前提
- 来源：《Computer Graphics Programming in OpenGL Using C++ 》by V Scott
Gordon John L Clevenger - 内容：程序5.1 Pyramid with Brick Texture，书P113页，PDF131/403
- 生成：
笔记
cull
- 默认情况下，如果三角形的三个顶点沿逆时针方向前进，则该三角形被认为是正面的
顺时针编号的三角形被认为是反面的，不会被渲染 - Winding order ：逆时针编号的面认为是正面，可以显式写成：
glFrontFace(GL_CCW)
如果设置顺时针编号的面是正面，写成：glFrontFace(GL_CW) - Cull ： 剔除部分三角形不需要渲染，来达到加快渲染的目的
glEnable(GL_CULL_FACE)： 开启cull
glDisable(GL_CULL_FACE)： 关闭cull
glCullFace(GL_BACK)： cull back-facing的三角形。关键词可以替换成GL_FRONT/
GL_FRONT_AND_BACK - back-face culling背面剔除通过确保OpenGL不花费时间光栅化和渲染那些永远不会被看到的表面来提高性能。
加载纹理
- 定义一个函数，加载图片中的纹理返回纹理所在的ID，将函数放在 Utils.cpp中方便调用
GLuint loadTexture(const char *texImagePath) {
GLuint textureID;
textureID = SOIL_load_OGL_texture
(texImagePath, SOIL_LOAD_AUTO, SOIL_CREATE_NEW_ID, SOIL_FLAG_INVERT_Y);
if (textureID == 0) cout << "could not find texture file" << texImagePath << endl;
return textureID; }
调用时，写作：
GLuint myTexture = Utils::loadTexture("image.jpg");
纹理坐标
- 纹理坐标成为 texels，来区分屏幕像素pixels。
点坐标是(x,y,z)，纹理坐标是(s,t) - 四面体的坐标示意图
- 纹理的坐标图示：
- 四面体和纹理的三角形对应关系
纹理图案的坐标：
float pyrTexCoords[36] = {
0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f, // top and right faces
0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f, // back and left faces
0.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f }; // base triangles
着色器部分的修改
- 在块着色（fragShader.glsl）中需要声明一个采样变量，以达到在硬件中处理纹理的目的。
layout (binding=0) uniform sampler2D samp;
这句话意思是，创建一个采样单元 samp，它是0号纹理单元（texture unit 0） - 纹理单元可以采样纹理对象，我们有不同的纹理图案，就要有各自对应的不同的纹理单元。如下，就是将 brickTexture 纹理图案和0号纹理单元进行绑定：
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, brickTexture);
- 在顶点着色器中，读取了纹理图案上的颜色：
tc = texCoord;（verShader.glsl），然后再赋值给四面体颜色，在片段着色器中运行：color = texture(samp, tc); （fragShader.glsl）
完整代码
程序5.1 Pyramid with Brick Texture
文件1：5.1 Texture.cpp
#include <string>
#include <iostream>
#include <fstream>
#include <cmath>
#include "glm\glm.hpp"
#include "glm\gtc\type_ptr.hpp"
#include "glm\gtc\matrix_transform.hpp"
#include "Utils\5.1 Utils.h"
using namespace std;
#define numVAOs 1
#define numVBOs 2
float cameraX, cameraY, cameraZ;
float pyrLocX, pyrLocY, pyrLocZ;
GLuint renderingProgram;
GLuint vao[numVAOs];
GLuint vbo[numVBOs];
GLuint brickTexture; // 纹理图片ID
// allocate variables used in display() function,
// so that they won’t need to be allocated during rendering
GLuint mvLoc, projLoc;
int width, height;
float aspect;
glm::mat4 pMat, vMat, mMat, mvMat;
void setupVertices(void) {
// pyramid with 18 vertices, comprising 6 triangles (four sides, and two on the bottom)
float pyramidPositions[54] =
{ -1.0f, -1.0f, 1.0f, 1.0f, -1.0f, 1.0f, 0.0f, 1.0f, 0.0f, // front face
1.0f, -1.0f, 1.0f, 1.0f, -1.0f, -1.0f, 0.0f, 1.0f, 0.0f, // right face
1.0f, -1.0f, -1.0f, -1.0f, -1.0f, -1.0f, 0.0f, 1.0f, 0.0f, // back face
-1.0f, -1.0f, -1.0f, -1.0f, -1.0f, 1.0f, 0.0f, 1.0f, 0.0f, // left face
-1.0f, -1.0f, -1.0f, 1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, // base – left front
1.0f, -1.0f, 1.0f, -1.0f, -1.0f, -1.0f, 1.0f, -1.0f, -1.0f // base – right back
};
// 设置纹理的顶点对应关系
float pyrTexCoords[36] = {
0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f,// 前面和右面
0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.5f, 1.0f,// 后面和左面
