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Image pixillation when downsizing in Affinity Photo


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Hello, I would like advice on how to downsize a photo without losing the sharpness and quality. I have images that are too large for my website - eg 4896x3264 px. I want to have them 900px on the horizontal dimension but when I use 'Resize Document' the quality is drastically reduced with lots of pixillation. I can use Unsharp Mask to improve it a little, but it is nothing like the larger image in quality.

Is there a way to downsize without this high level of quality loss? I have attached the two versions of my image for refrence

DSCF8094.JPG

DSCF8094 sm.jpg

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An unsharp mask, by using in fact a Gaussian blur (thus the "unsharp" component of the name) can indeed create the illusion of sharpness. But it is not true sharpness. Other sharpening methods, such as the RL (richardson-lucy) deconvolution that RawTherapee offers, may produce slightly superior results. Nonetheless, you cannot avoid the fact the a lower resolution inherently means pixelation, although I suspect you've already realized that.

Regardless I note that the intense JPEG compression that you are using seems to be creating something known as macro-blocking. 

Quote

Macroblock is a processing unit in image and video compression formats based on linear block transforms, such as the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform blocks, and may be further subdivided into prediction blocks. Formats which are based on macroblocks include JPEG, where they are called MCU blocks, H.261, MPEG-1 Part 2, H.262/MPEG-2 Part 2, H.263, MPEG-4 Part 2, and H.264/MPEG-4 AVC.[1][2][3][4] In H.265/HEVC, the macroblock as a basic processing unit has been replaced by the coding tree unit.[5] 

https://en.wikipedia.org/wiki/Macroblock?oldformat=true

In simple terms, macroblocks are large, large 'pixels.' This can create a very pixelated effect, even if you have a really large image. And, although it's primarily a video term, it looks like your still image has what is called "Mosquito noise," since the varied macroblocking allows detail around the edges of the boats, but "smooths" (i.e. macroblocks) large areas in the water. Thus you get the "mosquitoes" swarming around objects. 

You probably wouldn't believe me if I told you this was a slight crop in on an over 6k photo, but it indeed is...

image.thumb.png.8e9d091409be0009b8ceecf15bdd44e9.png

Look at the pixelation at the bottom. Those aren't actually pixels; each one of those squares, or macroblocks, consists of 64 pixels! Here's a "microscopic" view, zoomed into the pixels of the image, of what's going on;

image.png.fe625d203a594ad1356b27574f5430cf.png

And this is how it should look in about the same spot, with no compression:

 image.thumb.png.468bb2c3877290de2b2901eca8287a97.png

So, aside from exploring sharpening procedures that will merely fake sharpness, perhaps turn up the "quality" slider when your exporting your JPEG , even if it raises file size a bit. Doing a smaller image, like 600px, and using a higher JPEG quality, like 90%, might actually look better. PNG's are quite large, although they support lossless data compression. 

 

At the end of the day, though, there's no getting around the fact that a smaller image resolution is, well, lower resolution. 

 

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