OpenGL程式設計指南8-Transform Feedback例子理解
阿新 • • 發佈：2018-11-04
transform feedback 是，OpenGL管線中，的，頂點處理階段結束之後，圖元裝配和光柵化之前的一個步驟。transform feedback，可以重新捕獲即將裝配為圖元（點、線段、三角形）的頂點，然後將它們的部分或全部屬性傳遞到快取物件中。
transform feedback主要API
- void glGenTransformFeedbacks(GLsizei n, GLuint* ids)
- void glBindTransformFeedback(GLenum target, GLuint id)
- void glDeleteTransformFeedbacks(GLsizei n, GLuint* ids)
-
GLboolean glIsTransformFeedback(GLenum id)
-
void glBindBufferBase(GLenum target, GLuint index, GLuint buffer)
-
void glBindBufferRange(GLenum target, GLuint index, GLuint buffer, GLintptr offset, GLsizeiptr size)
-
void glTransformFeedbackVaryings(GLuint program, GLsizei count, const GLchar** varyings, GLenum bufferMode)
- void glBeginTransformFeedback(GLenum primitiveMode)
- void glEndTransformFeedback()
- void glPauseTransformFeedback()
- void glResumeTransformFeedback()
紅寶書（OpenGL程式設計指南8）中，transform feedback示例程式碼中的一些細節理解：
- 並沒有直接建立feedback物件，而是使用了系統預設的feedback
- 繪製粒子時，使用兩個VBO，一個用於當前繪製的資料，一個用於feedback，捕獲更新後的三角形頂點和速度資訊，用於下一幀的繪製資料
- 建立tbo，用於儲存三角形頂點資料，此處的tbo被同時繫結到兩個物件中，1. tbo被繫結到feedback中，用於捕獲模型繪製的三角形資料，2. tbo被繫結到紋理物件，用於例子系統中獲取三角形資料
// ch05-tranformFeedback.cpp
#include <vapp.h>
#include <vutils.h>
#include <vmath.h>
#include "vbm.h"
#include <loadShaders.h>
#include <stdio.h>
using namespace vmath;
const int point_count = 5000;
static unsigned int seed = 0x13371337;
static inline float random_float()
{
float res;
unsigned int tmp;
seed *= 16807;
tmp = seed ^ (seed >> 4) ^ (seed << 15);
*((unsigned int *)&res) = (tmp >> 9) | 0x3F800000;
return (res - 1.0f);
}
static vmath::vec3 random_vector(float minmag = 0.0f, float maxmag = 1.0f)
{
vmath::vec3 randomvec(random_float() * 2.0f - 1.0f, random_float() * 2.0f - 1.0f, random_float() * 2.0f - 1.0f);
randomvec = normalize(randomvec);
randomvec *= (random_float() * (maxmag - minmag) + minmag);
return randomvec;
}
BEGIN_APP_DECLARATION(TransformFeedback)
// override functions from base class
virtual void Initialize(const char* title);
virtual void Display(bool auto_redraw);
virtual void Finalize();
virtual void Reshape(int width, int height);
private:
float m_aspect;
GLuint m_baseProg; // 用於繪製物體
GLuint m_updateProg; // 用於繪製更新的粒子
GLuint m_VAO[2]; // 用於關聯粒子系統的VBO
GLuint m_VBO[2]; // 儲存粒子座標、速度資訊
//GLuint m_xfb; // transformFeedback 物件
GLuint m_feedback_VAO; // 用於關聯 給feedback使用胡tbo
GLuint m_geometry_tbo;
GLuint m_geometry_tex;
GLint m_base_model_matrix_loc;
GLint m_base_projection_matrix_loc;
GLint m_update_model_matrix_loc;
GLint m_update_projection_matrix_loc;
GLint m_time_step_loc;
GLint m_triangle_count_loc;
VBObject m_object;
END_APP_DECLARATION()
DEFINE_APP(TransformFeedback ,"ch05-transformFeedback")
void TransformFeedback::Initialize(const char* title)
{
glClearColor(1.0f,1.0f,1.0f,1.0f);
base::Initialize(title);
// init shader
static ShaderInfo base_shader_info[] =
{
{GL_VERTEX_SHADER,"Media/Shaders/5/base.vs.glsl"},
{GL_FRAGMENT_SHADER,"Media/Shaders/5/base.fs.glsl"},
{GL_NONE,NULL}
};
static ShaderInfo update_shader_info[] =
{
{GL_VERTEX_SHADER,"Media/Shaders/5/update.vs.glsl"},
