argv)
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB)
glutInitWindowSize(400, 350)
glutCreateWindow(b"OpenGL Window")
create_blocks(0, 0, 0)
glClearColor(0.0, 0.0, 0.0, 0.0)
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE)
glutDisplayFunc(display)
glutReshapeFunc(reshape)
glutMainLoop()

OpenGL VBO VAO EBO can run without error but no graphics

1 Answers1