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Finding the pixel

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Hey again, 

 

So I'm trying to find the pixel of a big bg image related to the pixel of another image on top of it.

 

In the example image. The dotted grey line around the shadow is "how big that image is"

 

What I'm trying to accomplish is choosing whether to use the shadow pixel or the pixel of the image below it (doesn't have to be a bg just using it as an example)

 

the shader should be giving me the texCoord of the shadow pixel (as I will run the shader on the shadow itself) but then yeah, I need to find the same pixel UNDER the shadow to compare the two on whether or not the shadow should show up.

 

I tried solving the lerp equation to give me the percentage as this:

lerp:

output = a + (b - a) * %;

 

to

 

1/% = a + ( b - a) / output

but I'm not sure if that's correct and where I would plug the texCoord in either...

 

Anyone got any idea?

 

Say I have a 1000w x 200h bg image. starts at 0,0

my shadow is at 300, 250 and is 20w,10h

 

how would I find out the percentage across the bg that the shadow is so i can try to find the same pixel under each shadow pixel?

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texture baseTexture; 
texture baseMaskTexture;
texture shadowTexture;

float2 shadowPos;
float2 shadowDim;
float2 basePos;
float2 baseDim;

sampler2D sampBase = sampler_state 
{ 
	Texture = <baseTexture>;
	MinFilter = Point;
	MagFilter = Point;
};   
sampler2D sampBaseAlpha = sampler_state
{
	Texture = <baseMaskTexture>;
	MinFilter = Point;
	MagFilter = Point;
};
sampler2D sampShad = sampler_state
{
	Texture = <shadowTexture>;
	MinFilter = Point;
	MagFilter = Point;
};

float4x4 MatrixTransform;

void SpriteVertexShader(inout float4 vColor : COLOR0, inout float2 texCoord : TEXCOORD0, inout float4 position : POSITION0) {
	position = mul(position, MatrixTransform);
}
 
// Pixel Shader Output 
float4 main( float2 texCoord : TEXCOORD0  ) : COLOR0  
{ 
	//To get the correct pixel under the shadow, we should take the texCoord and add the size of our shadow to it
	float2 newTC = float2(texCoord * shadowDim);

	//Add the new TC + shadowPos to get the pix coord below it
	float basePix = newTC + shadowPos;

	float4 aTex = tex2D(sampBase, basePix);
	float4 baseTex = tex2D(sampBase, basePix);
	float4 shadMask = tex2D(sampShad, texCoord);


	if (aTex.a == 0)
	{
		return float4( shadMask);
	}
		
	return float4(baseTex );
}  
//Technique Calls 
technique Clipping  
{  
   pass P0 
   {  
	   AlphaBlendEnable = true;
	   SrcBlend = SrcAlpha;
	   DestBlend = InvSrcAlpha;
	   VertexShader = compile vs_2_0 SpriteVertexShader();
	   PixelShader = compile ps_2_0 main();
   }  
} 



Is what I have so far...I'm wondering now if I have to give it a renderTarget to output the pixels to or something? although I thought this should work without one?

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