多边形常见操作
android api sample中的region 操作如下图：
本文基于opengl 的stencil buffer 实现这几种多边形运算。
OpenGL 模板测试实现
如下图：
如上图，
Source为原始半透明重叠多边形绘制效果，重叠部分因为融合的缘故 有增强效果。
Union
联合操作，需保证每个像素 当且仅当只属于一个多边形，为此通过模板测试实现的具体思路：
- 用0x00 清除模板缓冲区，开启模板测试
- 写模板缓冲区，同时进行模板测试，同时写颜色缓冲区：保证只有模板值为0的片源可以通过模板测试，而且通过模板测试后 模板值加1 操作。
Difference
做差操作，是两个矩形相减效果，顺序无关，具体实现：
- 0x00 清除模板缓冲区
- 禁止颜色缓冲区写入，禁止模板测试，画任意一个矩形，将模板值加1 操作。此时末班缓冲区中第一个矩形绘制区域为mask区域，该区域内不能绘图。
- 开启模板测试，只能在模板值为0x00的地方绘图，画第二个矩形，模板测试通过后模板值加1。
XOr
异或操作，两个矩形相交的区域不显示，不相交的区域显示，具体思路：
- 0x00 清除模板缓冲区，
- 禁掉模板测试，禁掉颜色缓冲区写入，将模板值加1操作，绘制两个矩形，相交区域的模板值为2，其他矩形区域模板值为1。
- 开启模板测试，允许写入颜色缓冲区，测试条件为模板值等于1，模板值为8位，此时掩码为0x01，二进制为00000001。绘制两个矩形。模板测试通过后将模板值置为0。
Intersection
相交操作，只显示两个矩形相交区域，具体思路：
- 0x00 清除模板缓冲区，
- 禁掉模板测试，禁掉颜色缓冲区写入，将模板值加1操作，绘制两个矩形，相交区域的模板值为2，其他矩形区域模板值为1。
- 开启模板测试，允许写入颜色缓冲区，测试条件为模板值等于2，模板值为8位，此时掩码为0x03，二进制为00000011。绘制任意一个矩形。模板测试通过后将模板值置为0。
GLUT框架下实现：
// @module: stencil render
// @file: opacityRectRender.cpp
// @author: peteryfren
// @date: 2013/3/28
// @version: 1.0
// @desc: 通过模板缓冲区实现半透明矩形的常见操作。
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <time.h>
#include <math.h>
#include <gl/glut.h>
const int windowWidth = 256;
const int windowHeight = 256;
GLenum checkForError(char *loc);
void opacitySourceRender(int width, int height);
void opacityRectXorRender(int width, int height);
void opacityRectUnionRender(int width, int height);
void opacityRectIntersectRender(int width, int height);
void opacityRectDifferenceRender(int width, int height);
void Redraw(void)
{
glCullFace(GL_BACK);
float destAlpha = 0.5;
glClearColor(0,0,0, 0.5); // 指定清理颜色值
glClearStencil(0x00); // 指定清理帧缓冲值
glClearDepth(0.0); // 指定清理深度缓冲值
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
// glEnable (GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
opacitySourceRender(windowWidth, windowHeight);
// opacityRectDifferenceRender(windowWidth, windowHeight);
// opacityRectIntersectRender(windowWidth, windowHeight);
// opacityRectUnionRender(windowWidth, windowHeight);
// opacityRectXorRender(windowWidth, windowHeight);
glutSwapBuffers();
checkForError("swap");
}
void opacitySourceRender(int width, int height)
{
// one way to solve this in generic OpenGL is to
// fill the framebuffer with your desired alpha channel,
// switch the blending mode to glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_DST_ALPHA)
// and draw the rectangles.
glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_DST_ALPHA);
float tileVertices[] = {0,height/2, width,height/2, width,height*3/4, 0,height*3/4};
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
float tileVertices1[] = {width/4,0, width/2,0, width/2,height, width/4,height};
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
}
void opacityRectDifferenceRender(int width, int height)
{
glEnable(GL_STENCIL_TEST);
// draw mask area
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
glStencilFunc(GL_ALWAYS, 0, 0xFF); // 禁模板测试
glStencilOp(GL_KEEP, GL_KEEP, GL_INCR); // 模板值加1
glColor4f(1, 0, 0, 0.1);
float tileVertices[] = {0,height/2, width,height/2, width,height*3/4, 0,height*3/4};
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
//static unsigned char data[windowWidth*windowHeight];
//glReadPixels(0, 0, windowWidth, windowHeight, GL_STENCIL_INDEX, GL_UNSIGNED_BYTE, data);
//SaveFileGrayBMP("stencil.bmp", data, windowWidth, windowHeight);
// draw main area
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glStencilFunc(GL_EQUAL, 0x00, 0x01); // 只能在模板为0的地方绘图
glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
glColor4f(1, 0, 0, 0.5);
float tileVertices1[] = {width/4,0, width/2,0, width/2,height, width/4,height};
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
glDisable(GL_STENCIL_TEST);
}
void opacityRectXorRender(int width, int height)
{
glEnable(GL_STENCIL_TEST);
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
// draw mask area
glStencilOp(GL_INCR, GL_INCR, GL_INCR);
glStencilFunc(GL_ALWAYS, 0x00, 0xFF); // 禁用模板
float tileVertices[] = {0,height/2, width,height/2, width,height*3/4, 0,height*3/4};
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 0, 0, 0.5);
float tileVertices1[] = {width/4,0, width/2,0, width/2,height, width/4,height};
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
//static unsigned char data[windowWidth*windowHeight];
//glReadPixels(0, 0, windowWidth, windowHeight, GL_STENCIL_INDEX, GL_UNSIGNED_BYTE, data);
