Limit Your Exposure Time
Your most immediate problem is likely that you're exposure-limited. At full resolution, the PR2's Prosilica can stream at 15fps. Since you're only getting 2fps, I'm guessing the environment is dark and auto-exposure is ballooning to ~0.5s to achieve an acceptably bright image.
Try playing with the exposure and gain settings - you can access these through dynamic_reconfigure (look for the /prosilica_driver group). Turn off auto_exposure and set the exposure time to 0.066 or less (for 15fps). Then adjust the gain level to get a sufficiently bright image, or let auto_gain do it for you. Gain is digital amplification; it gets you a brighter image at the cost of increased noise.
The downsampling can be done either in hardware or software.
Hardware Downsampling
This is called binning. When you use MxN binning, the camera combines each MxN block of pixels into one super-pixel. You can enable binning with the binning_x and binning_y parameters.
Besides reducing the image size, binning can increase frame rate in a couple of ways:
- For each frame there's less data going over the wire. At full resolution, the frame rate is bandwidth limited, so your max FPS goes up. For the PR2's Prosilica, with 2x2 binning you can achieve 25fps, and at 4x4 binning (612x512 image) you can get ~39fps.
- A pixel is basically an electron bucket, and by pooling adjacent buckets in hardware you get better sensitivity. So you can reduce exposure time and/or gain in low-light conditions.
There is one big drawback to hardware binning: you lose color. Color cameras generally work by tesselating a 2x2 color filter called a Bayer pattern over the sensor. Each pixel only detects one RGB channel. One of image_proc's duties is to interpolate (debayer) the raw image to get a full RGB image. But when you apply binning, each 2x2 Bayer element gets averaged into one monochrome super-pixel.
Software Downsampling
Downsampling in software gives you none of the frame rate and sensitivity advantages of hardware binning. You should only bother if you need a smaller image and at least one of the following is true:
- Your camera doesn't support hardware binning.
- You require color.
In ROS Electric, the image_proc package contains a new crop_decimate nodelet that applies software binning/ROI. It is not loaded as part of the standard image_proc node. You can try it out by doing:
rosrun nodelet nodelet standalone image_proc/crop_decimate camera:=prosilica camera_out:=prosilica_downsampled
rosrun image_view image_view image:=/prosilica_downsampled/image_raw
Then open /image_proc_crop_decimate in reconfigure_gui and set decimation_x and decimation_y to 2.
Conclusion
You may not be able to get everything you want from the PR2 Prosilica. If you don't need color, use hardware binning. If you need color but can live with 15fps, use software downsampling. If you absolutely need color at 30fps, the GC2450C is simply the wrong camera. Prosilica does have cheaper cameras at VGA resolution.
