Affinity Photo - Same resolution, format, quality and bit depth, but different file size?

[Alex_M]

There's something very peculiar I just found out. I have a document which If I resize prior to export, the exported image is quite a bit larger in file size compared to resizing it in the Export window. All quality settings are the same. When I open and compare both files, they look the same to me, no visible quality difference. The format I export to doesn't seem to matter. I tested PNG and JPG and they both exhibit this issue. Why does this happen?

Please check the AFPHOTO file and the video I recorded.

waves.afphoto

[Alex_M]

Ok, so I found another document where the opposite happens.  If I resize it in the Export Settings window, the file is larger compared to resizing prior to export. Afphoto attached below.

pattern1.afphoto

[NotMyFault]

If you resize before exporting, you will use bicubic resample twice. If you resize at export, you resample only once.

The exported files are visually similar, the actual pixel values differ numerically.

So i assume the root cause is that bicubic resampling will alter the pixel content even if the resolution stays the same.

 

[Alex_M]

22 hours ago, NotMyFault said:

If you resize before exporting, you will use bicubic resample twice. If you resize at export, you resample only once.

 

In both cases I resized the image only once - either in the Resize Document window or in the Export Settings window.

[NotMyFault]

Just now, Alex_M said:

In both cases I resized the image only once - either in the Resize Document window or in the Export Settings window.

Yes, but Affinity might apply resampling even if export has the same dimension. Why? The document could contain pixels outside the visible canvas, and depending on layer dpi these pixels might influence the export after resampling.

[NotMyFault]

I tried yesterday on Mac and got the size discrepancy. Today on iPad i get only the smaller size. Seems to be a Mac issue.

 

[Alex_M]

I'm on Windows. Also, there are no pixels outside the canvas in my files. Not sure why this would inflate the file size though given that for JPG and PNG Photo exports only what's inside the canvas.

[lacerto]

Obsolete.

[R C-R]

3 hours ago, lacerto said:

When you resample the image beforehand (using Document > Resize Document), the procedure will produce transparent pixels (no idea why), which you can verify by using e.g. the Info panel or Curves adjustment.

I may not be doing that correctly or checking it correctly but when I try that on the OP's waves.afphoto file, I do not seem to be getting any either partially or fully transparent pixels. 

[lacerto]

Obsolete.

[R C-R]

7 minutes ago, lacerto said:

You need to follow the example of the video when trying to reproduce OP's problem: use Bicubic resampling method when resizing the document. It does not happen if you use other than Bicubic or Lanczos 3 (separable).

OK. I see that too ... now that I know it depends on the resampling method. That was not clear until you mentioned it in your last post.

[NotMyFault]

Getting partial alpha by resample is the actual bug. I didn't get that on iPad. So platform dependent bug (Mac and Win).

This issue happens for all GREY/8 layers when resizing with bicubic.

Does not happen for RGB/8

Edited April 18, 2022 by NotMyFault

[lacerto]

Obsolete.

[iconoclast]

As I tested it, I got exactly the same file size. It seems to be important that you set the background to "White", also in the Export dialog (under "More") in both cases. Otherwise you get an additional channel - the Alpha channel - that causes the bigger file size, I think. That's at least my explanation for what might have happened.

[lacerto]

Obsolete.

[R C-R]

48 minutes ago, lacerto said:

It was however clear from the OP's initial video (and discussion thereafter).

It was not clear to me from the initial video, possibly because it only used Bicubic ... unless I missed that too.

[lacerto]

Obsolete.

[NotMyFault]

For me it seems just another rounding issue for 8 bit color depth, we had similar in multiple other situations ( blend mode leading to white instead of yellow, iPad unable to reach full white by brush). 
 

Bicubic resample (with resize) creates about 1/3rd of the pixels with wrong alpha of 254/255 instead of 255/255.

The content of the images does not matter, you could resample any fully opaque layer in GREY/8 and get the from alpha values.