graphics-snippets

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Python code snippets

Print buffer

glBindBuffer(GL_ARRAY_BUFFER, vbo)

no_of_floats = 12 # read 12 floats
float_array = (GLfloat * no_of_floats)()
glGetBufferSubData(GL_ARRAY_BUFFER, 0, no_of_floats * sizeof(GLfloat), float_array)
print(list(float_array))

Buffer to byte array

Use numpy.frombuffer to convert the string to a byte array:

import numpy

data = glReadPixels(0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE)
byte_array = numpy.frombuffer(data, dtype='uint8')
print(list(byte_array))

Or use ctypes.from_buffer_copy

import ctypes

data = glReadPixels(0,0, w, h, GL_RGB,GL_UNSIGNED_BYTE)
byte_array = (ctypes.c_ubyte * (w * h * 3)).from_buffer_copy(data)
print(list(byte_array))

Vector and matrix operations

Arithmetic

def add(a, b):
    return a[0]+b[0], a[1]+b[1], a[2]+b[2]

def sub(a, b):
    return a[0]-b[0], a[1]-b[1], a[2]-b[2]

def mul(v, s):
    return v[0]*s, v[1]*s, v[2]*s

Dot product

Dot product

def dot(a, b):
    return a[0]*b[0] + a[1]*b[1] + a[2]*b[2]

Cross product

Cross product

def cross(a, b):
    return (
        a[1]*b[2] - a[2]*b[1],
        a[2]*b[0] - a[0]*b[2],
        a[0]*b[1] - a[1]*b[0]
    )

Intersection

Point in triangle

def point_in_segment(px, p0, a, b):
    cp1 = cross(sub(b, a), sub(px, a))
    cp2 = cross(sub(b, a), sub(p0, a))
    return dot(cp1, cp2) >= 0 

def point_in_triangle(p, a, b, c):
    return point_in_segment(p, a, b, c) and point_in_segment(p, b, c, a) and point_in_segment(p, c, a, b)

Intersect ray and plane

def intersect_plane(o, d, p0, p1, p2):
    n = cross(sub(p1, p0), sub(p2, p0))
    t = dot(sub(p0, o), n) / dot(d, n)
    x = add(o, mul(d, t))
    return x, t

Intersect ray and triangle

The algorithm does not depend on the winding order.

def intersect_triangle(o, d, p0, p1, p2):
    x, t = intersect_plane(o, d, p0, p1, p2)
    return point_in_triangle(x, p0, p1, p2), x, t