This week we have a new feature - hot pixel removal - and a change in behaviour for live stacking along with the usual minor changes and bug fixes.
Hot Pixel Removal (without needing dark frames)
This is something that we've had a number of requests for recently. Part of the desire for this comes from the new generation of very low noise CMOS cameras (IMX571, IMX533, etc). These cameras suffer from practically no amp glow or thermal noise, so the need for dark frame subtraction is much reduced. However, they still show some hot/warm pixels, so being able to get rid of those without using dark frames would be an advantage.
You can enable the new hot pixel removal by using the special 'Hot Pixel Removal Only' option in the 'Subtract Dark' dropdown. This also causes a sensitivity slider to show which allows the strength of the hot pixel removal algorithm to be tuned.
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Perhaps the easiest way to see what the hot pixel removal does is to set your camera up to capture dark frames of maybe a few seconds, enable the main histogram functionality and try turning it on and off and adjusting the sensitivity.
Here's the histogram of a dark frame (RAW16) on an ASI071MC-Cool with hot pixel removal turned off:
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- Capture3.JPG (46.12 KiB) Viewed 5560 times
To give a bit more detail on what the hot pixel removal does, it looks for pixels that are significantly brighter than any of the neigbouring pixels (how significantly depends on the sensitivity setting). When a pixel is found that exceeds the threshold, the value from that pixel is discarded and a value is taken instead from a nearby pixel (of the same colour for a colour camera).
My testing suggests that at the default sensitivity of 5, the new hot pixel removal detects and removes about 2/3 of the hot pixels that get picked up by a dark frame. However, since it is better at detecting the hottest pixels, those 2/3 account for about 90-92% of the total brightness in hot pixels that a dark frame would remove. At this sensitivity, hot pixel removal tends to have hardly any incorrect hot pixel detections (false positives).
As you turn the sensitivity up to 10, you can get to detecting about 80% of the hot pixels found by a dark frame, and about 98% of the hot pixel brightness being removed, but by this level there will be a significant number of false positive hot pixels being detected - this may or may not be an issue. At minimum sensitivity, you may only remove about 30-40% of the hot pixels that a dark frame will remove, but even then, you will probably get 70-80% of the total hot pixel brightness removed.
If you are interested in how many hot pixels are being detected/removed by this functionality, you can turn on extra logging for 'Image Processing' and you will get entries in the SharpCap log along the lines of '300 hot pixels removed out of 2000000'.
Live Stacking Change
Up until this version, Live Stacking would apply a 'behind the scenes' additional stretch to the stacked data if the images being stacked were significantly below the brightest values that can come from the camera. For instance if the maximum possible pixel value was 65000, but the actual maximum in the frames was only 10000, the live stacking could would pretend that the maximum possible value was only 10000 and 'pre stretch' the data before showing the histogram, etc.
In one sense, this was a good thing - it made it easier to adjust the histogram stretch, since the data was less scrunched over to the left hand side. On the other hand, recent reports have made it clear that this behaviour has some rather negative side effects - basically, if there are hot pixels in the individual frames that are brighter than the stars then as more frames are added and the hot pixels get smeared out, the behind the scenes stretch gets stronger, leading to the histogram drifting to the right, which in turn makes constant histogram tweaks necessary to keep a decent image brightness level.
After some consideration, I decided that any workaround to try to keep the behind the scenes stretch and fix this problem was likely to be a partial fix at best - the right thing to do was to remove the behind the scenes stretch entirely.
For most people, this will not affect anything - if you have any saturated stars in your individual frames then there will be no effect from this change. It's only those who are using underexposed frames for stacking (and that might not be the best use of your camera anyway) that will notice a change.
Since the 'behind the scenes' stretch was added, there have been changes to the live stacking histogram - addition of logarithmic axis mode and zoom mode - that mean that it is possible to adjust the histogram nicely even if it is mostly at the very left of the graph. I have also increased the data point density for the histogram graphs when in logarithmic mode to make these adjustments easier.
Other stuff
Other stuff this week includes a performance tweak to reduce CPU usage when using PlayerOne cameras and some bugfixes (including a couple of new UI issues).
cheers,
Robin
