Hacker News Reader

Comment by spuz | Hacker News Reader

I'm just getting my feet wet with shaders myself. Could this fragment shader be used in a "real" 3D game? At the moment, the sphere is orthographicly projected onto the screen so the sphere will always appear as a circle rather than an ellipse. How easy would it be to use this shader in place of a spherical mesh as you might find in a typical geometry based 3D render? I.e. could you use the same technique and render multiple balls on the screen with positions in 3D space and rendered correctly taking into account any distortion caused by the camera?

View on HN

I'm also new to shaders, so I might be wrong: Camera distortion (/perspective) is done in the vertex shader, this post here presents a fragment shader. So if you take a quad, in the vertex shader then rotate it so it'll face the camera (i.e. all vertices have same z/depth value in clip space), you can then probably use this fragment shader to render the ball.

You have to rotate the quad to face the camera, since it has to cover the area where the ball may render (imagine the extreme case: if the quad is perpendicular to the camera, it looks like a line to the camera, you could only render the ball within that line, it wouldn't protrude to the left or right).

> You have to rotate the quad to face the camera

Or you only run the center point through the camera transform, and then render the quad in screen space. Should be cheaper in terms of compute.

It can be incorporated into a 3D scene as long as you can make a square billboard . You may have problems with clipping that' you need to address manually, but it might be worth it as you can get better visuals, assuming you are trying to render spheres.

As far as I know there shouldn't be any distortion issues. A sphere will always look like a circle from any perspective. Well that is unless it's really really big or your are really really tiny and you give the size difference you are relatively close to its surface. At that perspective it just look like a flat plane.

Parent may or may not know this, but "billboards" are textured quads that always face the camera in a 3D scene. Trees and foliage are things that usually turn into billboards in a typical game if they are far away.

Think of it similar to an impostor: in world space, a sphere is always a sphere, and you should do all your lighting in world-space, so you don't need to account for the distortion. You do need to project into the camera's viewing, typically typically the projection matrix. But since a sphere under rotation has the same silhouette, you can also rotate the quad to view different parts of the sphere, and use a circle inside, though the rotation math would have to view a different part of the sphere. To integrate with other objects, you likely would turn on depth writing, and use gl_FragDepth to write the front depth to the buffer, and discard when outside of the silhouette.

It could be used as a sprite that looks like real geometry with some tweaking.

Reply on news.ycombinator.com