Cheap transparent shadows

[Josh]

While testing some code I accidentally discovered a way to do cheap semi-transparent shadows. The shadow casting object needs a shadow shader with a fragment program. Just add this line to the fragment main function:

if ((int(gl_FragCoord.x + gl_FragCoord.y) % 2) == 1) discard;

This is what it looks like with shadow filtering disabled:

And this is what it looks like with normal shadow filtering. (Ambient light is black in this shot.)

[havenphillip]

Dude that's rad. I just tried it on a basic shader and got semi-transparency in-game. It's not z-sorted or anything.

 

[Josh]

Mod it by 4 instead of 2 and it will be more transparent.

[IceBurger]

Wouldn't the amount of transparency change depending on shadow resolution (Or overall render resolution as a model shader)?

[Josh]

1 minute ago, IceBurger said:

Wouldn't the amount of transparency change depending on shadow render resolution (Or overall render resolution as a model shader)?

Nope, it is discarding pixels in a uniform pattern, then the normal shadow blur filter mixes it all together.

[havenphillip]

Has a little more of a dithered look:

if ((int(gl_FragCoord.x + gl_FragCoord.y) % 4) == 1) discard;

 

 

[Josh]

I got that wrong, that would give you 75% opacity. This will make it more transparent:

f ((int(gl_FragCoord.x + gl_FragCoord.y) % 4) != 1) discard;

 

[Josh]

Are you doing the discard in the normal shader or the shadow shader?

[havenphillip]

Ok yeah that looks great on shadows if I turn up the lighting quality in the options. Seems like that would be the best use. I cranked everything up on the normal shader but it still looks all dithered. 2 looks pretty good, though, even with the lighting on low:

 

 

[havenphillip]

This is:
if ((int(gl_FragCoord.x + gl_FragCoord.y) % 4) != 1) discard; ...with the lighting quality up.

 

 

 

[reepblue]

I'll have to try this. I have a lot of glass in my game.

[reepblue]

It seems not to work with Z-Sort enabled and that's how my glass works. I don't want any dithering on the material as seen with @havenphillip's screenshots.

[Josh]

Maybe you could add a second object with a shader that discards every pixel in the scene render, but still renders the shadow.

[reepblue]

That's an idea, but it might register as another drawcall or a light effect. My glass needs a script attached to it anyway so it doesn't get collapsed with the rest of the scene., What I can do is make each glass plane an actor that'll create an invisible box with it's AABB size and have it use that shader. I can also toggle the effect to help performance if it's much of a hit.

I can also probably do this when my screne applies the correct cubemap to the shader too. I can pass the entity through a function that'll create the box on level load. Lemme try that first since that'll be easier. 

I'll see what I can come up with, thanks.

[Josh]

It works. You don't even have to modify your existing materials or objects, just add an invisible dummy block that creates the shadow.

glasstest.rar

 

[reepblue]

Thanks. I was still going to add the dummy boxes via code but if it can just be collapsed with the scene with no artifacts, I might add it manually.

[Josh]

Now I must be added to the credits of Cyclone (TM), the amazing successful game on Steam!

[reepblue]

I mean, you already are in the readme.txt because you wrote the engine. 🙂

[reepblue]

Most of the time, I want as much light coming through as possible, but cases like this is a great example of how to use this effect. 

Here we have the default setup with no shadows being cast.

Now with that brush hack applied.

Then we apply the cubemap during map load and this is the final result!

It took like 5 years, but now Leadwerks now has a solution for glass.