# Need help with some GLSL shader code
**URL:** https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655
**Category:** Coding Questions
**Created:** [June 7, 2020, 1:36pm UTC](https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655 "2020-06-07T13:36:22Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@zenglenn](https://discourse.processing.org/u/zenglenn)
#### Post date: [June 7, 2020, 1:36pm UTC](https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655/1 "2020-06-07T13:36:22Z")
Hi
I want to use Processing shaders to do some code art algorithms - but I want to do it all through a shader to take advantage of the high GPU power.
I know the basics of shader programming, but what I’m stuck on at the minute is,
how would I draw a bitmap on top of another bitmap (if I have 2 2dsamplers for example)
how could I get the individual pixel color from an x,y position on either of these bitmaps
how could I change / write the value of a new color into a bitmap at an x,y pixel position.
thanks!
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### Author:  [@uheinema](https://discourse.processing.org/u/uheinema)
#### Post date: [June 8, 2020, 5:14pm UTC](https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655/2 "2020-06-08T17:14:04Z")
1. Transparency
2. texelFetch(texMap,ivec2(ix,iy),0)
3. You can’t. The result of the fragment shader IS the new bitmap.
```auto
#version 320 es
#ifdef GL_ES
precision highp float;
precision mediump int;
#endif
in vec4 vertTexCoord;
out vec4 was_gl_FragColor;
uniform sampler2D texMap;
....
void main(void)
{
was_gl_FragColor=yourcode();
}
```
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### Author:  [@zenglenn](https://discourse.processing.org/u/zenglenn)
#### Post date: [June 9, 2020, 8:46pm UTC](https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655/3 "2020-06-09T20:46:12Z")
thanks, i’ve worked out other solutions…
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### Author:  [@uheinema](https://discourse.processing.org/u/uheinema)
#### Post date: [June 11, 2020, 11:09am UTC](https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655/4 "2020-06-11T11:09:18Z")
Good for you.
Which?
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### Author:  [@zenglenn](https://discourse.processing.org/u/zenglenn)
#### Post date: [June 12, 2020, 6:41am UTC](https://discourse.processing.org/t/need-help-with-some-glsl-shader-code/21655/5 "2020-06-12T06:41:13Z")
well,I didn’t really, just had compromise - I worked out texelfetch though, which was very useful - but I still want to know how to simply write / store a pixel into a texture already in GPU memory e.g. the uniform sampler2D texMap - how would i write a single pixel into an x,y there?