If you are working on Linux or OSX, then the exrtotiff executable may be causing you problems on HW3.
Please use /usr/class/cs348b/files/exrtotiff with parameter -bg 0.001. For example, to convert file.exr to file.tiff, use '/usr/class/cs348b/files/exrtotiff -bg 0.001 file.exr file.tiff'. Note that this is only for Linux. The OSX binary is at /usr/class/cs348b/files/exrtotiff.osx.gz..
There appears to be a bug that has to do with the way exrtotiff handles premultiplied alpha, which can generate large errors in output tiff pixel values. The output EXR file seems to be fine -- just the conversion to tiff produces problems. The following images illustrate the symptoms of the problem, to help you figure out whether this bug has been affecting you:
The left image was created with the version of exrtotiff in the pbrtsrc that you have been using since Homework 1. The right image was produced with the newer /usr/class/cs348b/files/exrtotiff with parameter -bg 0.001.
The original exr for both of these is exrtotiff.exr, and the corresponding tiff files are exrtotiff.tiff and exrtotiff.new.tiff.