OpenGL 加载DDS文件（压缩纹理）

想必很多人都见过DDS这种文件，它是一个“图片文件”，如果你安装了某些看图软件，你可以直接双击打开它来进行预览。

那么，这种DDS文件和我们常见的TGA/PNG之类的文件有何不同呢？

DDS和TGA/PNG/JPG之类的“图片文件” 一样，支持“压缩”，减少磁盘空间占用（把文件变小）。

通常我们要加载一个TGA或者PNG文件到OpenGL的时候，都要先把文件数据还原成RGB格式的像素数据，然后用glTexImage2D把像素数据传到显存。这个过程相当于“解压”，这通常非常消耗CPU资源，速度较慢。

但是DDS的压缩数据不需要“解压”就能直接传到显存，而且传到显存之后也不会解压，这极大减少了显存的使用量，并且提高了纹理加载速度，有绝对的优势。我们只需要读取好压缩数据，然后使用glCompressedTexImage2D（代替glTexImage2D）就可以直接把压缩数据传到显存，完成加载。

DDS可以保存许多种格式的像素数据，这里只讲最常用的3种（DXT1、DXT3、DXT5）。

* 当然DDS文件也能存储不压缩的像素数据。

为了在OpenGL中使用DDS压缩纹理（下文简称压缩纹理），我们需要一下2个OpenGL扩展：

GL_ARB_texture_compression

提供函数 “glCompressedTexImage2D”

GL_EXT_texture_compression_s3tc

提供以下格式的压缩纹理支持：
GL_COMPRESSED_RGB_S3TC_DXT1_EXT
GL_COMPRESSED_RGBA_S3TC_DXT3_EXT
GL_COMPRESSED_RGBA_S3TC_DXT5_EXT

完整的加载过程代码：

#include <stdio.h>
#include <gl/glut.h>
#include <gl/glext.h>

// Minimum and maximum macros
#define max(a,b) (((a) > (b)) ? (a) : (b))
#define min(a,b) (((a) < (b)) ? (a) : (b))

PFNGLCOMPRESSEDTEXIMAGE2DARBPROC glCompressedTexImage2DARB = NULL;

#pragma region DDS

#define DDPF_ALPHAPIXELS    0x000001
#define DDPF_ALPHA            0x000002
#define DDPF_FOURCC            0x000004
#define DDPF_RGB            0x000040
#define DDPF_YUV            0x000200
#define DDPF_LUMINANCE        0x020000

#define D3DFMT_DXT1    (('D'<<0)|('X'<<8)|('T'<<16)|('1'<<24))
#define D3DFMT_DXT3    (('D'<<0)|('X'<<8)|('T'<<16)|('3'<<24))
#define D3DFMT_DXT5    (('D'<<0)|('X'<<8)|('T'<<16)|('5'<<24))

typedef struct
{
    DWORD    dwSize;
    DWORD    dwFlags;
    DWORD    dwFourCC;
    DWORD    dwRGBBitCount;
    DWORD    dwRBitMask;
    DWORD    dwGBitMask;
    DWORD    dwBBitMask;
    DWORD    dwABitMask;
} DDS_PIXELFORMAT;

#define DDSD_CAPS            0x000001
#define DDSD_HEIGHT            0x000002
#define DDSD_WIDTH            0x000004
#define DDSD_PITCH            0x000008
#define DDSD_PIXELFORMAT    0x001000
#define DDSD_MIPMAPCOUNT    0x020000
#define DDSD_LINEARSIZE        0x080000
#define DDSD_DEPTH            0x800000

typedef struct
{
    DWORD            dwSize;
    DWORD            dwFlags;
    DWORD            dwHeight;
    DWORD            dwWidth;
    DWORD            dwPitchOrLinearSize;
    DWORD            dwDepth;
    DWORD            dwMipMapCount;
    DWORD            dwReserved1[];
    DDS_PIXELFORMAT    ddspf;
    DWORD            dwCaps;
    DWORD            dwCaps2;
    DWORD            dwCaps3;
    DWORD            dwCaps4;
    DWORD            dwReserved2;
} DDS_HEADER;

typedef struct
{
    DWORD        dwMagic;
    DDS_HEADER    Header;
} DDS_FILEHEADER;

// For a compressed texture, the size of each mipmap level image is typically one-fourth the size of the previous, with a minimum of 8 (DXT1) or 16 (DXT2-5) bytes (for
// square textures). Use the following formula to calculate the size of each level for a non-square texture:
#define SIZE_OF_DXT1(width, height)    ( max(1, ( (width + 3) >> 2 ) ) * max(1, ( (height + 3) >> 2 ) ) * 8 )
#define SIZE_OF_DXT2(width, height)    ( max(1, ( (width + 3) >> 2 ) ) * max(1, ( (height + 3) >> 2 ) ) * 16 )