//SaveFileGrayBMP("stencil.bmp", data, windowWidth, windowHeight);
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glStencilFunc(GL_EQUAL, 0x01, 0x01); // 模板值为1的区域才绘制
glStencilOp(GL_ZERO, GL_ZERO, GL_ZERO); // 用0替换当前值
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
glDisable(GL_STENCIL_TEST);
}
void opacityRectIntersectRender(int width, int height)
{
glEnable(GL_STENCIL_TEST);
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
// draw mask area
glStencilOp(GL_INCR, GL_INCR, GL_INCR);
glStencilFunc(GL_ALWAYS, 0x00, 0xFF); // 禁用模板
float tileVertices[] = {0,height/2, width,height/2, width,height*3/4, 0,height*3/4};
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 0, 0, 0.5);
float tileVertices1[] = {width/4,0, width/2,0, width/2,height, width/4,height};
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
//static unsigned char data[windowWidth*windowHeight];
//glReadPixels(0, 0, windowWidth, windowHeight, GL_STENCIL_INDEX, GL_UNSIGNED_BYTE, data);
//SaveFileGrayBMP("stencil.bmp", data, windowWidth, windowHeight);
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glStencilFunc(GL_EQUAL, 0x02, 0x03); // 模板值为2的区域才绘制 掩码0000 0011
glStencilOp(GL_ZERO, GL_ZERO, GL_ZERO);
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
glDisable(GL_STENCIL_TEST);
}
void opacityRectUnionRender(int width, int height)
{
// 模板缓冲区就一个字节
glEnable (GL_STENCIL_TEST);
// glStencilFunc(GL_ALWAYS,0,0xFF); // 禁用模板测试
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE); // 开启写入颜色缓冲区
// fail = KEEP 模版测试失败后保持模板缓冲区中值 不变
// zfail = KEEP 深度测试失败后保持模版缓冲区中值 不变
// zpass = INCR 深度测试通过后对模版缓冲区中值 加1操作
glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
// glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
// func = EQUAL 只有与参考值相等才能通过模版测试
// ref = 0x00 参考值
// mask = 0x01 只比较最低位
glStencilFunc(GL_EQUAL, 0x0, 0x1); // 开启模板测试
glEnable (GL_BLEND); // 开启混合
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// sfactor = 1 - dstA
// dfactor = dstA
// glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_DST_ALPHA);
float tileVertices[] = {0,height/2, width,height/2, width,height*3/4, 0,height*3/4};
glColor4f(1, 0, 0, 0.5);
glVertexPointer(2, GL_FLOAT, 0, tileVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 0, 0, 0.5);
float tileVertices1[] = {width/4,0, width/2,0, width/2,height, width/4,height};
glVertexPointer(2, GL_FLOAT, 0, tileVertices1);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glColor4f(1, 1, 1, 1);
//static unsigned char data[windowWidth*windowHeight];
//glReadPixels(0, 0, windowWidth, windowHeight, GL_STENCIL_INDEX, GL_UNSIGNED_BYTE, data);
//SaveFileGrayBMP("stencil.bmp", data, windowWidth, windowHeight);
glDisable (GL_STENCIL_TEST);
}
void Button(int button, int state, int x, int y)
{
if (state != GLUT_UP)
return;
switch (button)
{
case GLUT_LEFT_BUTTON:
glutPostRedisplay();
break;
case GLUT_RIGHT_BUTTON:
glutPostRedisplay();
break;
}
}
void Keyboard(unsigned char key, int x, int y)
{
glutPostRedisplay();
}
void Reshape(int width, int height)
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, (GLfloat) windowWidth, 0.0f,
(GLfloat) windowHeight, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glViewport(0, 0, windowWidth, windowHeight);
}
int main(int argc, char *argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE|GLUT_DEPTH|GLUT_STENCIL);
glutInitWindowSize(windowWidth, windowHeight);
glutCreateWindow("Region");
// set up world space to screen mapping
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, (GLfloat) windowWidth, 0.0f,
(GLfloat) windowHeight, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glViewport(0, 0, windowWidth, windowHeight);
glClearColor(1.0, 1.0, 1.0, 1.0);
glutDisplayFunc(Redraw);
glutReshapeFunc(Reshape);
glutMouseFunc(Button);
glutKeyboardFunc(Keyboard);
glEnable(GL_CULL_FACE);
glutMainLoop();
return 0;
}
GLenum checkForError(char *loc)
{
GLenum errCode;
const GLubyte *errString;
if ((errCode = glGetError()) != GL_NO_ERROR)
{
errString = gluErrorString(errCode);
printf("OpenGL error: %s",errString);
if (loc != NULL)
printf("(%s)",loc);
printf("\n");
}
return errCode;
}
参考
1. opengl 模板测试理论：
http://blog.youkuaiyun.com/xiajun07061225/article/details/7388417
2. glStencilFunc函数讲解：
http://blog.youkuaiyun.com/doing5552/article/details/2297280
3. opencsg
http://www.opencsg.org/
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