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I want to apply SNORM texture by using GL_TEXTURE_3D as a target, what will be the fragment shader for the same? Also, what should be the data type of texture data?

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When using RGBA_SNORM you should use GL_BYTE with four components per texel to fill it.

It is basically the same as normal UNORM textures only that you get a value between -1.0 and 1.0 when sampling. The signed byte value is mapped -127 -> -1.0 and +127 -> 1.0. -128 is dropped.

Sampler type is sampler3D (Floating-point also covers normalized integer formats).

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isampler is used for signed integer textures. so i am just confused between sample3D and isampler3D. – Roshan Dec 10 '12 at 18:54
can you tell me why can't we use isampler3D here? – Roshan Dec 10 '12 at 19:05
The samplers result is not an integer, its a float. If you want an integer you should use an integer texture format instead of a normalised one. Eg GL_RGBA8UI for an unsigned integer between 0 and 255. – Archy Dec 10 '12 at 19:50
When I use glbyte with sampler3d. Values less than 127 are taken as 0 and renders black for it, works correctly with 127 – Roshan Dec 12 '12 at 14:41
Values less than 127 represent negative numbers in a SNORM (signed normalized) texture. Negative numbers get clamped to 0->black when you write them into a unsigned render target eg the back buffer. – Archy Dec 12 '12 at 14:44
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