OpenGL学习之路7----插值

本文根据教程：ogldev进行扩充学习，一步步从零开始，记录学习历程

一、插值

插值是3D渲染管线中非常重要的部分，其实就是光栅化程序执行的从顶点着色器传过来的变量。
光栅化程序对三角形三个顶点之间执行插值处理：对三个顶点之间的每个像素执行片元着色器(即fragment shader),片元着色器通过由顶点着色器传来的插值变量确定每个像素的颜色，光栅化程序将颜色存放到颜色缓冲区中。
两种常见的依赖这种插值的变量是三角形法线和纹理坐标，这两个变量都是在顶点着色器中计算完成后传到片元着色器。(法线在以后会用在灯光效果中，纹理坐标用来将纹理覆盖在我们的图形上，都在以后会学习到)

这里可以简单对插值理解为：在顶点着色器(Vertex Shader)中计算出来的一个值，并从顶点着色器中传出，在片元着色器(Fragment Shader)中接收到这个值并使用它

二、程序解释

2.1 opengl_math.h:

#ifndef __OPENGL_MATH_H
#define __OPENGL_MATH_H

#include <math.h>
#include <string.h>

#define PI (3.14159265358979323846)
#define PI_DIV_180 (0.017453292519943296)
#define INV_PI_DIV_180 (57.2957795130823229)

#define DegToRad(x)  ((x)*PI_DIV_180)
#define RadToDeg(x)  ((x)*INV_PI_DIV_180)
//向量       
typedef float   Vector3f[3];                

//向量赋值

inline void LoadVector3(Vector3f v, const float x, const float y, const float z)
{
    v[0] = x; v[1] = y; v[2] = z;
}
//缩放向量
inline void ScaleVector3(Vector3f v, const float scale)
{
    v[0] *= scale; v[1] *= scale; v[2] *= scale;
}
//  4 * 4 矩阵：
//      0       4       8       12
//      1       5       9       13
//      2       6       10      14
//      3       7       11      15
typedef float Matrix44f[16];         
//4*4单位矩阵
inline void LoadIdentity44(Matrix44f m)
{
    m[0] = 1.0f; m[4] = 0.0f; m[8] = 0.0f;  m[12] = 0.0f;
    m[1] = 0.0f; m[5] = 1.0f; m[9] = 0.0f;  m[13] = 0.0f;
    m[2] = 0.0f; m[6] = 0.0f; m[10] = 1.0f; m[14] = 0.0f;
    m[3] = 0.0f; m[7] = 0.0f; m[11] = 0.0f; m[15] = 1.0f;
}
//4*4矩阵相乘
inline void MatrixMultiply44(Matrix44f product, const Matrix44f a, const Matrix44f b)
{
    unsigned int j, k;
    for (unsigned int i = 0; i < 16; i++) {
        j = i % 4;
        k = i / 4 * 4;
        product[i] = a[j] * b[k] + a[j + 4] * b[k + 1] + a[j + 8] * b[k + 2] + a[j + 12] * b[k + 3];
    }
}
//缩放变换
inline void ScaleMatrix44(Matrix44f m, float xScale, float yScale, float zScale)
{
    LoadIdentity44(m); m[0] = xScale; m[5] = yScale; m[10] = zScale;
}
//旋转变换
inline void RotationMatrix44(Matrix44f m, float angle, float x, float y, float z)
{
    LoadIdentity44(m);
    if (z == 1)//绕z轴
    {
        m[0] = cosf(angle); m[4] = -sinf(angle);
        m[1] = sinf(angle); m[5] = cosf(angle);
    }
    else if (y == 1)//绕y轴
    {
        m[0] = cosf(angle); m[8] = -sinf(angle);
        m[2] = sinf(angle); m[10] = cosf(angle);
    }
    else if (x == 1)//绕x轴
    {
        m[5] = cosf(angle); m[9] = -sinf(angle);
        m[6] = sinf(angle); m[10] = cosf(angle);
    }
}
inline void RotationMatrix44(Matrix44f m, float RotateX, float RotateY, float RotateZ)
{
    Matrix44f rx, ry, rz, temp;

    const float x = DegToRad(RotateX);
    const float y = DegToRad(RotateY);
    const float z = DegToRad(RotateZ);

    RotationMatrix44(rx, x, 1, 0, 0);
    RotationMatrix44(ry, y, 0, 1, 0);
    RotationMatrix44(rz, z, 0, 0, 1);

    MatrixMultiply44(temp, rz, ry);
    MatrixMultiply44(m,temp, rx);

}
//平移变换
inline void TranslationMatrix44(Matrix44f m, float x, float y, float z)
{
    LoadIdentity44(m); m[12] = x; m[13] = y; m[14] = z;
}
#endif

3d数学库头文件没有变化，跟上一节相同，如果看不太懂请移步上一节
OpenGL学习之路6—-平移，旋转和缩放变换
这章只是增加了插值变量的使用

2.2 main.cpp

#include <stdio.h>
#include <string>
#include<math.h>
#include <gl/glew.h>
#include <gl/freeglut.h>
#include<fstream>
#include<assert.h>
#include "opengl_math.h"

using namespace std;

GLuint VBO;
GLuint gWorldLocation;

const char* pVSFileName = "shader.vs";
const char* pFSFileName = "shader.fs";

bool ReadFile(const char* FileName, string &outFile)
{
    ifstream f(FileName);
    bool ret = false;

    if (f.is_open()) {
        string line;
        while (getline(f, line)) {
            outFile.append(line);
            outFile.append("\n");
        }
        f.close();
        ret = true;
    }
    else {
        fprintf(stderr, "%s:%d: unable to open file '%s'\n", __FILE__, __LINE__, FileName);
        system("pause");
    }
    return ret;
}

static void Render()
{
    glClear(GL_COLOR_BUFFER_BIT);

    static float Scale = 0.0f;
    Scale += 0.001f;

