Help Needed: Writing first shader

[merspieler]

So I've written (and partly copy-pasted) a shader which works fine in shadertoy.

I'm trying to port that to a compositor pipeline.

I've taken the ALS pipeline and changed the buffers to cubemaps

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  <buffer>
    <name>color</name>
    <type>cubemap</type>
    <width>2048</width>
    <height>2048</height>
    <format>rgba8</format>
  </buffer>
  <buffer>
    <name>depth</name>
    <type>cubemap</type>
    <width>2048</width>
    <height>2048</height>
    <format>depth32f</format>
  </buffer>


Then I've added another pass at the bottom with an effect:

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  <pass>
    <name>display</name>
    <type>quad</type>
    <binding>
      <buffer>color</buffer>
      <unit>0</unit>
    </binding>
    <effect>Compositor/Effects/projection</effect>
  </pass>


The effect currently looks like this(there are still some parameters missing... and maybe stuff, I've missed???):

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<?xml version='1.0' encoding='UTF-8'?>
<PropertyList>
        <name>Effects/Compositor/projection</name>
        <parameters>
                <display_xsize><use>/sim/startup/xsize</use></display_xsize>
                <display_ysize><use>/sim/startup/ysize</use></display_ysize>
                <hfov><use>/sim/current-view/field-of-view</use></hfov>
                <vfov>180</vfov><!-- TODO add parameter -->
                <auto_vfov>1</auto_vfov><!-- TODO add parameter -->
                <enable_overlay>1</enable_overlay><!-- TODO add parameter -->
        </parameters>
        <technique n="1">
                <pass>
                        <program>
                                <fragment-shader>Compositor/Shaders/ALS/projection.frag</fragment-shader>
                        </program>
                        <uniform>
                                <name>display_xsize</name>
                                <type>int</type>
                                <value><use>display_xsize</use></value>
                        </uniform>
                        <uniform>
                                <name>display_ysize</name>
                                <type>int</type>
                                <value><use>display_ysize</use></value>
                        </uniform>
                        <uniform>
                                <name>hfov</name>
                                <type>float</type>
                                <value><use>hfov</use></value>
                        </uniform>
                        <uniform>
                                <name>vfov</name>
                                <type>float</type>
                                <value><use>vfov</use></value>
                        </uniform>
                        <uniform>
                                <name>auto_vfov</name>
                                <type>int</type>
                                <value><use>auto_vfov</use></value>
                        </uniform>
                        <uniform>
                                <name>enable_overlay</name>
                                <type>int</type>
                                <value><use>enable_overlay</use></value>
                        </uniform>
                </pass>
        </technique>
</PropertyList>


In shadertoy to I've got this line:

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vec4 ret = texture( iChannel0, ray );


Now obviously in flightgear I need to use `texture3D()` instead.
`iChannel0` is a cubemap input.
So... how do I access the `color` buffer I've defined in the pipline from within my shader?

[Necolatis]

In the effect you need to define iChannel0 in parameters and then again inside the technique as an uniform.
See http://wiki.flightgear.org/Effect_Framework

[merspieler]

Okay... but it doesn't say anything what I've got to put into the parameter block, to access a buffer...

[icecode]

So... how do I access the `color` buffer I've defined in the pipline from within my shader?

You can expose buffers to passes via the binding tag. See http://wiki.flightgear.org/Compositor#Passes. The unit parameter specifies the texture unit where all shaders being used on that pass will be able to access the buffer.

[merspieler]

I've done that already... see my first post.

Then how do I access it in the shader?

[icecode]

The buffer is just a normal texture at this point... Use a sampler2D uniform with the texture unit you have chosen as its value. If you don't know how to do the Effect part, Necolatis did a nice write-up on the Effects framework. The $FG_ROOT/Effects directory is also filled with practical examples.

[merspieler]

it's sampler3D in my case

... !thanks for RTFM... I read it already... and I didn't find, what I was looking for.

So just tell me, how to get that buffer object as a uniform in the effect.

[icecode]

I don't understand what you are having trouble with...

Just write something like

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<uniform>
  <name>youruniformname</name>
  <type>sampler-3d</type>
  <value>yourtextureunit</value>
</uniform>

[merspieler]

Okay, I somehow expected to have another block in the parameters which I then reference in the <value>, like with the other uniforms.

So next issue I'm facing.

According to the wiki, fg_BufferSize are the window dimensions.
And according to GL docs, gl_FragCoord contains the coords of the fragment.

So the following should at least give me a transition of colors:

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vec2 uv = gl_FragCoord.xy / fg_BufferSize.xy;
vec4 ret = vec4(uv.x, uv.y, 0.0, 1.0);
gl_FragColor = ret;


But instead, I get a plain yellow window.

I've put the code into shadertoy and I get transitions with the corners being green, yellow, red and black (clockwise, starting top left).

What am I doing wrong?

[icecode]

fg_BufferSize comes from Rembrandt and is outdated. Use fg_ViewportSize instead.

[merspieler]

Okay, thanks. This got me a hugh step further.

My uniform sampler3D is still empty.

Is it possible, that I need to copy and modify all effects to use a cubemap instead?

That would be a nightmare to maintain...

[icecode]

That tediousness is solved by Effect schemes. They allow using different effects for the same model depending on the rendering pass. They also allow setting a default technique to use when an effect doesn't implement a particular scheme. This is a bit advanced and can be tricky to understand if you are just starting out with Effects. There aren't many practical examples either. Perhaps you can get something out of this.

[merspieler]

This doesn't answer the question, if I need to rewrite some shaders cause I'm rendering to a cubemap now.

[icecode]

It does answer your question, you just need to have more experience with Effects and shaders before delving further in this topic. You can propose another project for the virtual FSWeekend event and I can try to help you a bit more directly there.

[merspieler]

Well... if the answer is "no", I'll continue down this path and try to get it working.
If the answer is "yes", then I know enough, to fix my initiall issue, without adding any other new features.

As for the FSWeekend... if you say, I shouldn't do it then I'll probably just hop onto another idea... cause right now, I don't really have any other... except messing with the A320 or A350.