[New User] Checkerboard Pattern in Image?

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[New User] Checkerboard Pattern in Image?

Postby jimv » Fri Mar 18, 2016 1:43 am

Hello,

I'm a first-time StarTools user (version 1.3.5.289).

When I load an image and apply AutoDev a checkerboard pattern becomes evident (see attached screen capture).

What am I doing incorrectly?

Thanks.
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StarTools Checkerboard.jpg
StarTools Checkerboard.jpg (308.7 KiB) Viewed 5956 times
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Re: [New User] Checkerboard Pattern in Image?

Postby admin » Sun Mar 20, 2016 1:59 am

Hmmmm... I've never seen this one before!

What do you use to pre-process and debayer your images?
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Re: [New User] Checkerboard Pattern in Image?

Postby jimv » Sun Mar 20, 2016 2:56 pm

admin wrote:Hmmmm... I've never seen this one before!

What do you use to pre-process and debayer your images?


PixInsight v1.8.

This checkerboard pattern is not visible in PixInsight nor Nebulosity (v4) no matter how much stretching is applied to the image.
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Re: [New User] Checkerboard Pattern in Image?

Postby admin » Mon Mar 21, 2016 1:37 am

Would you be able at all to share the data with me?
I'm suspecting there is, in fact, a pattern in the data and that it will "go away" (e.g. be much less visible) when you zoom out to 100%. The way the zoom function works in StarTools, tends to bring out such anomalous patterns (for exactly that reason), as well as to keep the apparent noise level constant when zoomed out.

It's often caused by issues with the debayering (such as specifying an incorrect matrix for the RAW data). Can you tell me what device you used to acquire the data?

Thanks!
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Re: [New User] Checkerboard Pattern in Image?

Postby jimv » Mon Mar 21, 2016 4:28 am

admin wrote:Would you be able at all to share the data with me?


Sure. Let me know where I can place a 70mb FITS file!

admin wrote:It's often caused by issues with the debayering (such as specifying an incorrect matrix for the RAW data). Can you tell me what device you used to acquire the data?


The camera is a Starlight Xpress SXVR-M25c.
The BGGR matrix pattern was used in PixInsight.
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Re: [New User] Checkerboard Pattern in Image?

Postby admin » Mon Mar 21, 2016 5:05 am

jimv wrote:Sure. Let me know where I can place a 70mb FITS file!

Great! Most people like to use Dropbox, Google Drive or Microsoft OneDrive.

jimv wrote:The camera is a Starlight Xpress SXVR-M25c.
The BGGR matrix pattern was used in PixInsight.


Hmmm... that sounds about right. Is this a single frame or a stack?
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Re: [New User] Checkerboard Pattern in Image?

Postby jimv » Mon Mar 21, 2016 5:10 pm

Thanks for your interest in this.

admin wrote:Great! Most people like to use Dropbox, Google Drive or Microsoft OneDrive.


OK, there are now two, FITS files (integration.fit and integration (cropped).fit) at: https://onedrive.live.com/redir?resid=83E0AB8033CF6569!104&authkey=!AM4wwNnBaAvEAGk&ithint=folder%2cfit.

Both files were created by PixInsight. integration.fit exhibits the checkerboard pattern, in StarTools, when AutoDev is applied; integration (cropped).fit does not exhibit the checkerboard pattern when AutoDev is applied.

admin wrote:Hmmm... that sounds about right. Is this a single frame or a stack?

The files are stacks.
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Re: [New User] Checkerboard Pattern in Image?

Postby admin » Tue Mar 22, 2016 10:07 pm

Thanks Jim,

There is definitely something not quite right with the data;

integration(4).png
integration(4).png (85.79 KiB) Viewed 5895 times


As you can see in the 400% blow-up of a crop of the data, there is a clear zipper pattern visible, which in turn causes the StarTools preview to exhibit an anomalous pattern as well (e.g. the feature I highlighted above is doing its job and rightfully drew your attention to the problem)

Zipper artifacts such as these are text book examples of debayering artefacts. Something is going wrong in PixInsight or the data you feed is not quite right. Especially if this is a stack of multiple images, the debayering artefacts in this resulting dataset are excessive.

I'm afraid this is a pre-processing issue that is a little bit beyond the scope of StarTools, but the answer to your original query can be summed up as "StarTools shows an artificial pattern on the macro level due to the presence of an artificial pattern on the micro level".
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Re: [New User] Checkerboard Pattern in Image?

