Texture tiling without deformation

Hi there !

I am trying to achieve a special texture stretching effect in my shader. Here is how I define my uv before sampling my texture, nothing really special.

// uv_ST contains the tiling and offset values
uv.xy = uv.xy * uv_ST.xy + uv_ST.zw

This gives the standard tiling/stretching and offset behaviour when you tile/stretche a clamped texture as you can see in the image below. First is normal, second is offset and last is stretching.

But I want to avoid the deformation behaviour when stretching, I want to keep margins when stretching my texture or simply cut it in the middle and stretching it. Here is an illustration below.

How could I do that inside my shader when defining my uvs before sampling the texture ?

Thanks a lot !