GLSL:gl_FragCoord问题 [英] GLSL: gl_FragCoord issues

问题描述

我正在针对OpenGL ES 2.0使用GLSL进行实验.我有一个四边形和正在渲染的纹理.我可以这样成功完成:

I am experimenting with GLSL for OpenGL ES 2.0. I have a quad and a texture I am rendering. I can successfully do it this way:

//VERTEX SHADER
attribute highp vec4 vertex;
attribute mediump vec2 coord0;

uniform mediump mat4 worldViewProjection;

varying mediump vec2 tc0;

void main()
{
    // Transforming The Vertex
    gl_Position = worldViewProjection * vertex;

    // Passing The Texture Coordinate Of Texture Unit 0 To The Fragment Shader
    tc0 = vec2(coord0);
}

//FRAGMENT SHADER
varying mediump vec2 tc0;

uniform sampler2D my_color_texture;

void main()
{
    gl_FragColor = texture2D(my_color_texture, tc0);
}

到目前为止，一切都很好.但是，我想做一些基于像素的过滤，例如中位数.因此，我想使用像素坐标而不是规范化的(tc0)，然后将结果转换回规范化的坐标.因此，我想使用gl_FragCoord而不是uv属性(tc0).但是我不知道如何返回归一化的坐标，因为我不知道gl_FragCoords的范围.知道我怎么能得到吗?我已经做到了这一点，使用固定值进行规范化"，尽管它不能完美工作，因为它会导致拉伸和平铺(但至少会显示出一些东西):

So far so good. However, I'd like to do some pixel-based filtering, e.g. Median. So, I'd like to work in pixel coordinates rather than in normalized (tc0) and then convert the result back to normalized coords. Therefore, I'd like to use gl_FragCoord instead of a uv attribute (tc0). But I don't know how to go back to normalized coords because I don't know the range of gl_FragCoords. Any idea how I could get it? I have got that far, using a fixed value for 'normalization', though it's not working perfectly as it is causing stretching and tiling (but at least is showing something):

//FRAGMENT SHADER
varying mediump vec2 tc0;

uniform sampler2D my_color_texture;

void main()
{
    gl_FragColor = texture2D(my_color_texture, vec2(gl_FragCoord) / vec2(256, 256));
}

因此，一个简单的问题是，应该使用什么代替vec2(256, 256)，以便获得与使用uv坐标相同的结果.

So, the simple question is, what should I use in the place of vec2(256, 256) so that I could get the same result as if I were using the uv coords.

谢谢！

推荐答案

gl_FragCoord在屏幕坐标中，因此要获得规范化的坐标，需要除以视口的宽度和高度.您可以使用统一变量将信息传递给着色器，因为没有内置变量.

gl_FragCoord is in screen coordinates, so to get normalized coords you need to divide by the viewport width and height. You can use a uniform variable to pass that information to the shader, since there is no built in variable for it.

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