Shadow map projection issue

Question

I am using Unity but this question might not be proper to this engine.

I have projected the shadow map onto this sphere but there are grazing shadows. Is it possible to avoid it or to correct it as I am using the raw depth data of the texture.

Here is the pseudo-shader code.

uniform sampler2D _ShadowMap;
float4 shadowCoord : TEXCOORD1;

// vertex shader
o.shadowCoord = mul(unity_World2Shadow[0], mul(_Object2World, v.vertex));

// fragment shader
color.rgb = tex2D(_ShadowMap, i.shadowCoord);
return color;

Answer 1

One of the downsides to shadowmapping is that you need to offset, or bias, the point used for shadowmap-lookups. This is done in a variety of ways - typically by a constant (shadowmap-depth) distance or based on the slope of the casting triangle. Both of these tend to be done during the shadowmap-rendering pass.

I think what you need to solve your sphere-problem, is to offset the point you are shading a bit along its surface normal, before doing the shadowmap-lookup. This happens during shading, when sampling the shadowmap, not during shadowmap-rendering. Incidentally it is also part of the Unity standard rendering pipeline, so you can steal it from the shader include-files.

ref http://www.dissidentlogic.com/old/images/NormalOffsetShadows/GDC_Poster_NormalOffset.png ref http://www.digitalrune.com/Blog/Post/1765/Shadow-Acne