Getting Bounding Box For Sphere on Screen

BlueSpud · 2014-12-15T12:25:24

Hey, So I want to do a bit of optimization on my deferred renderer, in that currently I render all the lights as full screen quads, which wastes a lot of time on unnecessary pixels. Right now, all I have is point lights, so basically lights are represented with a position and a radius. I want to compute the bounding box for the sphere so that I can either render quads where the fragment shader is going to be preformed or tile-based shading, all though I'm not sure if tile based shading needs this kind of computation. Right now I've used an approximation algorithm to get the radius and the center on screen and then I can construct the bounding box by that. It goes something like this: float rad = glm::atan(lightRad, screenSpacePos.z); rad = rad*(glm::max(ENGINE_WIDTH, ENGINE_HEIGHT) / glm::radians(RENDERENGINE_FOV)); It kinda works, but, like I said, its only an approximation, and when the camera is rotated, things that it covers becomes uncovered, so I need a more accurate representation. My thought was to construct a bounding box of the sphere in world space and translate it to clip space, get the min and max of all the points and construct a bounding box that way. Is there any way to do this more optimally? Or is this going to be one of the best ways to make sure that this is fairly accurate, as well as include other shapes ultimately (because bounding boxes can be calculated for those too). Thanks.

Graphics and GPU Programming Programming

Started by BlueSpud December 13, 2014 11:35 PM

10 comments, last by Hodgman 9 years, 4 months ago

BlueSpud

651

Author

December 14, 2014 08:01 PM

Either flip the culling mode when the viewpoint is inside the sphere, or else just use a regular full-screen quad for that case.

Nah, there's an easier way...

EDIT: Tried it and just remember why I decided not to use it. I rendered the geometry with additive blending and it doubled up because the back was showing through. I enabled back face culling, but when Im inside the sphere, it then culls out the sphere, so nothing is rendered. Any ideas?

Draw the back faces, not the front faces.

That worked perfectly. Thanks everyone for their help, this works really well for what I'm going to need.

Hodgman

52,717

December 15, 2014 12:25 PM

Last time I did this, I just rendered a sphere :D I mean you're issuing a draw call anyway, who cares about a couple dozen extra polys...

All those poly-edges will reduce your pixel-quad efficiency though.

In the *artificial* worst case, there could be 75% wasted computation time (high poly sphere covering only a few pixels).
Some new cards support an extension for drawing axis-aligned 2D quads, as tiled based processing is getting popular, and regular methods always incur the penalties from triangles intersecting with the 2x2 pixel-quads.

On the flipside, drawing a quad instead of a sphere will have about 25% wastage too, from the corners that are outside of the sphere...

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Getting Bounding Box For Sphere on Screen

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Getting Bounding Box For Sphere on Screen

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