OpenGL缓冲区对象之Atomic Counter Object
简介
Atomic Counter是OpenGL 4.2版本通过GL_ARB_shader_atomic_counters扩展引入的新特性，它可以在各种着色语言中使用。Atomic Counter具体来说指的是在缓冲区对象（Buffer Object）中存储着一个或者多个可以用来计数的变量值，对这些变量定义了特定的操作方式，可以让它们在着色语言中进行加一和减一的操作，除此之后其他所有的操作都是非法的。
关于Atomic Counter的使用场景，一个简单的案例是统计场景中哪些像素会先进行渲染。具体的实现思路：当我们调用片元Shader对像素进行着色的时候，我们可以让atomic Counter变量值加1，然后将这个值转换成一种颜色来渲染我们的像素，这样我们就可以很清楚看到那些像素点是先渲染的，哪些像素点是后渲染的了。
使用
首先我们需要创建Atomic Counter的缓冲区，和其他类型的缓冲区的实现过程都是一样的。
GLuint atomicsBuffer; glGenBuffers(1, &atomicsBuffer); glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, atomicsBuffer); glBufferData(GL_ATOMIC_COUNTER_BUFFER, sizeof(GLuint) * 3, NULL, GL_DYNAMIC_DRAW); glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, 0);
上面的代码生成了一个Atomic Counter Object对象，并且在它的存储区中存放了三个Atomic Counter类型的变量（Atomic Counter本质上是一个GLuint类型的变量，占4个字节） 与其他缓冲区一样，更新和获取Atomic Counter缓冲区对象的方法如下所示：
- 更新
GLuint *userCounters;
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, atomicsBuffer);
userCounters = (GLuint*)glMapBufferRange(GL_ATOMIC_COUNTER_BUFFER,
0 ,
sizeof(GLuint) * 3,
GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT | GL_MAP_UNSYNCHRONIZED_BIT);
memset(userCounters, 0, sizeof(GLuint) *3 );
// unmap the buffer
glUnmapBuffer(GL_ATOMIC_COUNTER_BUFFER);
或者使用glBufferSubData的方式进行更新
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, m_AtomicCountersBuffer);
GLuint a[3] = {0,0,0};
glBufferSubData(GL_ATOMIC_COUNTER_BUFFER, 0 , sizeof(GLuint) * 3, a);
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, 0);
- 获取
GLuint *userCounters;
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, atomicsBuffer);
userCounters = (GLuint*)glMapBufferRange(GL_ATOMIC_COUNTER_BUFFER,
0,
sizeof(GLuint) * 3,
GL_MAP_READ_BIT
);
redPixels = userCounters[0];
greenPixels = userCounters[1];
bluePixels = userCounters[2];
glUnmapBuffer(GL_ATOMIC_COUNTER_BUFFER);
或者使用另一种方式来获取
GLuint userCounters[3];
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, m_AtomicCountersBuffer);
glGetBufferSubData(GL_ATOMIC_COUNTER_BUFFER, 0, sizeof(GLuint) * 3, userCounters);
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, 0);
redPixels = userCounters[0];
greenPixels = userCounters[1];
bluePixels = userCounters[2];
Shader中的设置
当我们在shader中需要使用Atomic Counter的时候，我们可以定义Atomic Counter类型的变量atomic_uint,在我们定义atomic_uint类型变量的时候我们需要设置这些变量对应到AtomicCounter缓冲区对象存储空间的位置和偏移量，一般来说定义Atomic Counter变量的方式如下：
layout (binding = 1, offset = 0) uniform atomic_uint atRed; layout (binding = 2, offset = 0) uniform atomic_uint atGreen; layout (binding = 2, offset = 4) uniform atomic_uint atBlue;
atRed变量对应的binding point是1，偏移量是0 atGreen对应的binding point是2，偏移量是0 atBlue对应的binding point是2，偏移量是4（正好越过atGreen的4个字节） 需要注意的是当我们设置两个变量同样的binding和offset，那么他们实际上是同一个atomic_uint
layout (binding = 1, offset = 0) uniform atomic_uint at1; layout (binding = 1, offset = 0) uniform atomic_uint at2;
at1和at2指向的是同一个变量
尽管atomic_uint类型的变量作为uniform来使用，但是它和samplers一样并不能用在Uniform Block之中
示例
