How to correctly add a light to make object get a better view with pygame and pyopengl

问题

I am following the tutorial available in https://pythonprogramming.net/opengl-pyopengl-python-pygame-tutorial/ where he teachs how to render a cube with pyOpenGL an pygame.

When rendering the cube, the tutorial set color to all the vertices of the cubes and then dispays it. However, im my project, i load object from a .obj file using the code provided in https://www.pygame.org/wiki/OBJFileLoader and most of my objects is fully white.

Conclusion: when i render it on screen, i only see full white, which is very ugly. So i tought to use a light to better view the object, but i cannot make this work.

I know very little of pyOpenGl and i cannot find a deeper tutorial of it.

Here is part of the code and the result provided in the tutorial. (vertices, edges, surfaces and color variables are tuple of tuples)

def Cube():
    glBegin(GL_QUADS)
    for surface in surfaces:
        x = 0
        for vertex in surface:
            x+=1
            glColor3fv(colors[x])
            glVertex3fv(verticies[vertex])
    glEnd()
    glBegin(GL_LINES)
    for edge in edges:
        for vertex in edge:
            glVertex3fv(verticies[vertex])
    glEnd()


def main():
    pygame.init()
    display = (800,600)
    pygame.display.set_mode(display, DOUBLEBUF|OPENGL)

    gluPerspective(45, (display[0]/display[1]), 0.1, 50.0)
    glTranslatef(0.0,0.0, -5)

    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                quit()      
        glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)
        glRotatef(1, 3, 1, 1)     
        Cube()
        pygame.display.flip()
        pygame.time.wait(10)

main()

i tried to edit the main function to insert a simple light, but the colors in the cube just disapeared:

def main():
    pygame.init()
    display = (800,600)
    pygame.display.set_mode(display, DOUBLEBUF|OPENGL)

    gluPerspective(45, (display[0]/display[1]), 0.1, 50.0)
    glTranslatef(0.0,0.0, -5)

    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                quit()      
        glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)
        glRotatef(1, 3, 1, 1)
        glEnable(GL_LIGHTING)
        glEnable(GL_LIGHT0)

        glPushMatrix()
        glTranslatef(0.0,0.0, 5)
        glLight(GL_LIGHT0, GL_POSITION,  (0, 1, 0, 1))
        glLightfv(GL_LIGHT0, GL_AMBIENT,  (0, 1.5, 1, 0))
        glPopMatrix()
        Cube()
        glDisable(GL_LIGHT0)
        glDisable(GL_LIGHTING)

        pygame.display.flip()
        pygame.time.wait(10)

What i want is the cube with its colors and iluminated by a light. What is wrong with my code and how to fix it?

回答1:

When lighting (GL_LIGHTING) is enabled, then the color is taken from the material parameters (glMaterial).

If you still want to use the current color, the you have to enable GL_COLOR_MATERIAL and to set the color material paramters (glColorMaterial).

The ambient light does not depend on the the direction of the light source. You've to define a diffuse and/or specular light. See glLightfv:

When the light position is set by glLightfv(GL_LIGHT0, GL_POSITION, pos), then the position is multiplied by the current model view matrix. So the light position in world space has to be set before the model transformation. Switch to the matrix mode GL_PROJECTION, before the projection matrix is set. Otherwise the light position would be multiplied by the projection matrix.

glMatrixMode(GL_PROJECTION)
gluPerspective(45, (display[0]/display[1]), 0.1, 50.0)

glMatrixMode(GL_MODELVIEW)
glTranslatef(0, 0, -5)

#glLight(GL_LIGHT0, GL_POSITION,  (0, 0, 1, 0)) # directional light from the front
glLight(GL_LIGHT0, GL_POSITION,  (5, 5, 5, 1)) # point light from the left, top, front
glLightfv(GL_LIGHT0, GL_AMBIENT, (0, 0, 0, 1))
glLightfv(GL_LIGHT0, GL_DIFFUSE, (1, 1, 1, 1))

glEnable(GL_DEPTH_TEST) 

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            quit()      

    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)

    glEnable(GL_LIGHTING)
    glEnable(GL_LIGHT0)
    glEnable(GL_COLOR_MATERIAL)
    glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE )

    glRotatef(1, 3, 1, 1)
    Cube()

    glDisable(GL_LIGHT0)
    glDisable(GL_LIGHTING)
    glDisable(GL_COLOR_MATERIAL)

    pygame.display.flip()

The diffuse (and specular) light depends on the Normal vector of the surface.

