moderated Question about image processing #misc


 

Yesterday I posted a message to my group that included two .png image files. The files I uploaded from my hard disk had these characteristics.

Image #1: 1043 x 543 pixels, 51,102 bytes
Image #2: 1011 x 614 pixels, 34,600 bytes

In the archived version of the message on the groups.io web site I find these file statisics.

Image #1: 640 x 327 pixels, 124,301 bytes
Image #2: 640 x 381 pixels, 89,969 bytes
(these images are stored in the Amazon "cloud")

In terms of display area, groups.io shrank the first image by 63.05% and the second by 60.72%. (The two percentages are different because of different aspect ratios.) But the file sizes increased: by 143.24% and 160.03%. respectively.

So here's my question. Why does a lower resolution image take up so much more disk space?

I have a hunch that it relates to color encoding and the program Mark is using to rescale every image that's more than 640 pixels wide. I had gone to some trouble to compress the images using a 256-color palette and 8-bit color codes. I used an open source program called "pngquant" (https://pngquant.org/). I suspect that the program which rescaled the images desstroyed the color palette and reverted the images to 24-bit color codes. Assuming that's what happened, is there an easy way around it? For instance, what if I resized the images to be 640 pixels wide before uploading them to the groups.io servers? Would the image processing algorithm leave them alone? Or would they still get a whole lot bigger? Are the file sizes increased by groups.io software? Or is that a "feature" of the Amazon "cloud"?

I'll do some experimenting to find out (by sending a private message to myself, and not to the whole group). It's not a big deal, so far as I'm concerned. I'm far, far under the 20 GB "free images" threshold for a premium group. Still, for somebody with a lot of stored images. it might make a difference in the amount the group is charged for image storage. People might even appreciate it if image compression via pngquant were offered as an opion on the groups.io web site. Email messages with smaller (in bytes) images would download faster, etc. So I thought I should mention it here.
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David Bryant
Canyon Lake, Texas
https://t-vog.groups.io/g/main    https://davidcbryant.net

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