为什么这个OpenGL着色器使用超过1.0的纹理坐标？
我正在尝试熟悉 opengl 中的着色器。这是我找到的一些示例代码（使用 openframeworks）。该代码只是分两次模糊图像，首先是水平模糊，然后是垂直模糊。这是水平着色器的代码。我唯一的困惑是纹理坐标。它们超过了 1。
void main( void )
{
vec2 st = gl_TexCoord[0].st;
//horizontal blur
//from http://www.gamerendering.com/2008/10/11/gaussian-blur-filter-shader/
vec4 color;
color += 1.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -4.0, 0));
color += 2.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -3.0, 0));
color += 3.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -2.0, 0));
color += 4.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -1.0, 0));
color += 5.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt , 0));
color += 4.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 1.0, 0));
color += 3.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 2.0, 0));
color += 2.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 3.0, 0));
color += 1.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 4.0, 0));
color /= 5.0;
gl_FragColor = color;
}
我无法从这段代码中得出结论。纹理坐标应该在 0 和 1 之间，我已经阅读了一些关于当它们大于 1 时会发生什么的内容，但这不是我看到的行为（或者我没有看到联系）。 BluAmnt 在 0.0 到 6.4 之间变化，因此 s 可以从 0 到 25.6。图像只是根据值或多或少地变得模糊，我没有看到任何重复的图案。
我的问题归结为：当调用texture2DRect时纹理坐标参数超过1时到底发生了什么？为什么尽管如此，模糊行为仍然可以完美地发挥作用？
I'm trying to get familiar with shaders in opengl. Here is some sample code that I found (working with openframeworks). The code simply blurs an image in two passes, first horizontally, then vertically. Here is the code from the horizontal shader. My only confusion is the texture coordinates. They exceed 1.
void main( void )
{
vec2 st = gl_TexCoord[0].st;
//horizontal blur
//from http://www.gamerendering.com/2008/10/11/gaussian-blur-filter-shader/
vec4 color;
color += 1.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -4.0, 0));
color += 2.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -3.0, 0));
color += 3.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -2.0, 0));
color += 4.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * -1.0, 0));
color += 5.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt , 0));
color += 4.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 1.0, 0));
color += 3.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 2.0, 0));
color += 2.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 3.0, 0));
color += 1.0 * texture2DRect(src_tex_unit0, st + vec2(blurAmnt * 4.0, 0));
color /= 5.0;
gl_FragColor = color;
}
I can't make heads or tails out of this code. Texture coordinates are supposed to be between 0 and 1, and I've read a bit about what happens when they're greater than 1, but that's not the behavior I'm seeing (or I don't see the connection). blurAmnt varies between 0.0 and 6.4, so s can go from 0 to 25.6. The image just gets blurred more or less depending on the value, I don't see any repeating patterns.
My question boils down to this: what exactly is happening when the texture coordinate argument in the call to texture2DRect exceeds 1? And why does the blurring behavior still function perfectly despite this?
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评论（2）
[0, 1] 纹理坐标范围仅适用于 GL_TEXTURE_2D 纹理目标。由于该代码使用texture2DRect（和samplerRect），因此它使用GL_TEXTURE_RECTANGLE_ARB纹理目标，该目标使用非标准化纹理坐标，范围为[0，宽度] x [0，高度]。
这就是为什么你有“奇怪”的纹理坐标。别担心，它们可以很好地处理这个纹理目标。
The [0, 1] texture coordinate range only applies to the GL_TEXTURE_2D texture target. Since that code uses texture2DRect (and a samplerRect), it's using the GL_TEXTURE_RECTANGLE_ARB texture target, that this target uses Unnormalized texture coordinates, in the range [0, width]x[0, height].
That's why you have "weird" texture coords. Don't worry, they work fine with this texture target.
取决于主机代码。如果您看到类似
的 内容
glTexParameteri (GL_TEXTURE_3D, GL_TEXTURE_WRAP_S, GL_CLAMP);
那么出界的维度将为零，IIRC。与 t 类似。
Depends on the host code. If you saw something like
glTexParameteri (GL_TEXTURE_3D, GL_TEXTURE_WRAP_S, GL_CLAMP);
Then the out of bounds s dimension will be zeros, IIRC. Similar for t.