When a shader uses 4 entity skin textures within the model, that means the shader/engine does 4 passes or 4 calls to the images? More calls/passes mean more rendering time correct?
mmm...hard to explain...there is an explanation of the D3D rendering pipeline into shader tutorials...
The graphic card calls the vertex shader once per vertex and calls the pixel shader once per screen pixel contained by the triangles projected into the screen. Backfacing triangles are clipped away inbetween so there can be not shown computed vertex and there can be screen pixels computed more than once because of triangles overlapping. There is no such a texture rasterization I guess your are thinking about. Just image samples for each projected pixel.
Linear interpolation on texture samples takes its time, yes. The more operations you perform the slower it will be, you know. You can speed up the samples by disabling the linear interpolation (MagFilter = Point;) but it shows the pixels.
Should I just make a new material?
Yes. A material for each rendering technique. You can define more than one technique inside the same effect file and share functions between techniques. For example:
float3 doDiffuse ( float3 inNormal )
{
float DiffuseFactor = pow ( ( 1.0f + dot(-vecSunDir.xyz, InNormal) ) * 0.5f, DiffuseGradient );
return lerp ( vecAmbient.rgb, vecSunColor.rgb, DiffuseFactor );
}
...
float3 texColor = texSample.rgb * doDiffuse ( inNormal );
