Uniformly sampled bidirectional tracer is noisier in mostly directly light situations.

On debugging: You're on the right track with the reference image comparison. Get ready for a boatload of pain though.

I really suggest you download the Intel compiler trial. It saves a lot of debugging time (as the generated code pretty much beats VC++ by 10-20% usually)

For BDPT, the weights for path configurations of a given path length must sum to 1. So, we can have any weights we want, as long as it sums to 1. Some things I would try

- render with path length = 3 only, compare uniform bdpt with reference
- render with path length = 3 only, render with 4 eye verts, 0 light verts weighted to 1,
- render with path length = 3 only, render with 3 eye verts, 1 light verts weighted to 1,

Also, I've found it extremely helpful to posterize the images in a photo editor (I use Paint.Net) and compare. It lets me see the lighting gradients very well.

Also, having a crapload of asserts is extremely helpful (check your understanding, detect undesirable nan/infs)

Also, I have some stuff in my blog when I implemented BDPT. (I have not yet written about MIS BDPT)

Here's my reference uniform BDPT code by the way
https://github.com/j...ghtway/bdpt.cpp

MIS BDPT (Does not handle specular / escaped ray)
https://github.com/j...way/bdptMIS.cpp

My implementation requires at least 2 eye vertices (so weights change slightly)

His implementation looks like uniform weighting (might be wrong). With standard path tracing, you have eye*-shadow-light connections, whereas in uniform BDPT you have a portion of paths requiring random hits vs the light (and higher variance). Thus, for mostly direct lighting (where eye*-shadow-light should be weighted more) uniform BDPT's increase variance should be very visible in a comparison with regular path tracing.

[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif]

[background=rgb(250, 251, 252)]if you have smaller lightsources[/background]

[/font]
[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif]

[background=rgb(250, 251, 252)][/quote][/background]

[/font]
[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif]

[background=rgb(250, 251, 252)]Are you by any chance referring to MIS path tracing? where we combine brdf sampling (good for specularish surfaces) and explicit light sampling (good for small lights)?[/background]

[/font]

MIS BDPT weights isn't easy to get right. Understanding the bdpt implementation section in the Veach thesis and fully grasp the P(i+1)/P(i) notation (i.e. what is i? What is a path with 0 light vertex? etc) is very important.

Also, you can try summing weights of a path length and see if it gives you 1 for fully expressable paths (e.g. eye/light paths are not too short for open cornell boxes)

Also, for MIS BDPT if you're using an open cornell box it will look different unless you somehow correctly account for escaped rays.