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When I use the Affinity Photo extension in Apple Photos, I get this message: "16MB change limit exceeded -- document will be flattened." This topic was raised in the forum a couple of years ago and some workarounds were suggested. However, nobody explained what "flattened" means. I'm using a 27-inch, 5K display, and I can't see the difference before and after the flattening. What exactly do I lose if I let Photos flatten the image?

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Hi Peter Brunette,

Welcome to the forums :)

14 hours ago, Peter Brunette said:

 I can't see the difference before and after the flattening. What exactly do I lose if I let Photos flatten the image?

There's no visual difference when flattening an image, simply you lose the editability of your document. When a document is flattened, all of the layers are merged into one pixel layer, for example:

Non flattened-

image.png

Flattened-

image.png

As can be seen, the image looks the same, however the layers panel shows only one layer after being flattened, as all of the adjustments have been merged into one.

This is known as 'destructive', as you can no longer go back to the image and change the adjustment parameters after the document has been flattened. One positive of flattening an image is file size, as it can reduce a multiple layer document to only one - rather similar to exporting to JPEG. I hope this helps!

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13 hours ago, Dan C said:

When a document is flattened, all of the layers are merged into one pixel layer ...

That's true, but I think the OP is asking what "flattened" means in the "16MB change limit exceeded -- document will be flattenedwarning we sometimes see in Apple Photos after editing photos from the Photos library using the Edit in Affinity Photo extension. This warning may appear in Apple Photos when the photo is sent back to it, so it seems pretty clear that Apple Photos (not Affinity Photo) is generating the message & it has something to do with a "16MB change limit" imposed by that app on Apple Photos extensions, but what is being flattened is not clear.

According to this recent Tidbits article (written by the leader of the Apple team that developed the editing engine for the Mac version of Photos, so he should know what he is talking about), Photos extensions are allowed only to send back to the app an 8 bit JPEG version of the edited photo -- which is inherently flat -- plus a set of editing instructions. (The original unedited photo is not altered so the Apple Photos "Revert to Original" feature still works.)

Also, as long as only the Apple Photos extension is used to edit the photo, Photos will send the JPEG & the extension's editing instructions back to the extension, so even though the JPEG is 'flat' the extension's edits are still accessible in the extension. So for example, after using the Edit in Affinity Photo extension to add a Vibrance adjustment, saving in Photos, & using the Extension again, the original Vibrance adjustment is still there in the Layers panel, & I can change its settings & send that reedited version back to Photos:

554582203_LayerspanelinAPhextension.jpg.49140bc99e330dcf84d5360ba394ec27.jpg

This does not generate the 16MB warning. This particular photo was a JPEG to begin with, but it usually works for any image format except for RAW formats, which at least for me always generates the warning.

So my guess is the 16MB limit applies to the set of editing instructions, not to the size of the original photo file, & for whatever reason the Edit in Affinity Photo extension always includes more than 16 MB of editing data when used to develop a RAW image.

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2 hours ago, R C-R said:

This warning may appear in Apple Photos when the photo is sent back to it, so it seems pretty clear that Apple Photos (not Affinity Photo) is generating the message & it has something to do with a "16MB change limit" imposed by that app on Apple Photos extensions, but what is being flattened is not clear.

Wouldn't you – if the message was initiated by Apple – expect to find other results than related to Affinity with an online search?
I get only 8 results with a search for "16MB change limit exceeded -- document will be flattened", all link to Affinity.

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1 hour ago, thomaso said:

Wouldn't you – if the message was initiated by Apple – expect to find other results than related to Affinity with an online search?

Maybe, but the message appears in the Apple Photos edit window, not in Affinity Photo, so one way or another I think it is a very strong indication that it is initiated by Apple's app. Why it occurs is a different matter, & as I said may have something to do with the amount of edit data Affinity Photo sends back to Apple Photos.

Regardless, there is no 16MB "change limit" I am aware of in Affinity Photo, not with RAW or any other image format.

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