feedbackvarying.py

"""Transliteration of https://open.gl/feedback into Python"""

from __future__ import print_function
import pygamegltest
from OpenGL.GL import *
from OpenGL.GL import shaders

vertex_shader = """#version 150 core
    in float inValue;
    out float outValue;

    void main() {
        outValue = sqrt(inValue);
    }
"""


@pygamegltest.pygametest(name="Geometry Shader Test")
def main():
    program = shaders.compileProgram(
        shaders.compileShader([vertex_shader], GL_VERTEX_SHADER),
    )

    buff = ctypes.c_char_p(b"outValue")
    c_text = ctypes.cast(ctypes.pointer(buff), ctypes.POINTER(ctypes.POINTER(GLchar)))
    # modifies the state in the linking of the program
    glTransformFeedbackVaryings(program, 1, c_text, GL_INTERLEAVED_ATTRIBS)

    # so have to re-link
    glLinkProgram(program)
    glUseProgram(program)

    vao = glGenVertexArrays(1)
    glBindVertexArray(vao)

    data = (GLfloat * 5)(1.0, 2.0, 3.0, 4.0, 5.0)

    vbo = glGenBuffers(1)
    glBindBuffer(GL_ARRAY_BUFFER, vbo)
    glBufferData(GL_ARRAY_BUFFER, ctypes.sizeof(data), data, GL_STATIC_DRAW)

    inputAttrib = glGetAttribLocation(program, "inValue")
    glEnableVertexAttribArray(inputAttrib)
    # Note the need to cast 0 to a GLvoidp here!
    glVertexAttribPointer(inputAttrib, 1, GL_FLOAT, GL_FALSE, 0, GLvoidp(0))

    tbo = glGenBuffers(1)
    glBindBuffer(GL_ARRAY_BUFFER, tbo)
    glBufferData(GL_ARRAY_BUFFER, ctypes.sizeof(data), None, GL_STATIC_READ)

    glEnable(GL_RASTERIZER_DISCARD)

    glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, tbo)

    glBeginTransformFeedback(GL_POINTS)
    glDrawArrays(GL_POINTS, 0, len(data))
    glEndTransformFeedback()

    glDisable(GL_RASTERIZER_DISCARD)

    glFlush()

    feedback = (GLfloat * len(data))()
    glGetBufferSubData(
        GL_TRANSFORM_FEEDBACK_BUFFER, 0, ctypes.sizeof(feedback), feedback
    )

    for value in feedback:
        print(value)
    print('OK')


if __name__ == "__main__":
    main()