texture minification filter in raytracing?

Question

can someone point me to a paper/algorithm/resource/whatever that tells me how to implement a texture minification filter (applies when texels are smaller than pixels) in a raytracer?

thanks!

Answer 1

I think you mean mipmap'ing.

Here is an article talking about using them.

But nether say how to chose which mipmap to use, but they are often blended (the bigger and smaller mipmap).

Here's a one more article about how Google Earth works, and it talks about how they mipmapping the earth.

Answer 2

Since you are using ray tracing I suspect you are looking for a high quality filtering that changes sampling dynamically based on the amount of "error". Based on this assumption I would say take a look at "ray differentials". There's a nice paper on this here: http://graphics.stanford.edu/papers/trd/ and it takes effects like refraction and reflection into account.

Answer 3

thank you guys for your answers, but since I didn't find any appropriate techinque i created something myself which turned out to work very well:

i assume my ray to be a cone with a coneradius of half a pixel on the imageplane. when the ray hits a surface, i calculate the ellipse which is projected onto the surface (the ellipse from the plane-cone intersection). Then, using the texturecoordinate derivatives at the intersection point, i project this ellipse into texturespace. now i know which part of the texture lies under my pixel and can subsample this area

I Also use RipMaps to improve the quality - and i chose the RipMap level based on the size of the ellipse in Texturespace