{GL_FRAGMENT_SHADER,"Media/Shaders/5/update.fs.glsl"},
{GL_NONE,NULL}
};
m_updateProg = LoadShaders(update_shader_info);
glUseProgram(m_updateProg);
m_update_model_matrix_loc = glGetUniformLocation(m_updateProg,"model_matrix");
m_update_projection_matrix_loc = glGetUniformLocation(m_updateProg,"projection_matrix");
m_triangle_count_loc = glGetUniformLocation(m_updateProg,"triangle_count");
m_time_step_loc = glGetUniformLocation(m_updateProg,"time_step");
static const char* varyings[] =
{
"position_out", "velocity_out"
};
/*glTransformFeedbackVaryings(GLuint program,
GLsizei count,
const GLchar *const* varyings,
GLenum bufferMode)
*/
// 使用交錯模式的快取
glTransformFeedbackVaryings(m_updateProg,2,varyings,GL_INTERLEAVED_ATTRIBS);
glLinkProgram(m_updateProg); // 重新link著色器
//glUseProgram(m_updateProg);
m_baseProg = LoadShaders(base_shader_info);
glUseProgram(m_baseProg);
m_base_model_matrix_loc = glGetUniformLocation(m_baseProg,"model_matrix");
m_base_projection_matrix_loc = glGetUniformLocation(m_baseProg,"projection_matrix");
static const char * varyings2[] =
{
"world_space_position"
};
glTransformFeedbackVaryings(m_baseProg,1,varyings2,GL_INTERLEAVED_ATTRIBS);
glLinkProgram(m_baseProg);
//glUseProgram(m_baseProg);
glGenVertexArrays(2,m_VAO);
glGenBuffers(2,m_VBO);
//此處沒有呼叫glGenTransformFeedbacks和glBindTransformFeedback函式，因為系統存在一個預設的feedback，可以直接使用
for(int i = 0 ; i < 2; ++i)
{
glBindBuffer(GL_TRANSFORM_FEEDBACK_BUFFER,m_VBO[i]);
/*
glBindBuffer(GLenum target, GLsizeiptr size, const void* data, GLenum usage);
*/
glBufferData(GL_TRANSFORM_FEEDBACK_BUFFER,
point_count * (sizeof(vec4) + sizeof(vec3)),
NULL,
GL_DYNAMIC_COPY);
// 初始化第一個buffer
if(i == 0)
{
struct buffer_t
{
vec4 position;
vec3 velocity;
} *buffer = (buffer_t*)glMapBuffer(GL_TRANSFORM_FEEDBACK_BUFFER,GL_WRITE_ONLY);
for(int j = 0; j < point_count; ++j)
{
buffer[j].velocity = random_vector();
buffer[j].position = vec4(buffer[j].velocity + vec3(-0.5f, 40.0f, 0.0f), 1.0f);
buffer[j].velocity = vec3(buffer[j].velocity[0], buffer[j].velocity[1] * 0.3f, buffer[j].velocity[2] * 0.3f);
}
glUnmapBuffer(GL_TRANSFORM_FEEDBACK_BUFFER);
}
glBindVertexArray(m_VAO[i]);
glBindBuffer(GL_ARRAY_BUFFER,m_VBO[i]);
/*
glVertexAttribPointer(GLuint index,
GLint size,
GLenum type,
GLboolean normalized,
GLsizei stride,
const void* pointer);
*/
glVertexAttribPointer(0,4,GL_FLOAT,GL_FALSE,
sizeof(vec4) + sizeof(vec3),0);
glVertexAttribPointer(1,3,GL_FLOAT,GL_FALSE,
sizeof(vec4) + sizeof(vec3),(GLvoid*)sizeof(vec4));
glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
glBindVertexArray(0);
glDisableVertexAttribArray(0);
glDisableVertexAttribArray(1);
}
// 建立tbo，用於儲存三角形頂點資料，此處的tbo被同時繫結到兩個物件中，
// 1. tbo被繫結到feedback中，用於捕獲模型繪製的三角形資料
// 2. tbo被繫結到紋理物件，用於例子系統中獲取三角形資料
glGenBuffers(1,&m_geometry_tbo);
glGenTextures(1,&m_geometry_tex);
glBindBuffer(GL_TEXTURE_BUFFER,m_geometry_tbo);
glBufferData(GL_TEXTURE_BUFFER,1024 * 1024 * sizeof(vec4),NULL,GL_DYNAMIC_COPY);
glBindTexture(GL_TEXTURE_BUFFER,m_geometry_tex);
/*
glTexBuffer(GLenum target, GLenum internalFormat, GLuint buffer);
*/
// 將快取區關聯到紋理物件上
