opengl中TGA图像加载技术

TGA格式图像是游戏中十分常见的一种图像格式，所以有必要了解其内部格式以及编程实现。

TGA图像一般有非压缩和压缩两种格式，下面分别进行介绍。

一、非压缩TGA图像

注：前面的标记绿色的部分（共12字节）表示对于所有的非压缩TGA格式图像值都是相同的！所以通常用来在读取数据时鉴别是否为TGA图像。

下面的程序实现了绘制一个立方体，并进行纹理贴图。

需要注意的是：TGA图像中数据存放的顺序是BGR(A)，而在OpenGL中顺序是RGB(A)，所以在进行纹理生成的时候必须先进行格式的转化。

在OpenGL中只能加载24位或者32位的TGA图像生成纹理。

TGATexture.h定义了一些结构体以及函数声明：

[cpp] view plain copy print?

#ifndef TGATEXTURE_H
#define TGATEXTURE_H
#include <GL/glut.h>
#include <iostream>
using namespace std;
//纹理结构体定义
typedef struct
{
GLubyte *p_w_picpathData;//图像数据
GLuint bpp;//像素深度
GLuint width;//图像宽度
GLuint height;//图像高度
GLuint texID;//对应的纹理ID
}TextureImage;
//加载TGA图像，生成纹理
bool LoadTGA(TextureImage *texture,char *fileName);
#endif

TGATexture.cpp则包含加载TGA图像生成纹理的函数具体实现：

[cpp] view plain copy print?

#include "TGATexture.h"
//加载TGA图像（无压缩格式），生成纹理
bool LoadTGA(TextureImage *texture, char *filename) // Loads A TGA File Into Memory
{
GLubyte TGAheader[12]={0,0,2,0,0,0,0,0,0,0,0,0}; // Uncompressed TGA Header
GLubyte TGAcompare[12]; // Used To Compare TGA Header
GLubyte header[6]; // First 6 Useful Bytes From The Header
GLuint bytesPerPixel; // Holds Number Of Bytes Per Pixel Used In The TGA File
GLuint p_w_picpathSize; // Used To Store The Image Size When Setting Aside Ram
GLuint temp; // Temporary Variable
GLuint type=GL_RGBA; // Set The Default GL Mode To RBGA (32 BPP)
FILE *file = fopen(filename, "rb"); // Open The TGA File
if( file==NULL || // Does File Even Exist?
fread(TGAcompare,1,sizeof(TGAcompare),file)!=sizeof(TGAcompare) || // Are There 12 Bytes To Read?
memcmp(TGAheader,TGAcompare,sizeof(TGAheader))!=0 || // Does The Header Match What We Want?
fread(header,1,sizeof(header),file)!=sizeof(header)) // If So Read Next 6 Header Bytes
{
if (file == NULL) // Did The File Even Exist? *Added Jim Strong*
return false; // Return False
else
{
fclose(file); // If Anything Failed, Close The File
return false; // Return False
}
}
texture->width = header[1] * 256 + header[0]; // Determine The TGA Width (highbyte*256+lowbyte)
texture->height = header[3] * 256 + header[2]; // Determine The TGA Height (highbyte*256+lowbyte)
//OpenGL中纹理只能使用24位或者32位的TGA图像
if( texture->width <=0 || // Is The Width Less Than Or Equal To Zero
texture->height <=0 || // Is The Height Less Than Or Equal To Zero
(header[4]!=24 && header[4]!=32)) // Is The TGA 24 or 32 Bit?
{
fclose(file); // If Anything Failed, Close The File
return false; // Return False
}
texture->bpp = header[4]; // Grab The TGA's Bits Per Pixel (24 or 32)
bytesPerPixel = texture->bpp/8; // Divide By 8 To Get The Bytes Per Pixel
p_w_picpathSize = texture->width*texture->height*bytesPerPixel; // Calculate The Memory Required For The TGA Data
texture->p_w_picpathData=(GLubyte *)malloc(p_w_picpathSize); // Reserve Memory To Hold The TGA Data
if( texture->p_w_picpathData==NULL || // Does The Storage Memory Exist?
fread(texture->p_w_picpathData, 1, p_w_picpathSize, file)!=p_w_picpathSize) // Does The Image Size Match The Memory Reserved?
{
if(texture->p_w_picpathData!=NULL) // Was Image Data Loaded
free(texture->p_w_picpathData); // If So, Release The Image Data
fclose(file); // Close The File
return false; // Return False
}
//RGB数据格式转换，便于在OpenGL中使用
for(GLuint i=0; i<int(p_w_picpathSize); i+=bytesPerPixel) // Loop Through The Image Data
{ // Swaps The 1st And 3rd Bytes ('R'ed and 'B'lue)
temp=texture->p_w_picpathData[i]; // Temporarily Store The Value At Image Data 'i'
texture->p_w_picpathData[i] = texture->p_w_picpathData[i + 2]; // Set The 1st Byte To The Value Of The 3rd Byte
texture->p_w_picpathData[i + 2] = temp; // Set The 3rd Byte To The Value In 'temp' (1st Byte Value)
}
fclose (file); // Close The File
// Build A Texture From The Data
glGenTextures(1, &texture[0].texID); // Generate OpenGL texture IDs
glBindTexture(GL_TEXTURE_2D, texture[0].texID); // Bind Our Texture
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); // Linear Filtered
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); // Linear Filtered
if (texture[0].bpp==24) // Was The TGA 24 Bits
{
type=GL_RGB; // If So Set The 'type' To GL_RGB
}
glTexImage2D(GL_TEXTURE_2D, 0, type, texture[0].width, texture[0].height, 0, type, GL_UNSIGNED_BYTE, texture[0].p_w_picpathData);
return true; // Texture Building Went Ok, Return True
}