transform_feedback.cpp

#include <GL/glew.h>
#include <SFML/Window.hpp>

// Shader macro
#define GLSL(src) "#version 150 core\n" #src

// Vertex shader
const GLchar* vertexShaderSrc = GLSL(
in float inValue;
out float outValue;

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

int main()
{
	sf::ContextSettings settings;
	settings.depthBits = 24;
	settings.stencilBits = 8;

	sf::Window window(sf::VideoMode(800, 600, 32), "Transform Feedback", sf::Style::Titlebar | sf::Style::Close, settings);

	// Initialize GLEW
	glewExperimental = GL_TRUE;
	glewInit();

	// Compile shader
	GLuint shader = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(shader, 1, &vertexShaderSrc, nullptr);
	glCompileShader(shader);

	// Create program and specify transform feedback variables
	GLuint program = glCreateProgram();
	glAttachShader(program, shader);

	const GLchar* feedbackVaryings[] = { "outValue" };
	glTransformFeedbackVaryings(program, 1, feedbackVaryings, GL_INTERLEAVED_ATTRIBS);

	glLinkProgram(program);
	glUseProgram(program);

	// Create VAO
	GLuint vao;
	glGenVertexArrays(1, &vao);
	glBindVertexArray(vao);

	// Create input VBO and vertex format
	GLfloat data[] = { 1.0f, 2.0f, 3.0f, 4.0f, 5.0f };

	GLuint vbo;
	glGenBuffers(1, &vbo);
	glBindBuffer(GL_ARRAY_BUFFER, vbo);
	glBufferData(GL_ARRAY_BUFFER, sizeof(data), data, GL_STATIC_DRAW);

	GLint inputAttrib = glGetAttribLocation(program, "inValue");
	glEnableVertexAttribArray(inputAttrib);
	glVertexAttribPointer(inputAttrib, 1, GL_FLOAT, GL_FALSE, 0, 0);

	// Create transform feedback buffer
	GLuint tbo;
	glGenBuffers(1, &tbo);
	glBindBuffer(GL_ARRAY_BUFFER, tbo);
	glBufferData(GL_ARRAY_BUFFER, sizeof(data), nullptr, GL_STATIC_READ);

	// Perform feedback transform
	glEnable(GL_RASTERIZER_DISCARD);

	glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, tbo);

	glBeginTransformFeedback(GL_POINTS);
	glDrawArrays(GL_POINTS, 0, 5);
	glEndTransformFeedback();

	glDisable(GL_RASTERIZER_DISCARD);

	glFlush();

	// Fetch and print results
	GLfloat feedback[5];
	glGetBufferSubData(GL_TRANSFORM_FEEDBACK_BUFFER, 0, sizeof(feedback), feedback);

	printf("%f %f %f %f %f\n", feedback[0], feedback[1], feedback[2], feedback[3], feedback[4]);

	glDeleteProgram(program);
	glDeleteShader(shader);

	glDeleteBuffers(1, &tbo);
	glDeleteBuffers(1, &vbo);

	glDeleteVertexArrays(1, &vao);
}