OpenGL---PBO

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PBO(Pixel Buffer Object),将像素数据存储在显存中。

优点：

1、快速的像素数据传递，它采用了一种叫DMA（Direct Memory Access）的技术，无需CPU介入

2、高效并不在于传输更快，而在于与硬件相关的异步调用方式，调用之后CPU即返回执行其它操作（使用DMA方式的传输、由OpenGL直接控制）

3、在单个PBO情况下并不能得到很好的效果，毕竟传输过程仍然存在（但速度可能变快，比如显存内部的数据传输），但其异步性就提供了双PBO实现的可能性，用双PBO来进行加速

左图是传统的加载一个纹理的示意图，可以看到，首先将纹理图像读到内存，然后再从内存拷贝到纹理对象中，这两个过程均需CPU的参与；而右图采用了PBO加载纹理，首先将像素数据加载到PBO中，然后通过DMA方式传送到纹理对象，最后一步不需要CPU参与。

Conventional texture loading Texture loading with PBO

创建PBO:

1. 用glGenBuffersARB()生成缓存对象；
2. 用glBindBufferARB()绑定缓存对象；
3. 用glBufferDataARB()将像素数据拷贝到缓存对象。

映射PBO:

void* glMapBufferARB(GLenum target, GLenum access);
GLboolean glUnmapBufferARB(GLenum target);

几个参数的释疑：

GL_PIXEL_PACK_BUFFER_ARB 将像素数据传给PBO
GL_PIXEL_UNPACK_BUFFER_ARB 从PBO得到像素数据

比如说，glReadPixel就是从帧缓存中读取数据，写到PBO中，可理解为“pack”；glDrawPixel是从PBO中读取数据，写到到帧缓存，可理解为“unpack”；glGetTexImage是从纹理对象到PBO，可理解为“pack”；glTexImage2d从PBO写到纹理对象（texture object），可理解为“unpack”。

[cpp] view plaincopy

#include <iostream>
#include <gl/glew.h>
#include <gl/glut.h>
using namespace std;
GLuint pboIds[2];
GLuint textureId; // Storage For 6 face Textures
//camera
float cameraAngleX;
float cameraAngleY;
float cameraDistance = -5.0;
//mouse
bool mouseLeftDown;
bool mouseRightDown;
float LastXPos;
float LastYPos;
const int IMAGE_WIDTH = 1024;
const int IMAGE_HEIGHT = 1024;
const int CHANNEL_COUNT = 4;
const int DATA_SIZE = IMAGE_WIDTH * IMAGE_HEIGHT * CHANNEL_COUNT;
GLubyte* imageData = 0;
void initGL()
{
glEnable(GL_TEXTURE_2D); // Enable Texture Mapping
glShadeModel(GL_SMOOTH); // Enable Smooth Shading
glClearColor(0.0f, 0.0f, 0.0f, 0.5f); // Black Background
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, IMAGE_WIDTH, IMAGE_HEIGHT, 0, GL_BGRA, GL_UNSIGNED_BYTE, (GLvoid*)imageData);
glBindTexture(GL_TEXTURE_2D, 0);
glGenBuffers(2, pboIds);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pboIds[0]);
glBufferData(GL_PIXEL_UNPACK_BUFFER, DATA_SIZE, 0, GL_STREAM_DRAW);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pboIds[1]);
glBufferData(GL_PIXEL_UNPACK_BUFFER, DATA_SIZE, 0, GL_STREAM_DRAW);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
}
void updatePixels(GLubyte* dst, int size)
{
static int color = 0;
if(!dst)
return;
int* ptr = (int*)dst;
// copy 4 bytes at once
for(int i = 0; i < IMAGE_HEIGHT; ++i)
{
for(int j = 0; j < IMAGE_WIDTH; ++j)
{
*ptr = color;
ptr++;
}
color += 257; // add an arbitary number (no meaning)
}
++color; // scroll down
}
void display()
{
static int index = 0;
int nextIndex = 0;
index = (index + 1) % 2;
nextIndex = (index + 1) % 2;
glBindTexture(GL_TEXTURE_2D, textureId);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pboIds[index]);
// copy pixels from PBO to texture object
// Use offset instead of ponter.
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, IMAGE_WIDTH, IMAGE_HEIGHT, GL_BGRA, GL_UNSIGNED_BYTE, 0);
// bind PBO to update pixel values
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pboIds[nextIndex]);
glBufferData(GL_PIXEL_UNPACK_BUFFER, DATA_SIZE, 0, GL_STREAM_DRAW);
GLubyte* ptr = (GLubyte*)glMapBuffer(GL_PIXEL_UNPACK_BUFFER, GL_WRITE_ONLY);
if(ptr)
{
// update data directly on the mapped buffer
updatePixels(ptr, DATA_SIZE);
glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER); // release pointer to mapping buffer
}
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
glClear(GL_COLOR_BUFFER_BIT);
glPushMatrix();
// tramsform camera
glTranslatef(0, 0, cameraDistance);
glRotatef(cameraAngleX, 1, 0, 0); // pitch
glRotatef(cameraAngleY, 0, 1, 0); // heading
glBindTexture(GL_TEXTURE_2D, textureId);
glColor4f(1, 1, 1, 1);
glBegin(GL_QUADS);
glNormal3f(0, 0, 1);
glTexCoord2f(0.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 0.0f);
glTexCoord2f(1.0f, 0.0f); glVertex3f( 1.0f, -1.0f, 0.0f);
glTexCoord2f(1.0f, 1.0f); glVertex3f( 1.0f, 1.0f, 0.0f);
glTexCoord2f(0.0f, 1.0f); glVertex3f(-1.0f, 1.0f, 0.0f);
glEnd();
glBindTexture(GL_TEXTURE_2D, 0);
glPopMatrix();
glutSwapBuffers();
}
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60.0f, (GLfloat)w/(GLfloat)h, 0.01f, 100.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void mouse(int button, int state, int x, int y)
{
if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
{
mouseLeftDown = true;
LastXPos = x;
LastYPos = y;
}
if (button == GLUT_RIGHT_BUTTON && state == GLUT_UP)
{
mouseRightDown = true;
}
}
void mouseMotion(int x, int y)
{
if (mouseLeftDown)
{
cameraAngleX += GLfloat(y - LastYPos)/GLfloat(20.0);
cameraAngleY += GLfloat(x - LastXPos)/GLfloat(20.0);
LastYPos = y;
LastXPos = x;
}
}
int main(int argc, char **argv)
{
imageData = new GLubyte[DATA_SIZE];
memset(imageData, 0, DATA_SIZE);
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowSize(480, 640);
glutInitWindowPosition(100, 100);
glutCreateWindow("PBO");
glewInit();
if(!glewIsSupported("GL_VERSION_2_0"))
{
fprintf(stderr, "ERROR: Support for necessary OpengGL extensions missing.");
}
initGL();
glutDisplayFunc(display);
glutIdleFunc(display);
glutReshapeFunc(reshape);
glutMouseFunc(mouse);
glutMotionFunc(mouseMotion);
glutMainLoop();
glDeleteBuffers(2, pboIds);
glDeleteTextures(1, &textureId);
delete []imageData;
return 0;
}

参考：http://www.songho.ca/opengl/gl_pbo.html

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