Transform Feedback#

Transform Feedback Example
transform_feedback.py#
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"""
Shows simple use of transform feedback.

Transforming is similar to rendering except that the output
or the shader is a buffer instead of a framebuffer/screen.

This examples shows a common ping-pong technique were we
transform a buffer with positions and velocities between
two buffers so we always work on the previous state.

* A list of N points are initialized with random positions and velocities
* A point of gravity is moving around on the screen affecting the points

Using transforms in this way makes us able to process
a system that is reacting to external forces in this way.
There are no predetermined paths and they system just lives on its own.
"""
from array import array
import math
import time
import random
import arcade
from arcade.gl import BufferDescription

# Do the math to figure out our screen dimensions
SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
SCREEN_TITLE = "Transform Feedback"


class MyGame(arcade.Window):

    def __init__(self, width, height, title):
        super().__init__(width, height, title, resizable=True)
        self.time = 0

        # Program to visualize the points
        self.points_progran = self.ctx.program(
            vertex_shader="""
            #version 330
            in vec2 in_pos;
            out vec3 color;
            void main() {
                // Let's just give them a "random" color based on the vertex id
                color = vec3(
                    mod((gl_VertexID * 100 % 11) / 10.0, 1.0),
                    mod((gl_VertexID * 100 % 27) / 10.0, 1.0),
                    mod((gl_VertexID * 100 % 71) / 10.0, 1.0));
                // Pass the point position to primitive assembly
                gl_Position = vec4(in_pos, 0.0, 1.0);
            }
            """,
            fragment_shader="""
            #version 330

            // Color passed in from the vertex shader
            in vec3 color;
            // The pixel we are writing to in the framebuffer
            out vec4 fragColor;

            void main() {
                // Fill the point
                fragColor = vec4(color, 1.0);
            }
            """,
        )

        # A program transforming points being affected by a gravity point
        self.gravity_program = self.ctx.program(
            vertex_shader="""
            #version 330

            // Delta time (since last frame)
            uniform float dt;
            // Strength of gravity
            uniform float force;
            // Position of gravity
            uniform vec2 gravity_pos;

            // The format of the data in our transform buffer(s)
            in vec2 in_pos;
            in vec2 in_vel;

            // We are writing to a buffer of the same format
            out vec2 out_pos;
            out vec2 out_vel;

            void main() {
                // Simplified gravity calculations
                vec2 dir = normalize(gravity_pos - in_pos) * force;
                vec2 vel = in_vel + dir / length(dir) * 0.01;

                // Write to the output buffer
                out_vel = vel;
                out_pos = in_pos + vel * dt;
            }
            """,
        )
        N = 50_000
        # Make two buffers we transform between so we can work on the previous result
        self.buffer_1 = self.ctx.buffer(data=array('f', self.gen_initial_data(N)))
        self.buffer_2 = self.ctx.buffer(reserve=self.buffer_1.size)

        # We also need to be able to visualize both versions (draw to the screen)
        self.vao_1 = self.ctx.geometry([BufferDescription(self.buffer_1, '2f 2x4', ['in_pos'])])
        self.vao_2 = self.ctx.geometry([BufferDescription(self.buffer_2, '2f 2x4', ['in_pos'])])

        # We need to be able to transform both buffers (ping-pong)
        self.gravity_1 = self.ctx.geometry([BufferDescription(self.buffer_1, '2f 2f', ['in_pos', 'in_vel'])])
        self.gravity_2 = self.ctx.geometry([BufferDescription(self.buffer_2, '2f 2f', ['in_pos', 'in_vel'])])

        self.ctx.enable_only()  # Ensure no context flags are set
        self.time = time.time()

    def gen_initial_data(self, count):
        for _ in range(count):
            yield random.uniform(-1.2, 1.2)  # pos x
            yield random.uniform(-1.2, 1.2)  # pos y
            yield random.uniform(-.3, .3)  # velocity x
            yield random.uniform(-.3, .3)  # velocity y

    def on_draw(self):
        self.clear()
        self.ctx.point_size = 2 * self.get_pixel_ratio()

        # Calculate the actual delta time and current time
        t = time.time()
        frame_time = t - self.time
        self.time = t

        # Set uniforms in the program
        self.gravity_program['dt'] = frame_time
        self.gravity_program['force'] = 0.25
        self.gravity_program['gravity_pos'] = math.sin(self.time * 0.77) * 0.25, math.cos(self.time) * 0.25

        # Transform data in buffer_1 into buffer_2
        self.gravity_1.transform(self.gravity_program, self.buffer_2)
        # Render the result (Draw buffer_2)
        self.vao_2.render(self.points_progran, mode=self.ctx.POINTS)

        # Swap around stuff around so we transform back and fourth between the two buffers
        self.gravity_1, self.gravity_2 = self.gravity_2, self.gravity_1
        self.vao_1, self.vao_2 = self.vao_2, self.vao_1
        self.buffer_1, self.buffer_2 = self.buffer_2, self.buffer_1


if __name__ == "__main__":
    window = MyGame(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)
    window.center_window()
    arcade.run()