# Drawing a shader generated PGraphics onto a non OpenGL PGraphics object
**URL:** https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264
**Category:** Coding Questions
**Created:** [December 15, 2020, 11:50am UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264 "2020-12-15T11:50:08Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@r.jaoui](https://discourse.processing.org/u/r.jaoui)
#### Post date: [December 15, 2020, 11:50am UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/1 "2020-12-15T11:50:08Z")
Hello everyone !
I am currently developping quite a big project, and for simplicity I have chosen to use Processing as a graphical engine. I am coding with Eclipse Neon.
The issue is that I have developped GLSL Fragment shaders to accelerate image processing, and then apply them to a P2D PGraphics object (which works fine), the general PApplet is also P2D but there are a few in-between PGraphics objects that aren’t P2D and that I can’t make P2D since for some reason this messes up the whole look of the project (and this represent quite a substantial amount of time).
So basically, I run a shader onto a P2D PGraphics object and I then want to draw the resulting image onto a non OpenGL PGraphics object, and for some reason this doesn’t work, even though I assumed that after rendering the shader on the first PGraphics object, I would simply have an image I could render elsewhere…
Thank you for your help !
---
### Author:  [@GoToLoop](https://discourse.processing.org/u/GoToLoop)
#### Post date: [December 15, 2020, 8:07pm UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/2 "2020-12-15T20:07:30Z")
Maybe transferring the contents of _pixels[]_ to another’s via **arrayCopy()** might work:
> **[PImage::pixels\[\] \\ Language (API) \\ Processing 3+](https://Processing.org/reference/PImage_pixels.html)**
> **[arrayCopy() \\ Language (API) \\ Processing 3+](https://Processing.org/reference/arrayCopy_.html)**
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### Author:  [@r.jaoui](https://discourse.processing.org/u/r.jaoui)
#### Post date: [December 16, 2020, 1:45pm UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/3 "2020-12-16T13:45:25Z")
@GoToLoop Thanks for your reply !
So I’ve tried adapting my code the following way :
```auto
public void render(){
this.image.beginDraw();
this.image.filter(alphaOverlayShader);
this.image.endDraw();
}
public PGraphics getImage() {
PGraphics out = context.createGraphics(w, h);
this.image.beginDraw();
this.image.loadPixels();
out.beginDraw();
out.loadPixels();
PApplet.arrayCopy(this.image.pixels, out.pixels);
out.updatePixels();
out.endDraw();
this.image.updatePixels();
this.image.endDraw();
return out;
}
```
But this still doesn’t work, or at least I am still doing something wrong…
---
### Author:  [@GoToLoop](https://discourse.processing.org/u/GoToLoop)
#### Post date: [December 16, 2020, 4:21pm UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/4 "2020-12-16T16:21:39Z")
> [@r.jaoui](#):
>
> But this still doesn’t work,
Merely stating it doesn’t work w/o describing exactly what happened doesn’t help.
And btW, we just use **updatePixels()** and/or **loadPixels()** for _pixels[]_.
Methods **beginDraw()** & **endDraw()** shouldn’t be used on that context.
And given you’re **createGraphics()** every time rather than using an existing 1 you can instead clone a PGraphics as a PImage calling method **get()**:
> **[get() / Reference](https://processing.org/reference/PImage_get_.html)**
>
> Reads the color of any pixel or grabs a section of an image. If no parameters are specified, the entire image is returned. Use the x and y parameters to get the value of one pixel. Get a…
---
### Author:  [@r.jaoui](https://discourse.processing.org/u/r.jaoui)
#### Post date: [December 16, 2020, 6:07pm UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/5 "2020-12-16T18:07:56Z")
> [@GoToLoop](#):
>
> Merely stating it doesn’t work w/o describing exactly what happened doesn’t help.
Yeah you’re right. So basically the sketch runs as if the shader wasn’t applied (this shader doesn’t modify the current **pixels** array, but it actually draws over it, in this case a checkerboard pattern) and the result here is that it’s as if this image is transparent.
> [@GoToLoop](#):
>
> And btW, we just use **updatePixels()** and/or **loadPixels()** for _pixels[]_ .
>
> Methods **beginDraw()** & **endDraw()** shouldn’t be used on that context.
Oh I didn’t know that, I sometimes run into NullPointerExceptions when **loadPixels()** is called without **beginDraw()** before but I think that this is due to the fact there was actually nothing drawn onto the **PGraphics** beforehand.
> [@GoToLoop](#):
>
> And given you’re **createGraphics()** every time rather than using an existing 1 you can instead clone a PGraphics as a PImage calling method **get()** :
This is also one of the things I tried before. This is why I tried recreating my issue on a simpler sketch :
```auto
PGraphics inBetween;
PGraphics shadedLayer;
PImage image;
void setup(){
size(1280, 720, P2D);
inBetween = createGraphics(1280, 720);
inBetween.beginDraw();
inBetween.background(255);
inBetween.endDraw();
shadedLayer = createGraphics(1280, 720, P2D);
image = loadImage("image.png");
}
void draw(){
shadeLayer();
method1();
//method2();
//method3();
//method4();
image(inBetween, 0, 0);
}
void method1(){
inBetween.beginDraw();
inBetween.image(shadedLayer, 0, 0);
inBetween.endDraw();
}
void method2(){
shadedLayer.loadPixels();
inBetween.loadPixels();
arrayCopy(shadedLayer.pixels, inBetween.pixels);
inBetween.updatePixels();
shadedLayer.updatePixels();
}
void method3(){
inBetween.beginDraw();
inBetween.image(shadedLayer.get(), 0, 0);
inBetween.endDraw();
}
void method4(){
shadedLayer.loadPixels();
inBetween.loadPixels();
for(int x = 0; x < shadedLayer.width; x++){
for(int y = 0; y < shadedLayer.height; y++){
inBetween.pixels[x + y * inBetween.width] = shadedLayer.pixels[x + y * shadedLayer.width];
}
}
inBetween.updatePixels();
shadedLayer.updatePixels();
}
void shadeLayer(){
shadedLayer.beginDraw();
shadedLayer.image(image, 0, 0);
shadedLayer.endDraw();
shadedLayer.filter(BLUR, 10);
}
```
But in this case all methods seem to work so… perhaps my issue isn’t even where I thought it was…
Thanks for your help @GoToLoop by the way.
---
### Author:  [@GoToLoop](https://discourse.processing.org/u/GoToLoop)
#### Post date: [December 16, 2020, 6:30pm UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/6 "2020-12-16T18:30:39Z")
I know nothing about Processing’s shader implementation.
I guess it’s tightly bound to a PGraphics and can’t be transferred.
---
### Author:  [@r.jaoui](https://discourse.processing.org/u/r.jaoui)
#### Post date: [January 9, 2021, 8:37pm UTC](https://discourse.processing.org/t/drawing-a-shader-generated-pgraphics-onto-a-non-opengl-pgraphics-object/26264/7 "2021-01-09T20:37:46Z")
**I am writing this answer to close subject** : apparently to apply a shader, something has to already have been drawn onto the PGraphics object, weither it is P2D or not. The simple fix was to apply a background color before applying the shader, weither it is a modifyer fragment shader or a “rendering” shader that draws “on top” of the previous pixels.