#pragma endregion

GLuint gl_load_dds(GLvoid *pBuffer)
{
    DDS_FILEHEADER    *header;
    DWORD            compressFormat;
    GLuint            texnum;
    GLvoid            *data;
    GLsizei            imageSize;

    header = (DDS_FILEHEADER *)pBuffer;

    if (header->dwMagic != 0x20534444) {
        printf("bad dds file\n");
        return ;
    }

    if (header->Header.dwSize != ) {
        printf("bad header size\n");
        return ;
    }

    if (!(header->Header.dwFlags & DDSD_LINEARSIZE)) {
        printf("bad file type\n");
        return ;
    }

    if (!(header->Header.ddspf.dwFlags & DDPF_FOURCC)) {
        printf("bad pixel format\n");
        return ;
    }

    compressFormat = header->Header.ddspf.dwFourCC;

    if (compressFormat != D3DFMT_DXT1 &&
        compressFormat != D3DFMT_DXT3 &&
        compressFormat != D3DFMT_DXT5) {
            printf("bad compress format\n");
            return ;
    }

    data = (GLvoid *)(header + );    // header data skipped

    glGenTextures(, &texnum);
    glBindTexture(GL_TEXTURE_2D, texnum);

    switch (compressFormat)
    {
    case D3DFMT_DXT1:
        imageSize = SIZE_OF_DXT1(header->Header.dwWidth, header->Header.dwHeight);
        glCompressedTexImage2DARB(GL_TEXTURE_2D, , GL_COMPRESSED_RGB_S3TC_DXT1_EXT, header->Header.dwWidth, header->Header.dwHeight, , imageSize, data);
        break;
    case D3DFMT_DXT3:
        imageSize = SIZE_OF_DXT2(header->Header.dwWidth, header->Header.dwHeight);
        glCompressedTexImage2DARB(GL_TEXTURE_2D, , GL_COMPRESSED_RGBA_S3TC_DXT3_EXT, header->Header.dwWidth, header->Header.dwHeight, , imageSize, data);
        break;
    case D3DFMT_DXT5:
        imageSize = SIZE_OF_DXT2(header->Header.dwWidth, header->Header.dwHeight);
        glCompressedTexImage2DARB(GL_TEXTURE_2D, , GL_COMPRESSED_RGBA_S3TC_DXT5_EXT, header->Header.dwWidth, header->Header.dwHeight, , imageSize, data);
        break;
    }

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glBindTexture(GL_TEXTURE_2D, );

    return texnum;
}

GLuint g_texnum;

void load_textures(void)
{
    FILE    *fp;
    int        size;
    void    *data;

    fp = fopen("028_dxt5.dds", "rb");
    if (!fp) {
        return;
    }

    fseek(fp, , SEEK_END);
    size = ftell(fp);
    fseek(fp, , SEEK_SET);

    data = malloc(size);
    if (!data) {
        fclose(fp);
        return;
    }

    if (fread(data, size, , fp) != ) {
        free(data);
        fclose(fp);
        return;
    }

    fclose(fp);

    // Load DDS to GL texture
    g_texnum = gl_load_dds(data);

    free(data);
}

void init(void)
{
    // GL_ARB_texture_compression
    // GL_EXT_texture_compression_s3tc
    glCompressedTexImage2DARB = (PFNGLCOMPRESSEDTEXIMAGE2DARBPROC)wglGetProcAddress("glCompressedTexImage2DARB");

    load_textures();
    glClearColor(, , , );
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    RECT rc;
    rc.left = ;
    rc.top = ;
    rc.right = rc.left + ;
    rc.bottom = rc.top + ;

    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, g_texnum);

    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glColor4f(, , , );

    glBegin(GL_QUADS);
        glTexCoord2f(, );
        glVertex2f(rc.left, rc.top);
        glTexCoord2f(, );
        glVertex2f(rc.right, rc.top);
        glTexCoord2f(, );
        glVertex2f(rc.right, rc.bottom);
        glTexCoord2f(, );
        glVertex2f(rc.left, rc.bottom);
    glEnd();

    glutSwapBuffers();
    glutPostRedisplay();
}

void reshape(int width, int height)
{
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(, width, height, );
    glMatrixMode(GL_MODELVIEW);

    glViewport(, , width, height);
}

int main(int argc, char **argv)
{
    glutInitWindowPosition(, );
    glutInitWindowSize(++, ++);
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
    glutCreateWindow("OpenGL DDS");
    init();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutMainLoop();

    return ;
}

关于如何制作一个DDS文件，可以使用Nvidia提供的DXT工具，下载地址：

https://developer.nvidia.com/legacy-texture-tools
http://pan.baidu.com/s/1pKKRL3P

以下是文件大小对比：

以下是图像质量对比：

原图（TGA，无压缩）：

DXT1（压缩比：1/8，无Alpha通道，但可以单色透明）：

DXT3（压缩包：1/4，Alpha通道还原较差）：

DXT5（压缩比：1/4，Alpha通道还原较好）：

参考：

http://msdn.microsoft.com/en-us/library/windows/desktop/bb943990(v=vs.85).aspx
http://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_texture_compression.txt
http://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_compression_s3tc.txt