Define an array of normal vector tuples (x, y, z). Note the following definition is an example. Since you draw a cube, which has 6 faces, you've to define 6 normal vectors, but the direction of the vectors depends on your vertex coordinates, which I do not know.

normals = [
    ( 0,  0, -1),  # surface 0
    (-1,  0,  0),  # surface 1
    ( 0,  1,  1),  # surface 2
    ( 1,  0,  0),  # surface 3
    ( 0,  1,  0),  # surface 4
    ( 0, -1,  0)   # surface 5
]

And set the proper normal vector when the object is drawn:

def Cube():
    glBegin(GL_QUADS)
    for i_surface, surface in enumerate(surfaces):
        x = 0

        glNormal3fv(normals[i_surface]) # set the normal vector the vertices of the surface

        for vertex in surface:
            x+=1
            glColor3fv(colors[x])
            glVertex3fv(verticies[vertex])
    glEnd()

    glColor3fv(colors[0])
    glBegin(GL_LINES)
    for edge in edges:
        for vertex in edge:
            glVertex3fv(verticies[vertex])
    glEnd()

Enable the Depth Test (glEnable(GL_DEPTH_TEST)) to get an animation like the following:

Full example code:

import pygame
from pygame.locals import *

from OpenGL.GL import *
from OpenGL.GLU import *

verticies = (
    ( 1, -1, -1), # 0
    ( 1,  1, -1), # 1
    (-1,  1, -1), # 2
    (-1, -1, -1), # 3
    ( 1, -1,  1), # 4
    ( 1,  1,  1), # 5
    (-1, -1,  1), # 6
    (-1,  1,  1), # 7
    )

surfaces = (
    (0,1,2,3),
    (3,2,7,6),
    (6,7,5,4),
    (4,5,1,0),
    (1,5,7,2),
    (4,0,3,6),
    )

normals = [
    ( 0,  0, -1),  # surface 0
    (-1,  0,  0),  # surface 1
    ( 0,  0,  1),  # surface 2
    ( 1,  0,  0),  # surface 3
    ( 0,  1,  0),  # surface 4
    ( 0, -1,  0)   # surface 5
]

colors = (
    (1,1,1),
    (0,1,0),
    (0,0,1),
    (0,1,0),
    (0,0,1),
    (1,0,1),
    (0,1,0),
    (1,0,1),
    (0,1,0),
    (0,0,1),
    )

edges = (
    (0,1),
    (0,3),
    (0,4),
    (2,1),
    (2,3),
    (2,7),
    (6,3),
    (6,4),
    (6,7),
    (5,1),
    (5,4),
    (5,7),
    )


def Cube():
    glBegin(GL_QUADS)
    for i_surface, surface in enumerate(surfaces):
        x = 0
        glNormal3fv(normals[i_surface])
        for vertex in surface:
            x+=1
            glColor3fv(colors[x])
            glVertex3fv(verticies[vertex])
    glEnd()

    glColor3fv(colors[0])
    glBegin(GL_LINES)
    for edge in edges:
        for vertex in edge:
            glVertex3fv(verticies[vertex])
    glEnd()


def main():
    global surfaces

    pygame.init()
    display = (800, 600)
    pygame.display.set_mode(display, DOUBLEBUF|OPENGL)
    clock = pygame.time.Clock()

    glMatrixMode(GL_PROJECTION)
    gluPerspective(45, (display[0]/display[1]), 0.1, 50.0)

    glMatrixMode(GL_MODELVIEW)
    glTranslatef(0, 0, -5)

    #glLight(GL_LIGHT0, GL_POSITION,  (0, 0, 1, 0)) # directional light from the front
    glLight(GL_LIGHT0, GL_POSITION,  (5, 5, 5, 1)) # point light from the left, top, front
    glLightfv(GL_LIGHT0, GL_AMBIENT, (0, 0, 0, 1))
    glLightfv(GL_LIGHT0, GL_DIFFUSE, (1, 1, 1, 1))

    glEnable(GL_DEPTH_TEST) 

    while True:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                quit()      

        glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)

        glEnable(GL_LIGHTING)
        glEnable(GL_LIGHT0)
        glEnable(GL_COLOR_MATERIAL)
        glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE )

        glRotatef(1, 3, 1, 1)
        Cube()

        glDisable(GL_LIGHT0)
        glDisable(GL_LIGHTING)
        glDisable(GL_COLOR_MATERIAL)

        pygame.display.flip()
        clock.tick(60)

main()

来源：https://stackoverflow.com/questions/56514791/how-to-correctly-add-a-light-to-make-object-get-a-better-view-with-pygame-and-py