atomic_int支持的操作非常有限，仅包含一下几种：
//获取 uint atomicCounter(atomic_uint c); // 减一并返回新值 uint atomicCounterDecrement(atomic_uint c); //加一并返回旧值 uint atomicCounterIncrement(atomic_uint c);
下面这个例子来自OSG Examples，我将它转换成OpenGL的方式实现： 实现的内容：记录一帧绘制中的像素绘制顺序，先绘制的颜色呈现淡黄色，后绘制的颜色黄色逐渐加深直至为(1,1,0)的黄色，实现代码如下：
#pragma comment(lib, "glew32.lib")
#pragma comment(lib, "freeglut.lib")
#include <gl/glew.h>
#include <gl/freeglut.h>
#include <iostream>
#include <vector>
#include <algorithm>
#include "glshadertools.h"
GLuint programID;
GLuint vboID;
GLuint eboID;
GLuint atomicCounterArrayRedAndGreen[2];
GLuint atomicCounterArrayBlue[1];
GLuint acboRedAndGreen;
GLuint acboBlue;
GLfloat invNumPixel;
//////////////////////////////////////////////////////////////////////////
GLfloat vertices[] = {
-1.0f, 1.0f,0.0f,
-1.0f, -1.0f,0.0f,
1.0f, -1.0f,0.0f,
1.0f, 1.0f, 0.0f
};
GLuint indices[] = {
0,1,2,
2,3,0
};
//////////////////////////////////////////////////////////////////////////
GLfloat xRot;
GLfloat yRot;
GLfloat zoom = -10.0f;
bool mouseLeftDown;
float mouseX, mouseY;
void init()
{
programID = gltLoadShaderProgram("atomiccounter.vert", "atomiccounter.frag");
glGenBuffers(1, &vboID);
glBindBuffer(GL_ARRAY_BUFFER, vboID);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glGenBuffers(1, &eboID);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, eboID);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
atomicCounterArrayBlue[0] = 0;
atomicCounterArrayRedAndGreen[0] = 0;
atomicCounterArrayRedAndGreen[1] = 0;
glGenBuffers(1, &acboRedAndGreen);
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, acboRedAndGreen);
glBufferData(GL_ATOMIC_COUNTER_BUFFER, sizeof(GLuint)*2, NULL, GL_STREAM_COPY);
glBufferSubData(GL_ATOMIC_COUNTER_BUFFER, 0, sizeof(GLuint)*2, atomicCounterArrayRedAndGreen);
glBindBufferBase(GL_ATOMIC_COUNTER_BUFFER, 0, acboRedAndGreen);
glGenBuffers(1, &acboBlue);
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, acboBlue);
glBufferData(GL_ATOMIC_COUNTER_BUFFER, sizeof(GLuint), NULL, GL_STREAM_COPY);
glBindBufferBase(GL_ATOMIC_COUNTER_BUFFER, 2, acboBlue);
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, 0);
GLint invNumPixelLocation;
invNumPixelLocation = glGetUniformLocation(programID, "invNumPixel");
glProgramUniform1f(programID, invNumPixelLocation, 1.0f/(800.0f*600.0f));
glUseProgram(programID);
}
void reshape(int w, int h)
{
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60.0f, (float)(w)/h, 0.1f, 1000.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void display()
{
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(1.0f, 0.0f, 1.0f);
glLoadIdentity();
glTranslatef(0, 0, zoom);
glRotatef(xRot, 1, 0, 0); // pitch
glRotatef(yRot, 0, 1, 0); // heading
//更新蓝色成分缓冲区中的数据为0，让绘制结果呈现黄色
GLuint *userCounters;
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, acboBlue);
userCounters = (GLuint*)glMapBufferRange(GL_ATOMIC_COUNTER_BUFFER,
0,
sizeof(GLuint) * 1,
GL_MAP_WRITE_BIT
);