glTexBuffer(GL_TEXTURE_BUFFER,GL_RGBA32F,m_geometry_tbo);
//
glGenVertexArrays(1, &m_feedback_VAO);
glBindVertexArray(m_feedback_VAO);
glBindBuffer(GL_ARRAY_BUFFER,m_geometry_tbo);
glVertexAttribPointer(0,4,GL_FLOAT,GL_FALSE,0,NULL);
glEnableVertexAttribArray(0);
glBindVertexArray(0);
glDisableVertexAttribArray(0);
// init object
std::string path = Utils::instance()->getMediaPath() + "Media\\armadillo_low.vbm";
m_object.LoadFromVBM(path.c_str(), 0, 1, 2);
//m_object.BindVertexArray();
// glBindVertexArray(0);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
}
void TransformFeedback::Reshape(int width, int height)
{
glViewport(0,0,width,height);
m_aspect = float(height)/float(width);
}
void TransformFeedback::Display(bool auto_redraw)
{
static int frame_count = 0;
float t = float(GetTickCount() & 0x3FFFF) / float(0x3FFFF);
static float q = 0.0f;
static const vmath::vec3 X(1.0f, 0.0f, 0.0f);
static const vmath::vec3 Y(0.0f, 1.0f, 0.0f);
static const vmath::vec3 Z(0.0f, 0.0f, 1.0f);
vmath::mat4 projection_matrix(vmath::frustum(-1.0f, 1.0f, -m_aspect, m_aspect, 1.0f, 5000.0f) * vmath::translate(0.0f, 0.0f, -100.0f)); // project and view matrix
vmath::mat4 model_matrix(vmath::scale(0.3f) *
vmath::rotate(t * 360.0f, 0.0f, 1.0f, 0.0f) *
vmath::rotate(t * 360.0f * 3.0f, 0.0f, 0.0f, 1.0f));
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
// Setup
glEnable(GL_CULL_FACE);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
// Active instancing program
glUseProgram(m_baseProg);
glUniformMatrix4fv(m_base_model_matrix_loc, 1, GL_FALSE, model_matrix);
glUniformMatrix4fv(m_base_projection_matrix_loc,1,GL_FALSE, projection_matrix);
glBindVertexArray(m_feedback_VAO);
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER,0,m_geometry_tbo);
// 捕獲三角形頂點資料
glBeginTransformFeedback(GL_TRIANGLES);
m_object.Render();
glEndTransformFeedback();
glUseProgram(m_updateProg);
model_matrix = vmath::mat4::identity();
glUniformMatrix4fv(m_update_model_matrix_loc,1,GL_FALSE,model_matrix);
glUniformMatrix4fv(m_update_projection_matrix_loc,1,GL_FALSE,projection_matrix);
glUniform1i(m_triangle_count_loc,m_object.GetVertexCount()/3);
// 時間控制
if( t > q)
{
glUniform1f(m_time_step_loc,(t-q)*2000.0f);
// printf("time:%f\n",(t-q)*2000.0f);
}
q = t;
// 每隔一幀，交換VBO，使用兩個VBO，一個用於當前繪製的資料，一個用於feedback，捕獲更新後的三角形頂點和速度資訊
if((frame_count & 1) != 0)
{
glBindVertexArray(m_VAO[1]); // 啟用用於繪製的VBO，即m_VBO[1];
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER,0,m_VBO[0]); // 使用m_VBO[0]捕獲粒子系統的當前位置和速度
// printf("frame:%d\n",frame_count);
}
else
{
glBindVertexArray(m_VAO[0]);// 啟用用於繪製的VBO，即m_VBO[0];
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER,0,m_VBO[1]);
}
glBeginTransformFeedback(GL_POINTS);
glDrawArrays(GL_POINTS,0,min(point_count,(frame_count >> 3)));
glEndTransformFeedback();
glBindVertexArray(0);
glUseProgram(0);
frame_count++;
base::Display();
}
void TransformFeedback::Finalize()
{
glUseProgram(0);
glDeleteBuffers(2, m_VBO);
glDeleteBuffers(1,&m_geometry_tbo);
glDeleteVertexArrays(2, m_VAO);
glDeleteVertexArrays(1,&m_feedback_VAO);
}