How to use ROI (Region Of Interest)

Your Altair camera can generate a very large video file very quickly in video mode. At full resolution you can generate over a Gigabyte in 30 seconds with some cameras.

Think of each frame in the video as an individual image. A camera running at 640×480 pixel resolution or “0.3 Mega-Pixels” will produce a rapid succession of images, each weighing in at around 0.3mb. But the camera is running at 60FPS or Frames Per Second, so multiply that by 60 seconds and you’ve got a 1.1GB file in a minute.

On the other hand, a camera of 1280×960 pixel resolution (1.2mp or “HD”) produces a 1.2mp image. If you run that at 30 fps, it would produce a 2.2GB file. Try that at 60FPS, and in a minute you’ll have a hard drive crunching 3.6 gigs of video data. Big files not only eat up hard-drive space, they also take longer to process – and are generally quite unwieldy things.

These are rough approximations and don’t take into account data transfer overheads, processing overheads and lossless compression, but they serve to demonstrate the relationship between these three main factors.

So the faster the frame rate (FPS or Frames Per Second), and the higher the resolution in pixels, the larger the resulting output file. They’re all related in a kind of “triangle”.

The good news is that you can manage and control these factors in AltairCapture to suit your imaging target and keep video file size down if you want to.

Is there really a need for such a large file if you’re filming a planet sitting in the middle of the field, with nothing but black space around it? Perhaps you’re interested in one small solar feature like a sunspot or flare, and want a faster frame rate to record these quick changes.

ROI or “Region Of Interest” is a feature available on most Altair Cameras. You can select the area on the video preview you’re most interested in (in real-time) and tell the camera to ignore the rest. By ignoring part of the image, you get a smaller file size, and also a higher frame rate (provided the exposure duration allows). Higher frame rate isn’t always necessary, but in some situations it can be helpful.

To use ROI click on the “ROI” menu and move the handles around the blue box to frame the object you want to isolate. Click “Apply”. If you’re in video preview mode, you’ll see the screen cropped down to that size, and the frame rate will increase if the exposure duration is short enough.

Open ROI menu and blue box will appear. Drag handles to select ROI size. (Click to enlarge in new window).


Click “Apply” and ROI will be activated. Frame rate will increase. (Click to enlarge in new window).

ROI Active

To go back to normal non-ROI mode screen size click “Default” and ROI is switched off. The frame rate will then decrease to the “normal” levels for your individual PC.

PLEASE NOTE: The method we use for frame rate indication is to divide “# all frames captured (since the camera is connected)” by “amount of time elapsed to gather all these frames” (All frames Captured/Time=Frame Rate). This means it takes a little time for the frame rate indicator to catch up with the real camera speed but it provides a more stable rate report, without gaps and glitches – for example when you change the exposure duration.