    Matrix44f World;
    ScaleMatrix44(World, sinf(Scale), sinf(Scale), sinf(Scale));

    glUniformMatrix4fv(gWorldLocation, 1, GL_FALSE, &World[0]);

    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glDisableVertexAttribArray(0);

    glutSwapBuffers();
}

static void InitializeGlutCallbacks()
{
    glutDisplayFunc(Render);
    glutIdleFunc(Render);
}

static void CreateVertexBuffer()
{
    Vector3f Vertices[3];
    LoadVector3(Vertices[0], -1.0f, -1.0f, 0.0f);
    LoadVector3(Vertices[1], 1.0f, -1.0f, 0.0f);
    LoadVector3(Vertices[2], 0.0f, 1.0f, 0.0f);

    glGenBuffers(1, &VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
}

static void AdderShader(GLuint ShaderProgram, const char* pShaderText, GLenum ShaderType)
{
    GLuint ShaderObj = glCreateShader(ShaderType);
    if (!ShaderObj) {
        fprintf(stderr, "Error creating shader ype %d\n", ShaderType);
        system("pause");
        exit(1);
    }

    const GLchar* p[1];
    p[0] = pShaderText;
    GLint Lengths[1];
    Lengths[0] = strlen(pShaderText);
    glShaderSource(ShaderObj, 1, p, Lengths);
    glCompileShader(ShaderObj);
    GLint success;
    glGetShaderiv(ShaderObj, GL_COMPILE_STATUS, &success);
    if (!success) {
        GLchar InfoLog[1024];
        glGetShaderInfoLog(ShaderObj, 1024, NULL, InfoLog);
        fprintf(stderr, "Error compiling shader type %d: '%s'\n", ShaderType,InfoLog);
        system("pause");
        exit(1);
    }
    glAttachShader(ShaderProgram, ShaderObj);
}

static void CompileShaders()
{
    GLuint ShaderProgram = glCreateProgram();
    if (ShaderProgram == 0) {
        fprintf(stderr, "Error creating shader program\n");
        system("pause");
        exit(1);
    }

    string vs, fs;

    if (!ReadFile(pVSFileName, vs)) {
        exit(1);
    }
    if (!ReadFile(pFSFileName, fs)) {
        exit(1);
    }

    AdderShader(ShaderProgram,vs.c_str(),GL_VERTEX_SHADER);
    AdderShader(ShaderProgram, fs.c_str(), GL_FRAGMENT_SHADER);

    GLint Success = 0;
    GLchar ErrorLog[1024] = { 0 };

    glLinkProgram(ShaderProgram);
    glGetProgramiv(ShaderProgram, GL_LINK_STATUS, &Success);
    if (!Success) {
        glGetProgramInfoLog(ShaderProgram, sizeof(ErrorLog), NULL, ErrorLog);
        fprintf(stderr, "Error linking shader program: '%s'\n", ErrorLog);
        exit(1);
    }

    glValidateProgram(ShaderProgram);
    glGetProgramiv(ShaderProgram, GL_VALIDATE_STATUS, &Success);
    if (!Success) {
        glGetProgramInfoLog(ShaderProgram, sizeof(ErrorLog), NULL, ErrorLog);
        fprintf(stderr, "Invalid shader program: '%s'\n", ErrorLog);
        exit(1);
    }

    glUseProgram(ShaderProgram);

    gWorldLocation = glGetUniformLocation(ShaderProgram, "gWorld");
    assert(gWorldLocation != 0xFFFFFFFF);
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
    glutInitWindowPosition(10, 10);
    glutInitWindowSize(1024, 768);
    glutCreateWindow("Interpolation");

    InitializeGlutCallbacks();

    GLenum res = glewInit();
    if (res != GLEW_OK) {
        fprintf(stderr, "Error: '%s'\n", glewGetErrorString(res));
        system("pause");
        return 1;
    }

    printf("GL version: %s \n", glGetString(GL_VERSION));

    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    CreateVertexBuffer();

    CompileShaders();

    glutMainLoop();

    return 0;

}

主程序也没有什么改变，唯一值得一提的是我们回调的渲染函数中

static float Scale = 0.0f;
Scale += 0.001f;

Matrix44f World;
ScaleMatrix44(World, sinf(Scale), sinf(Scale), sinf(Scale));

这里可以看出来，我们会让我们绘制的图形进行一个缩放的工作

2.3 着色器

2.3.1 shder.vs(顶点着色器)

#version 330

layout (location=0) in vec3 Position;

uniform mat4 gWorld;

out vec4 Color;

void main()
{
    gl_Position = gWorld * vec4(Position,1.0);

    Color=vec4(clamp(Position,0.0,1.0),1.0);
}

可见声明了一个四维的输出向量Color
Color的值,XYZ三个分量由Position获得，W分量设为1.0
clamp()这个内置函数保证数值在0.0到1.0范围之内
三角行左下角顶点坐标是(-1,-1)，使用clamp()函数会被转化乘0，所以左下角会呈现黑色

2.3.1 shder.fs(片元着色器)

#version 330

in vec4 Color;

out vec4 FragColor;

void main()
{
    FragColor=Color;
}

声明了一个输入四维向量Color，即是接收刚才顶点着色器传来的输出变量
FragColor=Color把这个颜色变量直接传递下去

三、运行结果

这里写图片描述

可以看到三角形根据位置不同呈现不同的颜色，并且不断的缩放放大