I've made a similar change locally. It's worth noting that if your problem gets big enough COLMAP will fall-back to the CPU version of PBA which is ~an order of magnitude slower. You can remove the check in COLMAP to always use the GPU if you are comfortable with risking the worse numerical stability.

Think you mean in controller/incremental_mapper.cc:68 checking for ParallelBundleAdjuster::IsSupported?

I also found that if the camera mode is single, pba is also not used. But using single camera itself can reduce a lot of time.

No I don’t mean that. There’s a check on the number of residuals in the problem hardcoded. I’ll have a look for it on Monday if you can’t find it.

Since IncrementalMapperOptions has many other useful options, I would prefer to instead provide a flexible mechanism to modify any of these options from the command line, just like COLMAP accepts --Mapper.ba_global_pba_gpu_index 1. This could be later extended to the ImageReaderOptions.

This should be solved by #210. Can you please confirm that it works for you?