How does an image'alpha transfer so much information to other nodes in Unity Shadergraph?

Question

I have a image:

The upper part of this image, which alpha value is 1 (or 255 in RGBA)

The lower part of this image, which alpha value is 0.3, I used it for shadow in game.

So When I import it to Unity ShaderGraph as a _MainTex, when I split it alpha, it looks like this:

imported alpha

My first questions is:

"alpha" is actually a VECTOR 1 type in Unity Documention, but as I could see from the preview, there are three colors, black indicates alpha's value 0, hard white for alpha's value 1 and soft white for alpha's value 0.3, how can one single value transfer so much messages?

My first understanding is:

each pixel's alpha value is stored in the images already, the "alpha" in the shadergraph is just

like a global parameter to control them based every pixel.[I dont know if this is correct]

but when I give alpha a smoothstep node, I

am going to set the pixels's alpha under 0.3 to 0, I found it worked like this:

smoothstep added to the alpha, as you can see, 0.3<0.99, so

the translucent of the image is removed!

So here comes my second question:

Since "alpha" in the input works like a global parameter, how does it affect a picture separately?

My second understanding is:

"alpha" is just like an one-dimensional array, it stores transparency likes this:

{1,1,1,0.3,0.3,0.3}

and when it calculated by smoothstep,its value will be changed like this:

{1,1,1,0,0,0}

But it comes to my first question, ALPHA IS A VECTOR1 TYPE, it only has one value to edit

in the node, it can not be an array!

So, How does an image'alpha transfer so much information to other nodes in Unity Shadergraph?

https://docs.unity3d.com/Packages/com.unity.shadergraph@6.9/manual/Data-Types.html https://docs.unity3d.com/Packages/com.unity.shadergraph@6.9/manual/Smoothstep-Node.html

Someone who can help me really appreciated!

Answer 1

Shaders work in parallel: for any given vertex or pixel you only get data local to this element. Also critically here 'pixel' (or 'fragment') is a screen pixel, not a texel, which relates a texture's pixel.

In this context, the output of the texture node is a single rgba Vector4 (4 scalar values) at the provided coordinate. This is disconnected from how textures are stored: filtering, compression and mipmapping will come into play (and the control over this comes from the sampler, which you can also provide to the node even though it's most of the time implicit).

Smoothstep is a function that can remap a value - a vector (like the rgba output of the tex node), or a scalar (like the alpha) - into another range. More specifically it does it with smoothing both ends of the spectrum so that the slope is 0 at min and max. The linear equivalent is inverse lerp (which doesn't have a built in instruction in hlsl). You can read about the breakdown on the wikipedia page: https://www.wikiwand.com/en/Smoothstep