Why is the gobo filter limited to spot lights?

[AJUKking]

Why is this the case? Unless I'm failing to think of a better method, I believe this means that a spot light with a gobo filter can't easily be restricted to a certain area / certain shape like a rectangular projection: the spot light will give you a circle projection if you shine it perpendicular to a plane. But what if you want it to project as a rectangle? My best guess is using a light blocker to cut off the edges but that just seams silly. A gobo filter on a quad light would solve this but this isn't allowed.

I made another post just before this one where I figured out a full workaround by using a light blocker in a certain way but that's a whole other write up so go check it out.

[peter.horvath6V6K3]

What about changing the Roundness of the spot light?

[AJUKking]

@Peter Horvath This does work to give a square projection but doesn't account for a perfect perpendicular projection of light that a quad light would. I'm referring to the fact that spot light is producing light from a point while quad light is producing light from a square/rectangle. i.e. A spot light with no roundness/square projection pointed perpendicular to plane shows a square projection - but moving the plane towards the spot light will show the projection decrease in size since the distance between light and plane decreases. In the case where the plane is instead geometry that has raised polygons then the spot light projection will also be incorrect ('incorrect' in this context meaning not what the user wants due to art direction), regardless of the 0 roundness. See link. (I would attach the image directly to this post but I keep getting a file parse error). See how the height difference leads to ruined attempt at seamless light repetition. Of course now the answer is to use the quad light but gobo isn't supported on that, thus why I posted this problem.

[peter.horvath6V6K3 · Accepted solution]

What about connecting the image directly in the color of the area light? This probably gives you less features than the Gobo filter, yet it should produce a similar result. Is there a specific effect you are unable to achieve this way?

[Andrew_Wilkins1]

Not sure if this helps, but have you tried increasing the lens radius of the spotlight?

May require some tweaking, but a lower value on the cone angle (around 30) and a high value on the lens radius (200-300) and then adjust light exposure to compensate cause it will get darker.

[AJUKking]

I can't believe I didn't know about this feature. Thank you for telling me about it. So simple and yet somehow I never knew.

I was simply trying to achieve easy tiling so that I can create a motion graphics loop video.

What's the worst I'm missing out on by not using gobo in this case? The volumetric-like filter functionality?

I've put in a shader network into the color of the area light instead of just an image to create the fake chromatic aberration (from one of my other forum posts).

Everything is working great now, thanks!

[AJUKking]

Just tried it out and it's pretty finicky. Not easy to tell when you're getting a perfect perpendicular projection since what shows up in the viewport isn't always what you see in the IPR, especially when the lens radius is changed. Changing the height of the light in world space also changes the size of the projection on a plane so that's not handy in my situation. Alternatively, Peter's answer works great and is really simple and I'm surprised I didn't know about it. Also I'm not entirely sure what use case lens radius would have.

[Andrew_Wilkins1]

Cool, good luck. Agreed!
I've only really used the lens radius to make volumetric lighting from a spotlight more perpendicular/parallel and not emitting from a single point.