Mirror rendering optimization

7892

As you can see in the picture I have a simple mirror in my game. The setup is pretty simple. Render a second camera on the mirrored position of your camera and project the texture on your mirror plane based on screen uvs. But right now I'm rendering the whole screen twice even if I just need to render the size of the mirror plane. Any ideas on how I can optimize this to render only what I can actually see in the mirror?

cybereality

The camera should do frustum culling by default, so the stuff behind the mirror (or outside the view) will not be rendered. The physical size of the mirror makes no difference. However, the resolution of the second Viewport does, as this affects the performance (how many pixels are shaded). So the easiest solution is to simply lower the resolution of the mirror. Most games use a half size or even a quarter size (half on each axis) and it can still look okay, as long as it's not a core part of the gameplay.

7892

I already have a render scale value for my mirror. Lets take a look at the picture above. Say the camera in the image is the mirror camera and ignore the car. Now I'm rendering everything as the mirror camera and the output would be the whole picture. But I only need everything covered by the mirror plane. Everything outside the mirror plane is just a wast of rendered pixels. And in the image the mirror covers less than 1/4 of the screen so why should I render the full resolution then?

7892

Here's a quick image to show what I mean. The left plane is the mirror and on the right plane you can see the base texture for the mirror.

cybereality

Oh, I understand. In that case, you'll need to change the FOV of the mirror camera so it matches the size you want to display (that way you are showing the full camera buffer on the mirror, not a portion of it). It doesn't seem like you can render to a region in Godot (this would be useful) so the FOV may not match exactly. But it would gain you performance and may look good enough.

7892

@cybereality said: Oh, I understand. In that case, you'll need to change the FOV of the mirror camera so it matches the size you want to display (that way you are showing the full camera buffer on the mirror, not a portion of it). It doesn't seem like you can render to a region in Godot (this would be useful) so the FOV may not match exactly. But it would gain you performance and may look good enough.

Changing the FOV is one thing but I think I need to change the position too. Maybe someone knows how to do this the correct way without guessing any values until its close enough?

newmodels

Maybe you can also change the viewport size that the second camera is in? I have not done work on mirror images, or rear views.

7892

@newmodels said: Maybe you can also change the viewport size that the second camera is in? I have not done work on mirror images, or rear views.

Do you mean the resolution? That's what cybereality also suggested. If you mean size like aspect ratio: That's on my to-do but right now the mirror is 16:9 to simplify testing.

I did a quick test in Blender trying to find a camera angle that fits the whole mirror without wasting anything around it but it seems impossible to get anything close to right.

cybereality

For it to work, the camera needs to be behind the mirror. You should be able to figure it out with code, it is the distance from the real camera to the mirror position, but behind. Then you set the cull mask to not render the mirror in the mirror camera. I believe this would get the right look without changing the FOV.

7892

@cybereality said: For it to work, the camera needs to be behind the mirror. You should be able to figure it out with code, it is the distance from the real camera to the mirror position, but behind. Then you set the cull mask to not render the mirror in the mirror camera. I believe this would get the right look without changing the FOV.

Thats what I have now. The camera transform is mirrored based on the mirror normal. But then you are rendering more than you actually need.

cybereality

If the FOV of the camera touches the edge of the mirror, then I think that should be correct. But I've never tried this, I'm just guessing.

7892

@cybereality said: If the FOV of the camera touches the edge of the mirror, then I think that should be correct. But I've never tried this, I'm just guessing.

Could be but I'm not sure. I think the uv projection would also change. I will stay with my current system for now because that sounds difficult to implementand and there is no difference other than a better performance :D But thanks for your help!

newmodels

You might also try, frame skipping.