Downsize/resample image, Why? - Forums

Topic starter

Mar 15, 2023

I have seen some people downsizing the image before uploading it to social media, why do they do that? Are there any benefits or do they just want to save disk space?

If there are benefits, how do I know how much to downsize/resample? I have esprit 100 ed and asi 1600mm pro

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Kevin Morefield

IOTD Staff

Mar 15, 2023

Daniel Renner:
I have seen some people downsizing the image before uploading it to social media, why do they do that? Are there any benefits or do they just want to save disk space?

If there are benefits, how do I know how much to downsize/resample? I have esprit 100 ed and asi 1600mm pro

In many cases, with today's small pixels, we are oversampled. Meaning there is no resolution gain by having all of these pixels but if you down-sample (same concept as binning in processing) you can increase the SNR of the image.

Your Esprit 100 with the ASI1600 is probably not oversampled however. I'm not sure the Focal length but I imagine your image scale is around 1.5" per pixel. I wouldn't want to downsample unless your FWHM was >4 pixels or more, maybe 5 pixels. So you'd have to has some truly atrocious seeing or other issue before you could downsample without some loss of detail.

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Doug Summers

Mar 15, 2023

FWIW, I agree with the above post, but maybe the suggested FWHM measurement is a bit tight. You could take the advice that BXT gives and use 8 pixels as your limit for downsizing. I use PixInsight's FWHMEccentricity script (under scripts/ImageAnalysis) to measure, and IntegerResample with a factor2 when downsizing is desired.

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Chris Bailey

Mar 16, 2023

Most social media platforms have a native resolution and will downsize to that. Some make a bit of a hash of it and I suspect include a degree of sharpening. If you carry out the resampling yourself, you have a bit more control.

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dkamen

Mar 16, 2023

Facebook will do its own (rather agressive and optimized for daylight photos and portraits) lossy compression if your file exceeds 1MB. Best to do your own downsampling, especially for astrophotos.

That said, downsampling improves SNR, contrast and simply makes sense if an image is to be displayed on a mobile device.

I always downsample because my monitor is much smaller than 3000x3000 which is the IMX 533's resolution so what's the point?

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Jesse Priolo

Mar 16, 2023

To hide issues in optical train/acquisition/processing that show up when you pixel peep at 100%

James Peirce

Mar 24, 2023

I can think of several reasons that may be applicable.

But first, I suspect the main reason: service-size downsampling and compression. A good example of this is Facebook. As with other social media, Facebook does various things to murder and bludgeon images into some manner of form that meets their goals for storage and presentation. One of those steps involves a very sloppy downsampling and recompression stage that is applied to images that are larger, in resolution, than 2048px on the long side of the image. If you have a 3200px image, for example, you can improve the quality of what will be displayed on Facebook by downsampling it yourself to 2048px on the long side with no compression (e.g. JPG 100% or PNG 24-Bit). Facebook will still recompress the image (aggressively), convert it to JPG, and do other various things for different sorts of presentation, but the resulting quality will be noticeably higher than if Facebook had been entrusted with the initial downsampling stage.

And a few other reasons come to mind:

Limits: Various services may have file size and resolution limits. Downsampling/resizing can be a good way to balance quality in the image relative to resolution in meeting those limits for the sake of balancing quality. Say an image is uploaded to Cloudy Nights with its 500 kb file size limit. The image is going to look a lot better if it is downsampled in balance with compression. Form-based upload fields can also fail if file sizes are too large.

Sharing Resolution: If the service allows high-resolution downloads and doesn’t offer a service-side way to limit resolution (like Flickr does, for example) downsampling can be a way to retain the high resolution versions of the file for personal use and remove them from circulation for those who are concerned about such things.

Bandwidth: Self explanatory, for those who have very slow internet connections.

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