0.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f, 1.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f // 底面的三角形
};
glGenVertexArrays(1, vao);
glBindVertexArray(vao[0]);
glGenBuffers(numVBOs, vbo);
// 四面体
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
glBufferData(GL_ARRAY_BUFFER, sizeof(pyramidPositions), pyramidPositions, GL_STATIC_DRAW);
// 纹理
glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
glBufferData(GL_ARRAY_BUFFER, sizeof(pyrTexCoords), pyrTexCoords, GL_STATIC_DRAW);
}
void init(GLFWwindow* window) {
renderingProgram = createShaderProgram("add/5.1 vertShader.glsl", "add/5.1 fragShader.glsl");
cameraX = 0.0f; cameraY = 0.0f; cameraZ = 8.0f;
pyrLocX = 2.0f; pyrLocY = -2.0f; pyrLocZ = 0.0f; // shift down Y to reveal perspective
setupVertices();
brickTexture = loadTexture("add/brick1.jpg"); // 加载纹理的图片
}
void display(GLFWwindow* window, double currentTime) {
glClear(GL_DEPTH_BUFFER_BIT);
glUseProgram(renderingProgram);
// get the uniform variables for the MV and projection matrices
mvLoc = glGetUniformLocation(renderingProgram, "mv_matrix");
projLoc = glGetUniformLocation(renderingProgram, "proj_matrix");
// build perspective matrix
glfwGetFramebufferSize(window, &width, &height);
aspect = (float)width / (float)height;
pMat = glm::perspective(1.0472f, aspect, 0.1f, 1000.0f); // 1.0472 radians = 60 degrees
// build view matrix, model matrix, and model-view matrix
vMat = glm::translate(glm::mat4(1.0f), glm::vec3(-cameraX, -cameraY, -cameraZ));
// vbo[0]
mMat = glm::translate(glm::mat4(1.0f), glm::vec3(pyrLocX, pyrLocY, pyrLocZ));
mvMat = vMat * mMat;
// copy perspective and MV matrices to corresponding uniform variables
glUniformMatrix4fv(mvLoc, 1, GL_FALSE, glm::value_ptr(mvMat));
glUniformMatrix4fv(projLoc, 1, GL_FALSE, glm::value_ptr(pMat));
// associate VBO with the corresponding vertex attribute in the vertex shader
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(0);
// 纹理
glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(1);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, brickTexture);
// adjust OpenGL settings and draw model
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
glFrontFace(GL_CCW);// 锥体的三角形是逆时针的面认为是正方向
glDrawArrays(GL_TRIANGLES, 0, 18);
}
int main(void) { // main() is unchanged from before
if (!glfwInit()) { exit(EXIT_FAILURE); }
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
GLFWwindow* window = glfwCreateWindow(600, 600, "Chapter 4 - program 31", NULL, NULL);
glfwMakeContextCurrent(window);
if (glewInit() != GLEW_OK) { exit(EXIT_FAILURE); }
glfwSwapInterval(1);
init(window);
while (!glfwWindowShouldClose(window)) {
display(window, glfwGetTime());
glfwSwapBuffers(window);
glfwPollEvents();
}
glfwDestroyWindow(window);
glfwTerminate();
exit(EXIT_SUCCESS);
}
文件2： 5.1 vertShader.glsl
#version 430
layout (location=0) in vec3 pos;
layout (location=1) in vec2 texCoord;
out vec2 tc; // texture coordinate output to rasterizer for interpolation
uniform mat4 mv_matrix;
uniform mat4 proj_matrix;
layout (binding=0) uniform sampler2D samp; // not used in vertex shader
void main(void)
{ gl_Position = proj_matrix * mv_matrix * vec4(pos,1.0);
tc = texCoord;
}
文件3： 5.1 fragShader.glsl
#version 430
in vec2 tc; // interpolated incoming texture coordinate
out vec4 color;
uniform mat4 mv_matrix;
