My “simple light” scene is brighter and noisier than expected

[heyajulia]

I’m currently stuck on a perplexing issue: my “simple light” scene (AKA image 2.16) looks quite different from the book, and I don’t know why.

I expected this (this is just a hotlink to the image from the book):

But instead, I get this:

^{(In case it’s useful: image.ppm.zip)}

As you can see, it’s noticeably brighter and noisier. Is that correct?

How do I find out if there’s a bug and where it is?

My code is here: https://github.com/heyajulia/Raytracer. The specific commit is https://github.com/heyajulia/Raytracer/commit/d3f6ba65bba718035d97bb881e6aeb087db64bb3.

Update

The bug is in my code. I know this because I just tried running the reference implementation and got an image similar to the one in the book. (I never thought the bug wasn’t in my code, but it seems prudent to spell this out, and it means there’s one less place to look.) The search continues…