Help on my glow shader!

Question

I'm trying to create a glow shader for a neon style game but I'm having a little trouble with rendertargets. Right now all I want is something VERY simple where I change the rendertarget, draw to it and then pass it into the shader. A lot of the examples I've looked at do the first part but don't include how to pass it all through a shader. Right now I'm not really concerned about the glow part, I just want to get it passing through a shader so I can work on the glow part.

edit - new code:

graphicsDevice.SetRenderTarget(renderTarget);

..
..
..

graphicsDevice.SetRenderTarget(null);

renderTargetTexture = (Texture2D)renderTarget;
loader.GlowShader.Parameters["glowTexture"].SetValue(renderTargetTexture);

for (int l = 0; l < loader.GlowShader.CurrentTechnique.Passes.Count; l++)
{
    loader.GlowShader.CurrentTechnique.Passes[l].Apply();
}

renderQuad.Draw();

And the glow shader (this is probaly really messy but I'm not really fuzzed about that right now, I just want the basic thing to work):

float4x4 World;
float4x4 View;
float4x4 Projection;

Texture glowTexture;

sampler glowSampler = sampler_state 
{ 
texture = <glowTexture>;
magfilter = LINEAR;
minfilter = LINEAR;
mipfilter = LINEAR;
AddressU = clamp;
AddressV = clamp;
};

struct VertexShaderInput
{
float4 Position : POSITION0;
};

struct VertexShaderOutput
{
float4 Position : POSITION0;
};

VertexShaderOutput VSOutline(VertexShaderInput input)
{
VertexShaderOutput output;

float4 worldPosition = mul(input.Position, World);
float4 viewPosition = mul(worldPosition, View);

output.Position = mul(viewPosition, Projection);

return output;
}

float4 PSOutline(VertexShaderOutput input, float2 pixel:VPOS) : COLOR0
{
float2 uvPixel = (pixel + 0.5) * float2(1.0 / 1600, 1.0 / 900);
    return tex2D(glowSampler, uvPixel);
}

technique Outline
{
pass Pass1
{
    AlphaBlendEnable = TRUE;
    DestBlend = INVSRCALPHA;
    SrcBlend = SRCALPHA;

    VertexShader = compile vs_3_0 VSOutline();
    PixelShader = compile ps_3_0 PSOutline();
}
}

On a side note, if I try running this and drawing it using sprite batches it works so I think up until "rendertargetTexture = (Texture2D)renderTarget" everything is correct. It's just when I try to use the quads that nothing appears still.

        graphicsDevice.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.DarkSlateBlue, 1.0f, 0);
        using (SpriteBatch sprite = new SpriteBatch(graphicsDevice))
        {
            sprite.Begin();
            sprite.Draw(renderTargetTexture, new Vector2(0, 0), null, Color.White, 0, new Vector2(0, 0), 1, SpriteEffects.None, 1);
            sprite.End();
        }

Answer 1

Assuming that graphicsDevice, renderTarget and loader are properly set I think you are missing the apply() of the glow pass and a fullscreen quad draw. If you don't draw the fullscreen quad, the glow pixel shader cannot apply the effect since there is nothing being draw to the default render target. I think that's why nothing appears on the screen.

graphicsDevice.SetRenderTarget(renderTarget);
//
//...
//
graphicsDevice.SetRenderTarget(null);
renderTargetTexture = (Texture2D)renderTarget;
whateverEffectToGlow.whateverTechniqueToGlow.pass[0].apply;
RenderFullScreenQuad();

You can use this example to write RenderFullScreenQuad(): http://projectvanquish.wordpress.com/tag/fullscreen-quad/

Answer 2

What you are missing out is actualy where you do your "posteffects". The first part all seems alright.

A breakdown of glow & post effects would be like this :

1 : Render all your models to offscreen rendertarget ( this you have done )

2 : Render your offscreen rendertarget to a fullscrean quad, that will cover the entire screen. this way you will get a texture that looks like your world over the screen, and can manipulate the pixels.

3 : once you are rendering the quad, you can manipulate the orginal texture to do diffrent stuff with the pixels, like bloom or glow.

4 : Once all this is done, you got a new rendertarget ( or backbuffer ) that have an applied glow onto it.

All in all, you only miss the part where you are drawing the fullscreen quad with the postpass texture. I think this article : Good article about lightprepass