atomicCounterArrayBlue[0] = 0;
glUnmapBuffer(GL_ATOMIC_COUNTER_BUFFER);
//将蓝色成分缓冲区中的数据设置为0
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, acboBlue);
glBufferSubData(GL_ATOMIC_COUNTER_BUFFER, 0, sizeof(GLuint), &atomicCounterArrayBlue);
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, 0);
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_INDEX_ARRAY);
glBindBuffer(GL_ARRAY_BUFFER, vboID);
glVertexPointer(3, GL_FLOAT, 0, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, eboID);
glIndexPointer(GL_UNSIGNED_INT, 0, 0);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_INDEX_ARRAY);
glutSwapBuffers();
//////////////////////////////////////////////////////////////////////////
//一帧结束之后的操作
//获取前一帧绘制的像素总数(记录在蓝色成分的AtomicBuffer之中)
//并将这个值设置给invNumPixel
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, acboBlue);
GLubyte *src = (GLubyte*)glMapBuffer(GL_ATOMIC_COUNTER_BUFFER, GL_READ_ONLY);
if (src)
{
memcpy((void*)atomicCounterArrayBlue, src, 4);
}
glUnmapBuffer(GL_ATOMIC_COUNTER_BUFFER);
unsigned int numPixel = std::max(1u, atomicCounterArrayBlue[0]);
GLint invNumPixelLocation;
invNumPixelLocation = glGetUniformLocation(programID, "invNumPixel");
GLfloat invNumPixelValue;
glGetUniformfv(programID, invNumPixelLocation, &invNumPixelValue);
glProgramUniform1f(programID, invNumPixelLocation, 1.0f / static_cast<float>(numPixel));
//将表示红绿成分的AtomicBuffer设置为0
glBindBuffer(GL_ATOMIC_COUNTER_BUFFER, acboRedAndGreen);
userCounters = (GLuint*)glMapBufferRange(GL_ATOMIC_COUNTER_BUFFER,
0,
sizeof(GLuint) * 2,
GL_MAP_READ_BIT
);
atomicCounterArrayRedAndGreen[0] = 0;
atomicCounterArrayRedAndGreen[1] = 0;
glUnmapBuffer(GL_ATOMIC_COUNTER_BUFFER);
}
void mouse(int button, int state, int x, int y)
{
mouseX = (float)x;
mouseY = (float)y;
switch (button)
{
case GLUT_LEFT_BUTTON:
{
if (state == GLUT_DOWN) {
mouseLeftDown = true;
} else if (state == GLUT_UP) {
mouseLeftDown = false;
}
}
break;
case GLUT_RIGHT_BUTTON:
{
if (state == GLUT_DOWN) {
}
}
break;
default:
break;
}
}
void mouseMove(int x, int y)
{
if(mouseLeftDown)
{
yRot += (x - mouseX);
xRot += (y - mouseY);
mouseX = (float)x;
mouseY = (float)y;
}
glutPostRedisplay();
}
void mouseWheel(int wheel, int direction, int x, int y)
{
switch (direction)
{
case 1: //means wheel up
{
zoom -= 1.0f;
}
break;
case -1: //means wheel down
{
zoom += 1.0f;
}
break;
default:
break;
}
glutPostRedisplay();
}
void keyboard(unsigned char key, int x, int y)
{
switch(key)
{
case 27: // ESCAPE
exit(0);
break;
default:
break;
}
}
void idle()
{
glutPostRedisplay();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
glutInitWindowSize(640, 480);
glutCreateWindow(argv[0]);
if (glewInit()) {
std::cerr << "Unable to initialize GLEW ... exiting" << std::endl;
exit(EXIT_FAILURE);
}
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMouseFunc(mouse);
glutMotionFunc(mouseMove);
glutMouseWheelFunc(mouseWheel);
glutKeyboardFunc(keyboard);
glutIdleFunc(idle);
glutMainLoop();
}