uniform mat4 proj_matrix;
layout (binding=0) uniform sampler2D samp;
void main(void)
{ color = texture(samp, tc);
}
文件4：5.1 Utils.cpp
#include "Utils/5.1 Utils.h"
#include "GL/glew.h"
#include "GLFW/glfw3.h"
#include "SOIL2/SOIL2.h"
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
GLuint createShaderProgram(const char* a_Path, const char* b_Path) {
GLint vertCompiled;
GLint fragCompiled;
GLint linked;
string vertShaderStr = readShaderSource(a_Path); // 文件在add文件夹中
string fragShaderStr = readShaderSource(b_Path);
const char *vertShaderSrc = vertShaderStr.c_str();
const char *fragShaderSrc = fragShaderStr.c_str();
GLuint vShader = glCreateShader(GL_VERTEX_SHADER);
GLuint fShader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(vShader, 1, &vertShaderSrc, NULL);
glShaderSource(fShader, 1, &fragShaderSrc, NULL);
// 在编译着色器时，捕捉错误
glCompileShader(vShader);
checkOpenGLError();
glGetShaderiv(vShader, GL_COMPILE_STATUS, &vertCompiled);
if (vertCompiled != 1) {
cout << "vertex compilation failed" << endl;
printShaderLog(vShader);
}
glCompileShader(fShader);
checkOpenGLError();
glGetShaderiv(fShader, GL_COMPILE_STATUS, &fragCompiled);
if (fragCompiled != 1) {
cout << "fragment compilation failed" << endl;
printShaderLog(fShader);
}
GLuint vfProgram = glCreateProgram();
glAttachShader(vfProgram, vShader);
glAttachShader(vfProgram, fShader);
glLinkProgram(vfProgram);
checkOpenGLError();
glGetProgramiv(vfProgram, GL_LINK_STATUS, &linked);
if (linked != 1) {
cout << "linking failed" << endl;
printProgramLog(vfProgram);
}
return vfProgram;
}
string readShaderSource(const char *filePath) {
string content;
ifstream fileStream(filePath, ios::in);
string line = "";
while (!fileStream.eof()) {
getline(fileStream, line);
content.append(line + "\n");
}
fileStream.close();
return content;
}
bool checkOpenGLError() {
bool foundError = false;
int glErr = glGetError();
while (glErr != GL_NO_ERROR) {
cout << "glError: " << glErr << endl;
foundError = true;
glErr = glGetError();
}
return foundError;
}
void printProgramLog(int prog) {
int len = 0;
int chWrittn = 0;
char *log;
glGetProgramiv(prog, GL_INFO_LOG_LENGTH, &len);
if (len > 0) {
log = (char *)malloc(len);
glGetProgramInfoLog(prog, len, &chWrittn, log);
cout << "Program Info Log: " << log << endl;
free(log);
}
}
void printShaderLog(GLuint shader) {
int len = 0;
int chWrittn = 0;
char *log;
glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &len);
if (len > 0) {
log = (char *)malloc(len);
glGetShaderInfoLog(shader, len, &chWrittn, log);
cout << "Shader Info Log: " << log << endl;
free(log);
}
}
GLuint loadTexture(const char *texImagePath) {
GLuint textureID;
textureID = SOIL_load_OGL_texture(texImagePath,
SOIL_LOAD_AUTO, SOIL_CREATE_NEW_ID, SOIL_FLAG_INVERT_Y);
if (textureID == 0) cout << "could not find texture file" << texImagePath << endl;
return textureID;
}
文件5：5.1 Utils.h
#pragma once
#ifndef UTILS_H
#define UTILS_H
#include <string>
#include "GL\glew.h"
#include "GLFW\glfw3.h"
using namespace std;
GLuint createShaderProgram(const char* a_Path, const char* b_Path);
string readShaderSource(const char *filePath);
bool checkOpenGLError();
void printProgramLog(int prog);
void printShaderLog(GLuint shader);
GLuint loadTexture(const char *texImagePath);
#endif // !UTILS_H