IPad export preview issue

[NotMyFault]

In iPad, the export preview renders completely wrong.

It shows a full green/red/blue layer depending on position of the pattern layer.

To reproduce:

• open attached file
• goto export>png>preview
• 1/4 size preview is show in one primary color.
• if you use move tool to shift pattern layer by 1 in x or y position, the preview color changes to another primary color
• it seems the preview does a sub-sampling and uses only pixels with odd x/y position for preview
• same file renders correctly on Windows (muddy gray)

 

Update:

The file "preview 4" shows the 4 possible alignments in the 4 quadrants of a single document.

 

export previw wrong ipad.afphoto

ipad export preview 4.afphoto

Edited September 28, 2021 by NotMyFault

[Paul Mudditt]

24 minutes ago, NotMyFault said:

In iPad, the export preview renders completely wrong.

It shows a full green/red/blue layer depending on position of the pattern layer.

To reproduce:

• open attached file
• goto export>png>preview
• 1/4 size preview is show in one primary color.
• if you use move tool to shift pattern layer by 1 in x or y position, the preview color changes to another primary color
• it seems the preview does a sub-sampling and uses only pixels with odd x/y position for preview
• same file renders correctly on Windows (muddy gray)

export previw wrong ipad.afphoto 44.17 kB · 1 download

Yes looks just like anti-aliasing/under sampling below Nyquist-Shannon frequency which is basic physics occurring as the reduced size image is rendered, don’t think this could be classed as a bug more a feature of your image and how it was constructed,  same effect happens if you resize the document as shown. Important factor is the actual exported file is correct.

[NotMyFault]

1 hour ago, NotMyFault said:

same file renders correctly on Windows (muddy gray)

You may have overlooked that sentence. On Windows the rendering is correct. So it is 

1. Possible to do it correctly
2. Photo’s behavior differs between OS platforms.

This facts lead to my conclusion that it is a bug.

Image: Preview on Windows renders correctly

Edited September 28, 2021 by NotMyFault
added Windows for comparison

[NotMyFault]

@Paul Mudditt

Update:

When resizing, the pattern layer gets resized, too. As it was only 2*2px, resampling by factor 10 reduces the pattern layer to 1*x1px, so you get only one color. Never the less: Getting red is still wrong: it should get muddy gray.

Possible root cause:

The document must be (implicitly) rasterized / flattened before down-sampling for preview. Maybe the iPad version forgets this required step,

preview doesn't properly resample for zoom. preview stays constant independent from zoom level.

[Paul Mudditt]

1 hour ago, NotMyFault said:

@Paul Mudditt

Update:

When resizing, the pattern layer gets resized, too. As it was only 2*2px, resampling by factor 10 reduces the pattern layer to 1*x1px, so you get only one color. Never the less: Getting red is still wrong: it should get muddy gray.

Possible root cause:

The document must (implicitly) rasterized / flattened before down-sampling for preview. Maybe the iPad version forgets this required step,

I disagree, to get a good representation of a signal comprising multiple frequencies under sampled you must apply a low pass filter to avoid aliasing frequencies folding back on the nyquist frequency. Yes I am describing electromagnetic theory after spending many years sampling and processing voltage and current waveforms from the electricity distribution network in real time for protection relays but, the theory works the same for light waves which are also in the electromagnetic spectrum.

Without applying a low pass filter the effect you are seeing on windows is likely a minor calculated value difference which is typical of aliasing effects.

I guess you could argue that the developers should apply a low pass filter before showing the reduced size image but unless it was a repetitive pattern such as this it wouldn’t be a problem.

[NotMyFault]

8 minutes ago, Paul Mudditt said:

I guess you could argue that the developers should apply a low pass filter before showing the reduced size image but unless it was a repetitive pattern such as this it wouldn’t be a problem.

No.

Please have a look at the screenshot from Windows, Export, preview i've added to my former post. It gives the expected result. Or better, try it yourself on Windows.

iPad should give the same result as Windows.

When you zoom the preview, it becomes even more obvious:

• Windows correctly shows the re-sampling for every zoom level.
• iPad shows one (wrong) primary color at every zoom level.

Edited September 28, 2021 by NotMyFault

[Gabe]

Hi both,

It does look like a bug with the export preview on iPad. Issue logged.