Response to https://gamedev.stackexchange.com/questions/165514/texture-atlas-and-hlsl-shader-for-sprite-tiling

Atlas with wrapping

float4x4 WorldViewProj : WorldViewProjection;

texture ColorTexture;
sampler2D ColorSampler = sampler_state {
    Texture = <ColorTexture>;
    FILTER = MIN_MAG_MIP_LINEAR;
    AddressU = Wrap;
    AddressV = Wrap;
};  

float2 CellCount;
float2 CellIndex;
float2 TCRepeat;

struct VertexInput 
{
    float4 Position : POSITION0;
    float2 TextureCoordinates : TEXCOORD0;
};

struct VertexOutput {
    float4 Position : POSITION0;
    float2 TextureCoordinates   : TEXCOORD0;
};

VertexOutput mainVS(VertexInput input) 
{
    VertexOutput output;
	
	float2 cellSize = 1.0f / CellCount;
	float2 cellOffset = cellSize * CellIndex;
	
    output.TextureCoordinates = (input.TextureCoordinates * cellSize + cellOffset) * TCRepeat;
    output.Position = mul(input.Position, WorldViewProj);
    return output;
}

float4 mainPS(VertexOutput input)  : COLOR0
{
	float2 cellSize = 1.0f / CellCount;
	float2 cellOffset = cellSize * CellIndex;	
	
    float2 tc = input.TextureCoordinates;
    tc = frac((tc - cellOffset) / cellSize) * cellSize + cellOffset;
    float4 sample = tex2D(ColorSampler, tc);
    return sample;
}

technique technique0 {
    pass p0 {
        CullMode = None;
        VertexShader = compile vs_3_0 mainVS();
        PixelShader = compile ps_3_0 mainPS();
    }
}