```sampler``` specifies the sampler to which the texture from which texels will be retrieved is bound.
```coord``` specifies the texture coordinates at which texture will be sampled.
```bias``` specifies an optional bias to be applied during level-of-detail computation.
### Description
The texture2D function returns a texel, i.e. the (color) value of the texture for the given coordinates. The function has one input parameter of the type sampler2D and one input parameter of the type ```vec2``` : sampler, the uniform the texture is bound to, and coord, the 2-dimensional coordinates of the texel to look up.
There is an optional third input parameter of the type float: bias. After calculating the appropriate level of detail for a texture with mipmaps the bias is added before the actual texture lookup operation is executed.
Side note: On iOS devices texture lookup functionality is only available in the fragment shader.
