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Article : Realistic Soft Edged Water In XNA

Smoothing the Shoreline

Our next step is to add the logic to the pixel shader in order to get a fade factor from zero water to fully opaque water. Replace your pixel shader with this one.

float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0
{
    float2 screenTexCoords = 0.5 * (float2(input.ClipPos.x, input.ClipPos.y) / input.ClipPos.w) + 0.5;
    screenTexCoords.y = 1 - screenTexCoords.y;

    // Get the terrain depth
    float sceneViewZ = tex2D(DepthSampler, screenTexCoords).r;

    // Get the water plane depth
    float waterViewZ = length(input.ViewPos) / FarPlane;

    // Get the range of depth from waterplane to terrain
    float depthRange = (sceneViewZ - waterViewZ);

    const float shoreFalloff = 2.0f;
    const float shoreScale = 5.0f;

    // calculate a transparency value using a power function
    float alpha = max(pow(depthRange , shoreFalloff) * FarPlane * shoreScale, 0);

    return float4(0, 0, 1, saturate(alpha));
}

The shoreFalloff and shoreScale constants here can be made global to the shader, and set from C#. A value of 1.0f would mean no falloff, and higher values mean a longer a falloff. When using a higher falloff you probably want a higher scale, otherwise the falloff wont fade quick enough.

Also, since we are now alpha blending for the moment, we need to set the blend source and destination states. In the Planar Water draw method, set AlphaBendEnable and AlphaTestEnable to true, and the source and destination blend states to SourceAlpha and InverseSourceAlpha, as follows:

renderState.AlphaTestEnable = true;
renderState.AlphaBlendEnable = true;
renderState.SourceBlend = Blend.SourceAlpha;
renderState.DestinationBlend = Blend.InverseSourceAlpha;

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