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[ogl] Cube Map Texturing

2009-04-25 16:06:11| 分类： 3d | 标签： |举报 |字号大中小订阅

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Preface
red book 上对于一些内容的讲解，确实比较简略，比如 cube map texturing。
GL 1.2 时，cube map texturing 还是作为 extension 存在，目前 2.1/3.0 早已成为正式标准。

这里有详细的介绍，看完就懂了。
http://developer.nvidia.com/object/cube_map_ogl_tutorial.html

Cube Mapping Explained
对于金属直感的物体，会映射出周围场景的环境（environment mapping），如何在程序中模拟出这个效果呢，cube map texturing 是一种方法。其他方法如：sphere mapping、dual paraboloid mapping 等等。

对于 cube map texturing，其实就是用6张贴图（6 targets），一个 cube，作为当前物体的环境贴图（environment），在绘制物体时，不同的点，会对应到不同的 target 上，取得不同的 tex pixel 信息。

对于 (x, y, z)，哪个值最大，就选取那个为 mayor(ma)。
major axis direction     target                              sc      tc    ma ----------          --------------------------------- ---       ---    --- +rx         GL_TEXTURE_CUBE_MAP_POSITIVE_X_EXT    -rz      -ry     rx -rx               GL_TEXTURE_CUBE_MAP_NEGATIVE_X_EXT   +rz    -ry    rx +ry         GL_TEXTURE_CUBE_MAP_POSITIVE_Y_EXT     +rx    +rz   ry -ry               GL_TEXTURE_CUBE_MAP_NEGATIVE_Y_EXT   +rx    -rz    ry +rz             GL_TEXTURE_CUBE_MAP_POSITIVE_Z_EXT    +rx    -ry     rz
-rz            GL_TEXTURE_CUBE_MAP_NEGATIVE_Z_EXT    -rx    -ry     rz

s   =   ( sc/|ma| + 1 ) / 2 t   =   ( tc/|ma| + 1 ) / 2 sc / |ma| = -1.0 ~ 1.0 (sc / |ma| + 1)/2 = 0.0 ~ 1.0 对于每个 target surface，s/t 会映射为： (0.0, 0.0)        +-----------------------------+         |                       |         |                       |         |                       |        +-----------------------------+                            (1.0, 1.0)
Sample
---- init ----
glGenTextures(1, &texName);
glBindTexture(GL_TEXTURE_CUBE_MAP, texName);

glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_REPEAT);
glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X, 0, GL_RGBA, TEX_WIDTH, TEX_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, mytex1);
glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_X, 0, GL_RGBA, TEX_WIDTH, TEX_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, mytex2);
glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_Y, 0, GL_RGBA, TEX_WIDTH, TEX_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, mytex3);
glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_Y, 0, GL_RGBA, TEX_WIDTH, TEX_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, mytex4);
glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_Z, 0, GL_RGBA, TEX_WIDTH, TEX_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, mytex5);
glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_Z, 0, GL_RGBA, TEX_WIDTH, TEX_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, mytex6);

glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_NORMAL_MAP);
glTexGeni(GL_T, GL_TEXTURE_GEN_MODE, GL_NORMAL_MAP);
glTexGeni(GL_R, GL_TEXTURE_GEN_MODE, GL_NORMAL_MAP);
glEnable(GL_TEXTURE_GEN_S);
glEnable(GL_TEXTURE_GEN_T);
glEnable(GL_TEXTURE_GEN_R);

---- draw it ----
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_TEXTURE_CUBE_MAP);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glBindTexture(GL_TEXTURE_CUBE_MAP, texName);

glutSolidSphere(1.0, 30, 10);

对于 mytex1 ~ 6 就是简单的 64x64 single-color texture。使用 cube mapping 也只是设置到 target 即可。

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