Noise filter preview inconsistencies

[DavidMac]

The attached image shows the effect of applying the noise filter to two documents. In both cases the document was a single layer of 50% grey to which I have added noise. The only difference being that document no 1 was 600 x 400px and document no 2 was 6000 x 4000px. The attached image is a screen shot of the lower right corner. Each screenshot represents exactly the same area of the screen with viewed at 100%.  

 

In the smaller document the applied noise is (as one would hope and expect) the same as it's preview. However, in the large document the applied noise is completely different to it's preview.

 

It seems to me there are two possible explanations.

 

1) Noise is generated with a pre-set absolute resolution irrespective of document size and resolution. If this is correct it is going to appear finer as document sizes get larger. Which may be what is happening here. If however this is the case then surely that should be reflected in the preview which it is not.

 

2) Noise is generated exactly as per preview but is not getting properly applied.

 

Unless there is some other rational explanation here that I am completely missing, it would seem to me that, either way above, AP is not behaving as one would expect.

 

 

[MEB]

Hi DavidMAc,

Noise, Filters etc are dynamic and their preview on screen changes according to the zoom level. To ensure you are viewing/previewing the result accurately make sure your zoom level is set to 100%.

[James Ritson]

Hi David, what MEB said is the case. Photo generates a set of mipmaps for each image you open so it can improve performance by using lower resolution versions at different zoom levels. The downside of this is that you can end up previewing particular filters (noise-based filters especially, as well as defringing) on a lower resolution version of the image - therefore, the previewed effect will not represent its behaviour when applied to the full resolution version. Previewing at 100% will ensure the filter preview is being applied to the full resolution image. Many users encounter this with noise removal, where they comment that the noise "returns" once they actually apply the denoise filter. What's happening is that the noise removal settings they're using, whilst suitable for the lower resolution image, are less aggressive when applied to the full resolution image.

 

Apologies that this isn't made clearer within the app itself. However, this preview issue has been mentioned and an in-app clarification/explanation is being considered by the developers. Hope that helps.

[DavidMac]

Yes that's quite clear. I .... erm .... think.  ;)

 

If I understand correctly the preview is at 100% even if the image is zoomed at a different level. Therefore it is best to have the image zoomed at 100% as well so the two accurately reflect one another.

[James Ritson]

Yes that's quite clear. I .... erm .... think.  ;)

 

If I understand correctly the preview is at 100% even if the image is zoomed at a different level. Therefore it is best to have the image zoomed at 100% as well so the two accurately reflect one another.

 

Yes and no  ;)

 

If you're not zoomed to at least 100%, the filter preview is actually applied to a lower resolution version of the image. With some filters the difference is negligible, but with others, their results may vary. Think of it as the filters being applied to a sparser, less dense set of pixels.

 

(Long winded bit coming up...)

 

A more concrete way of actually seeing what's going on here is to take a copy of your image (Select all, copy, then File - New from Clipboard) and resample it to exactly half the resolution, preferably using bilinear resampling. Then, apply a destructive filter to both of them through the Filters menu: convolution filters like blurring, sharpening/clarity will illustrate this best. Use the same filter values for both images.

 

By using a destructive filter which "bakes" the filter effect in, Photo will recreate the mipmaps and you'll be able to see (however subtle) how the resolution has affected the filter outcome. You could zoom the full resolution image to 100%, then zoom the half resolution image to 200% and switch between them - you should at the very least observe some difference between them. Using unsharp mask with a moderate radius and factor should yield quite a noticeable difference.

 

Apologies for the waffling, hope that's helped clear it up? :ph34r:

[DavidMac]

So Affinity works in these instances with what, in the movie business, we call proxies, whose resolution may be lower than that of the image itself in the case of large or hi-res images?

But from a purely subjective practical point of view the solution is to apply filters with the image zoomed at 100% in which case the preview should accurately reflect the final application. 

 

Am I getting there?   :huh:

[James Ritson]

Pretty much - luckily though, Photo doesn't have to decode complex video frames with temporal and spatial compression ;) , so its "proxy" generation goes unnoticed in the background when you open an image.

 

For what it's worth, you'll encounter this issue with other software - some present a warning or tip about it, some don't. Hopefully, though, providing an in-app explanation is something we'll consider, as it's an issue that crops up for many users (as mentioned before, particularly with noise reduction)..

[DavidMac]

Thanks James. I see you are marked as staff. How come you are allowed a dog in Affinity's cat dominated team.  ;)

[sankarvichu]

what is the workaround for vertical image, this 100% preview is not working.