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Q&A OpenGL: Pass a double vector from vertex shader to fragment shader

Found a workaround: Instead of doing the calculating in the vertex shader, do it in the fragment shader. The precision error only occurs because the different vertexes have a uv value that is clos...

posted 1y ago by H_H‭ · edited 1y ago by H_H‭

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#2: Post edited by

H_H‭ · 2023-08-22T17:56:02Z (about 1 year ago)

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Found a workaround: Instead of doing the calculating in the vertex shader, do it in the fragment shader.
The precision error only occurs because the different vertexes have a `uv` value that is close together relative to the absolute value of `uv`. Which causes multiple fragments to have the same `uv` coordinates which causes this artefacts. By passing `uv` directly, with coordinates that only have the values `-1` `1`, each fragment shader instance gets its own `uv` value that is different from the others. The calculation inside the fragment shader can be done with double precision.
The good thing is, the main program can pass `double` values, when the correct extensions are supported.
The new fragment shader:
```
#version 400
//Moved zoom from the vertex shader. This is set by the main program.
// with glUniformMatrix3dv()
uniform dmat3 zoom;
//uv values are not transformed.
in vec2 uv;
...
void main()
{
dvec2 uv_double = uv;
uv_double = ( zoom * dvec3(uv_double,1)).xy ;
//us uv_double from here on and not uv
...
}
```
~~Disadvantage is that `glUniformMatrix3dv()` has to be supported and a other very minor disadvantage is that this is probably slower.~~

Found a workaround: Instead of doing the calculating in the vertex shader, do it in the fragment shader.
The precision error only occurs because the different vertexes have a `uv` value that is close together relative to the absolute value of `uv`. Which causes multiple fragments to have the same `uv` coordinates which causes this artefacts. By passing `uv` directly, with coordinates that only have the values `-1` `1`, each fragment shader instance gets its own `uv` value that is different from the others. The calculation inside the fragment shader can be done with double precision.
The good thing is, the main program can pass `double` values, when the correct extensions are supported.
The new fragment shader:
```
#version 400
//Moved zoom from the vertex shader. This is set by the main program.
// with glUniformMatrix3dv()
uniform dmat3 zoom;
//uv values are not transformed.
in vec2 uv;
...
void main()
{
dvec2 uv_double = uv;
uv_double = ( zoom * dvec3(uv_double,1)).xy ;
//us uv_double from here on and not uv
...
}
```
Disadvantage is that `glUniformMatrix3dv()` is an extension any may not be supported everywhere. A other very minor disadvantage is that this is probably slower.

#1: Initial revision by

H_H‭ · 2023-08-22T17:55:05Z (about 1 year ago)

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Found a workaround: Instead of doing the calculating in the vertex shader, do it in the fragment shader.

The precision error only occurs because the different vertexes have a `uv` value that is close together relative to the absolute value of `uv`. Which causes multiple fragments to have the same `uv` coordinates which causes this artefacts. By passing `uv` directly, with coordinates that only have the values `-1` `1`, each fragment shader instance gets its own `uv` value that is different from the others. The calculation inside the fragment shader can be done with double precision.

The good thing is, the main program can pass `double` values, when the correct extensions are supported.

The new fragment shader:
```
#version 400

//Moved zoom from the vertex shader. This is set by the main program.
// with glUniformMatrix3dv()
uniform dmat3 zoom;

//uv values are not transformed.
in vec2 uv;

...

void main()
{
  dvec2 uv_double = uv;
  uv_double = ( zoom * dvec3(uv_double,1)).xy ;
  //us uv_double from here on and not uv
  ...
}
```

Disadvantage is that `glUniformMatrix3dv()` has to be supported and a other very minor disadvantage is that this is probably slower.

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