What's the default value of -ab option of rtrace

Hi everyone,

I read manual pages for rtrace, for the "-ab ", it show as follows:
This value defaults to 1 in photon mapping mode (see -ap below), implying that global photon irradiance is always computed via one ambient bounce; this behaviour applies to any positive number of ambient bounces, regardless of the actual value specified.

Based on my understanding, this means that in mapping mode the default value of “-ab” is 1. But for the command I use as follows, what is the default value of “-ab”?

cat pixels.pts | rtrace -h -I -ov scene1.oct | rcalc -e '$1=$1*179'

Thanks in advance for your time and help. I appreciate it.

Kind regards,


rcalc doesn’t have a -ab option. For the default settings of rtrace, run rtrace -defaults. See Section 4.1 in the Radiance Tutorial.

1 Like

Hi Axel,

Thank you so much for your help. You are right, it is my fault. The question should be the default value of -ab for rtrace. I have revised the title of this topic.

Besides, I have another question. If the setting of -ab is 0, it will not consider the ambient bounces. Based on my knowledge of Radiance, this means that it will not consider the reflectance. Am I getting right?

Thanks again for your time and efforts.



We generally don’t mark topics as “solved” as this quells further discussion. You can just “like” responses you find helpful.

With -ab 0 and -I+ options, only the direct light arriving at a point is considered, so you are correct in your surmise.


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Hi Greg,

Thank you so much for reminding me about the rules of the forum. Also, your reply is constructive which helps me a lot to understand the Radiance.

Warm regards,