Ok, in my GLSL fragment shader I want to be able to calculate the distance of the fragment from a particular line in space.
The result of this is that I am first trying to use a varying vec2 set in my vertex shader to mirror what ends up in gl_FragCoord
:
varying vec2 fake_frag_coord;
//in vertex shader:
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
fake_frag_coord=(gl_ModelViewProjectionMatrix * gl_Vertex).xy;
Now in the fragment shader I expect:
gl_FragCoord.xy==fake_frag_coord
But it does not. What operation does the pipeline do on gl_Position
to turn it into gl_FragCoord
that I am neglecting to do on fake_frag_coord
?
The input variable gl_FragCoord is an input variable that allows us to read screen-space coordinates and get the depth value of the current fragment, but it is a read-only variable. We can't influence the screen-space coordinates of the fragment, but it is possible to set the depth value of the fragment.
GLSL also allows user defined varying variables. These must be declared in both the vertex and fragment shaders, for instance: varying float intensity; A varying variable must be written on a vertex shader, where we compute the value of the variable for each vertex.
Each fragment represents a sample-sized segment of a rasterized Primitive. The size covered by a fragment is related to the pixel area, but rasterization can produce multiple fragments from the same triangle per-pixel, depending on various multisampling parameters and OpenGL state.
There are three types of variables in shaders: uniforms, attributes, and varyings: Uniforms are variables that have the same value for all vertices - lighting, fog, and shadow maps are examples of data that would be stored in uniforms. Uniforms can be accessed by both the vertex shader and the fragment shader.
gl_FragCoord is screen coordinates, so you'd need to have information about the screen size and such to produce it based on the Position. The fake_frag_coord you've produced will correspond to a point in the screen in normalized device coordinates I think (-1 to 1). So if you were to multiply by half the screen's size in a particular dimension, and then add that size, you'd get actual screen pixels.
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