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#!/usr/bin/env python3
'''Simple example of using tessellation to render a cubic Bézier curve'''
import numpy as np
import moderngl
from _example import Example
class Tessellation(Example):
title = "Tessellation"
gl_version = (4, 0)
def __init__(self, **kwargs):
super().__init__(**kwargs)
self.prog = self.ctx.program(
vertex_shader='''
#version 400 core
in vec2 in_pos;
void main() { gl_Position = vec4(in_pos, 0.0, 1.0); }
''',
tess_control_shader='''
#version 400 core
layout(vertices = 4) out;
void main() {
// set tessellation levels, TODO compute dynamically
gl_TessLevelOuter[0] = 1;
gl_TessLevelOuter[1] = 32;
// pass through vertex positions
gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;
}
''',
tess_evaluation_shader='''
#version 400 core
layout(isolines, fractional_even_spacing, ccw) in;
// compute a point on a bezier curve with the points p0, p1, p2, p3
// the parameter u is in [0, 1] and determines the position on the curve
vec3 bezier(float u, vec3 p0, vec3 p1, vec3 p2, vec3 p3) {
float B0 = (1.0 - u) * (1.0 - u) * (1.0 - u);
float B1 = 3.0 * (1.0 - u) * (1.0 - u) * u;
float B2 = 3.0 * (1.0 - u) * u * u;
float B3 = u * u * u;
return B0 * p0 + B1 * p1 + B2 * p2 + B3 * p3;
}
void main() {
float u = gl_TessCoord.x;
vec3 p0 = vec3(gl_in[0].gl_Position);
vec3 p1 = vec3(gl_in[1].gl_Position);
vec3 p2 = vec3(gl_in[2].gl_Position);
vec3 p3 = vec3(gl_in[3].gl_Position);
gl_Position = vec4(bezier(u, p0, p1, p2, p3), 1.0);
}
''',
fragment_shader='''
#version 400 core
out vec4 frag_color;
void main() { frag_color = vec4(1.0); }
'''
)
# four vertices define a cubic Bézier curve; has to match the shaders
self.ctx.patch_vertices = 4
self.ctx.line_width = 5.0
vertices = np.array([
[-1.0, 0.0],
[-0.5, 1.0],
[0.5, -1.0],
[1.0, 0.0],
])
vbo = self.ctx.buffer(vertices.astype('f4'))
self.vao = self.ctx.simple_vertex_array(self.prog, vbo, 'in_pos')
def render(self, time, frame_time):
self.ctx.clear(0.2, 0.4, 0.7)
self.vao.render(mode=moderngl.PATCHES)
if __name__ == '__main__':
Tessellation.run()
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