So my problem is , what i need to somehow access a mask which is generated inside my shader , but i don’t know how can i do so , basically i need an output value of my subgraph. Is it possible to create a property with input field? Or a custom node which can set value in property?
No.
Generally, a shader’s only output is the final color value written to the current render target. Technically a shader if properly setup can output to multiple targets or with DX11 or better to arbitrary compute buffers, but Shader Graph has no support for either.
If you have a mask you want to use in your shader, you either have to recalculate it every time in every shader, or render it out to a render texture before hand using command buffers.
This might work for me , but how do i render it , i am pretty new to a shaders
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Try searching Google for Unity command buffer tutorials?