Region of Interest (ROI) Limitations?

james1ca

Firecapture allows you to define just about any size of ROI in any location within the field of view and I'm wondering how that is done with the ZWO cameras like the ASI178MM. In fact, I have read that software may use binning and line skipping to create these non-standard ROIs and if that is true then it would seem that they might degrade the image quality produced by that ROI (if the binning and line skipping happens within the captured area of the frame).

Thus, are there limits on the size and location of an ROI in order to make certain that you always get the best possible image quality? Does the ROI always have to be taken from the upper left corner of the field (to maintain full quality)? Does Firecapture's "Center ROI" option have any effect on the captured image quality?

For the ASI178MM I see the following values for the "Default" (standard?) ROIs in Firecapture:

100 x 100

200 x 200

300 x 300

400 x 400

640 x 480

800 x 400

600 x 600

1024 x 768

Are all of these values directly supported by the IMX178 sensor? Can these "default" ROIs be repositioned anywhere within the frame without affecting the captured image quality?

In any case, I just want to know what ROIs (and locations within the frame) are actually supported by the ASI178MM hardware.

Sam@ZWO

ROI is just ROI

no binning no skipping

you can tell from the fps

fps grow if you decrease the ROI height

all is done by hardware

but there is minimal size required for ROI

I think any size bigger than 128 is supported

james1ca

So, if I place a 128 x 128 pixel ROI in the CENTER of the ASI178MM frame then the captured result will be exactly the same pixels that exist at that same location on the sensor, no modifications whatever?

However, if you think that the minimum size supported by the hardware is 128 pixels then how is Firecapture showing a 100 x 100 pixel ROI in the default set? Do they just request the nearest 128 x 128 pixels and trim the excess? That would be okay, but I wouldn't want them to capture at 128 x 128 and then resize the result to 100 x 100.

Similar issue with the other ROIs that aren't an even multiple of 4 (let alone 128). How about the 200 x 200, 300 x 300, etc.

bdeclerc

james1ca wrote:
So, if I place a 128 x 128 pixel ROI in the CENTER of the ASI178MM frame then the captured result will be exactly the same pixels that exist at that same location on the sensor, no modifications whatever?

However, if you think that the minimum size supported by the hardware is 128 pixels then how is Firecapture showing a 100 x 100 pixel ROI in the default set? Do they just request the nearest 128 x 128 pixels and trim the excess? That would be okay, but I wouldn't want them to capture at 128 x 128 and then resize the result to 100 x 100.

Similar issue with the other ROIs that aren't an even multiple of 4 (let alone 128). How about the 200 x 200, 300 x 300, etc.

The ROI defaults in Firecapture are from Firecapture itself, you can change all of them in the configuration. They aren't invented by the camera or the camera driver.

I suspect that if you pick 100x100 you'll get 100x100 but since the minimum hardware ROI is 128x128, you won't be able to get a higher framerate by using even smaller ROI's, so it would probably be advantageous to not pick less than 128x128 in Firecapture, but use "cutout" to keep the captures area to 100x100.

At no point will Firecapture resize the image in software, so I wouldn't worry about that.