How to make a moving rectangle using OPenGL in python

Question

It is easy to draw a fixed position rectangles using OpenGL. But the problem is how to draw a moving rectangle.

In graphical programs, things "move" because they are re-drawn, in a different position each time, according to their velocity. Use code to decide the new position, and make sure that everything is redrawn in your main game loop. — Karl Knechtel, May 23 '22 at 18:12

score 0 · Accepted Answer · answered May 23 '22 at 19:59

The scene is redrawn every frame. This means that the viewport is cleared every frame and all meshes are drawn every frame. Drawing the mesh in a different position in each frame creates the illusion of movement. The position of the mesh is defined by the vertex transformations in the vertex shader. Usually the position of the message depends on the model, view and projection matrix e.g.:

#version 460 core

layout (location = 0) in vec2 a_position;

layout (location = 0) uniform mat4 u_projection;
layout (location = 1) uniform mat4 u_model_view;

void main() 
{
    gl_Position = u_projection * u_model_view * vec4(a_position, 0.0, 1.0);
}

See also LearnOpenGL - Transformations.

Minimal Python, (Py)OpenGL example:

from OpenGL.GLUT import *
from OpenGL.GL import *
import OpenGL.GL.shaders
import glm
import math

sh_vert = """
#version 460 core

layout (location = 0) in vec2 a_position;

layout (location = 0) uniform mat4 u_projection;
layout (location = 1) uniform mat4 u_model_view;

void main() 
{
    gl_Position = u_projection * u_model_view * vec4(a_position, 0.0, 1.0);
}
"""

sh_frag = """
#version 460 core

out vec4 frag_color;

void main()
{
    frag_color = vec4(1.0, 0.0, 0.0, 1.0);
}
"""

def display():
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
    
    projection = glm.ortho(-aspect, aspect, -1, 1, -1, 1)
    if aspect < 1:
        projection = glm.ortho(-1, 1, 1/-aspect, 1/aspect, -1, 1)

    elapsed_ms = glutGet(GLUT_ELAPSED_TIME)
    angle = elapsed_ms * math.pi * 2 / 5000.0
    model_view = glm.translate(glm.mat4(1), glm.vec3(math.cos(angle)*0.5, math.sin(angle)*0.5, 0))
    model_view = glm.scale(model_view, glm.vec3(0.2, 0.2, 1))
    glUniformMatrix4fv(0, 1, GL_FALSE, glm.value_ptr(projection))
    glUniformMatrix4fv(1, 1, GL_FALSE, glm.value_ptr(model_view))

    glDrawArrays(GL_TRIANGLE_FAN, 0, 4)

    glutSwapBuffers()
    glutPostRedisplay()

aspect = 1
def reshape(width, height):
    global aspect
    glViewport(0, 0, width, height)
    aspect = width / height  

glutInit(sys.argv)
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH | GLUT_MULTISAMPLE)
glutSetOption(GLUT_MULTISAMPLE, 8)
glutInitWindowSize(640, 480)
glutCreateWindow(b"OpenGL Window")
glutDisplayFunc(display)
glutReshapeFunc(reshape)

attributes = (GLfloat * 8)(-1,-1, 1,-1, 1, 1, -1, 1) 

vao = glGenVertexArrays(1)
vbo = glGenBuffers(1)
glBindVertexArray(vao)
glBindBuffer(GL_ARRAY_BUFFER, vbo)
glBufferData(GL_ARRAY_BUFFER, attributes, GL_STATIC_DRAW)
glVertexAttribPointer(0, 2, GL_FLOAT, False, 2*4, None)
glEnableVertexAttribArray(0)

program = OpenGL.GL.shaders.compileProgram(
    OpenGL.GL.shaders.compileShader(sh_vert, GL_VERTEX_SHADER),
    OpenGL.GL.shaders.compileShader(sh_frag, GL_FRAGMENT_SHADER)
)
glUseProgram(program)

glEnable(GL_DEPTH_TEST)
glClearColor(0.0, 0.0, 0.0, 0.0)
glutMainLoop()

How to make a moving rectangle using OPenGL in python

1 Answers1