Postby jimv » Wed Mar 23, 2016 12:15 am

Thanks for taking the time to analyze this.

admin wrote:As you can see in the 400% blow-up of a crop of the data, there is a clear zipper pattern visible, which in turn causes the StarTools preview to exhibit an anomalous pattern as well (e.g. the feature I highlighted above is doing its job and rightfully drew your attention to the problem)


I must be missing something here, because I do not see a pattern in the sense of discernible regularity.

admin wrote:Zipper artifacts such as these are text book examples of debayering artefacts. Something is going wrong in PixInsight or the data you feed is not quite right. Especially if this is a stack of multiple images, the debayering artefacts in this resulting dataset are excessive.


Upon further inspection, each de-bayered image exhibits a micro pattern of some sort so it may not be the result of integration. Such patterns are visible regardless of whether Nebulosity or PixInsight is used to de-bayer.

The PixInsight debayering configuration used was:
    Bayer/mosaic pattern: BGGR
    Debayer method: VNG
    Evaluate noise using multi-resolution support

Is the use of noise evaluation where I went astray?

admin wrote:I'm afraid this is a pre-processing issue that is a little bit beyond the scope of StarTools, but the answer to your original query can be summed up as "StarTools shows an artificial pattern on the macro level due to the presence of an artificial pattern on the micro level".


This is confusing to me. How does an irregular, micro pattern (visible at 400% magnification) become the regular, macro checkerboard pattern (visible at 100% magnification) after application of AutoDev. Are you saying AutoDev generates a regular artificial pattern that has no relation to the underlying (irregular) pattern it detects.

At the risk of being repetitious, neither the PixInsight nor the Nebulosity viewers show this regular pattern at any level of stretching.
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Re: [New User] Checkerboard Pattern in Image?

Postby admin » Wed Mar 23, 2016 1:40 am

jimv wrote:This is confusing to me. How does an irregular, micro pattern (visible at 400% magnification) become the regular, macro checkerboard pattern (visible at 100% magnification) after application of AutoDev. Are you saying AutoDev generates a regular artificial pattern that has no relation to the underlying (irregular) pattern it detects.


Correct! (but you're incorrect that the micro pattern is irregular). You're looking at "intentionally unsuppressed aliasing effects" :)
Aliasing can cause micro patterns to be visible as macro patterns ( see Moiré patterns for example)

First off, it's important to note that the 'pattern' that AutoDev shows (not introduces!) is merely cosmetic and is not the result of stretching (or any other operation for that matter). You will see the same macro pattern in any other module in StarTools - it's not inherent to the functionality of the module. Again, at 100% zoom it will go away. Save the image, and it won't be visible (unless viewed within StarTools or another program that uses intentional nearest neighbour scaling). It's a visualisation perk of StarTools;

The way you're seeing AutoDev rendering your data to the screen is purely because of the way the scaling algorithm (for the purpose of fitting the whole image on the screen) works; it is intentionally coded in such a way that visualisation brings out the patterns by way of aliasing artifacts/patterns (which is what you're looking at), if such patterns are present on a mico-level. It also keeps perceptual noise constant at any zoom factor, rather than smoothing it out when zoomed out. It's a feature, not a bug. It helps to 'unhide' issues with your data that you might have otherwise missed because you were looking at the data on another scale/zoom level.

If you're interested in how this visualisation feature works, here is an explanation; for a Moiré pattern to become visibile, it requires that two patterns are superimposed over eachother. StarTools uses this fact to its advantage by artificially providing the first pattern. If your data - at any scale, zoomed in or not - contains the other pattern required to make up the Moire pattern, the Moiré pattern becomes visible. If there is no pattern in your data, the conditions for a Moiré pattern to become visible are not met and no Moiré pattern will be visible.

Again, look at the data at 100% zoom and you will not see the aliasing pattern (but you should see the artifacts causing the pattern in the first place). Save the image, and it still won't be visible (unless viewed within StarTools or another program that uses intentional nearest neighbour scaling).

Hope this helps.

As for what is causing the excessive mazing/zippering artefacts, I'm not sure. What I can tell you, however is that even if those mazing/zippering artifacts were present in the individual frames, they should've long been stacked out by the stacker. It's almost like we're looking at a single frame here. Are you sure you're not just left with a single frame after rejection?
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