How to know if a pixel is above or under a the geometrie?

I have this question for a long time. Let's say you have a plane and you want to display the pixels under the plane in black, and the ones above the plane in white. How I can know in the fragment shader at the time I calculate the color of the pixel, that its position is above or under the geometry?

If someone found a solution to a similar problem I will be glad to heart about it.

Or it can be simple as, if the pixel is located in the 3d space <0 I display it in black and in white otherwise.

 

If you only need to know if a 'pixel' (fragment, actually) is above or below a simple plane in the y direction, you can pass that value as a simple shader property, and compare it with the worldPos.

 

Daniel Brauer@: Interesting idea, but I need to have the Y coordinates, it is about water. I want to darker under the water, and maybe shade under the water depending on the Camera/Geometrie orientation... not something easy to do!

rc-1290@: I see what you mean, but if I pass the Y threshold from C# or JS, how do I know in the Fragment shader that the current pixel is above or under. I do not have the x,y,z position of the pixel as if it was a raytracer for example... I only have to x/y screen coordinates, the normal, or the vertex x,y,z...

If I want to have a water separation between the top and the bottom of the water, it has to be pixel by pixel. Now if I use a grid with animated waves, it gets even more complex...

So I would like to find a way to estimate the Y coordinate for each pixel I draw... it is maybe impossible without Ray Tracing?

 

Thanks Meth11, I will make some tests and if I find a solution that works will post the piece of code here.

 

For a moving water surface you'll need to pass more than one value, so the technique I was talking about won't work, because the height values are different everywhere.

I briefly used the built-in shadow casting functionality in my watersimulation. It had a pretty interesting effect, but it was a bit odd because I didn't add any underwater lighting, so the effect was too dark, and looked odd at the maximum range of the shadow. Improving the water surface graphics themselves had a bigger impact on the 'prettiness'.

 

oK, if I do

In the vertex shader:

and in the pixel shader I do

It is not working of course: here is what I get:



But what I want is more complex... I want to blur only what is under the water. Like this



A little bit more tricky... I have to find a smart way to do it because I want it to be fast...
Any idea?

 

Or another idea, is to make the water a cube instead of a plane, and check if I am inside the cube (draw inside the cube black for example)...