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Question about applying multiple decals in 2D via shader

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I'm working on a 2d topdown game (C#, 2d slimdx-based lib Gorgon), and I'm trying to get a way to apply decals to individual sprites when they are drawn. I'd like to be able to apply more than one decal per sprite. The decal sprites are all in a single texture atlas. The base sprites may or may not be in an atlas.

I've attached the shader I'm currently using.

Right now, I've got it working so that I can apply a decal to a sprite in a little test app by feeding in some coordinates and a constant to transform texcoord sizes and display only the relevant part of the sprite.

The problem is that it seems I can only set the parameters once from within the application, causing the same decal to be applied in the same place every time I draw a sprite in the frame.

How do I specify a parameter that I can change for every sprite I draw in a single frame? I was looking at how to do uniform top level shader parameters, but I just wasn't able to make complete sense of it.

Also, what would be the most efficient way to feed in an array of parameters so that I can draw multiple decals per execution of the shader?

Here's the C# code I'm using to generate and set the parameters:

public void Draw()
Gorgon.CurrentShader = decalShader.Techniques["drawWithDecal"];


private Vector4D decalParms1(Vector2D decalBToffset, Vector2D baseTexAtlasSize, Vector2D decalSize)
float BTXDTL_x = decalBToffset.X / baseTexAtlasSize.X;
float BTXDTL_y = decalBToffset.Y / baseTexAtlasSize.Y;
float BTXDBR_x = (decalBToffset.X + decalSize.X) / baseTexAtlasSize.X;
float BTXDBR_y = (decalBToffset.Y + decalSize.Y) / baseTexAtlasSize.Y;
return new Vector4D(BTXDTL_x, BTXDTL_y, BTXDBR_x, BTXDBR_y);

private Vector4D decalParms2(Vector2D decalAtlasSize, Vector2D baseTexAtlasSize, Vector2D decalAtlasOffset)
float CFx = (float)decalAtlasSize.X / (float)baseTexAtlasSize.X;
float CFy = (float)decalAtlasSize.Y / (float)baseTexAtlasSize.Y;
float DOtc_xtl = (float)decalAtlasOffset.X / (float)decalAtlasSize.X;
float DOtc_ytl = (float)decalAtlasOffset.Y / (float)decalAtlasSize.Y;
return new Vector4D(CFx, CFy, DOtc_xtl, DOtc_ytl);

Any help rendered is much appreciated!

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