[Lowfer] Hints for Compressing Graphic Files

[Cliff Sojourner]

I'm missing something:  why is 32kB the goal?  Is that some arbitrary limit for attachments on this email list?  Does such a low number make sense anymore in the 21st century?  Thanks, Cliff K6CLS CM87 

On November 12, 2023 3:19:36 PM PST, J D <listread at lwca.org> wrote:
>Some of you may remember my posting some rather large seeming chunks of Argo captures in the past...even, one time, an enlarged view of another member's earlier attachment in a more readable image size and yet with 25 or 30% lower file size. Here are some quick tips on how to do that sort of thing. A couple may seem counter-intuitive at first, but there are good reasons for them.
>
>First and foremost, DON'T let Argo save .jpg images! If you need to keep capture sizes to a minimum on your hard drive or other storage device, use GIF instead.  Reason: GIF may limit color rendition to a 256-color palette, but it is lossless so far as detail resolution is concerned. JPEG images substitute patterns of pixels that minimize the amount of data needed to fool the eye into seeing something similar to the original lines and edges in the image. But that's a one-time-only benefit! If you edit and save a .jpg file again, those blocks of pixels get treated as a whole new pattern by the algorithm, which actually _increases_ file size with each generation, even as the lines and edges themselves grow fuzzier.
>
>Therefore, start with as clean and uncompressed image as you can. Get it into .bmp form ASAP and keep it that way through ALL stages of editing, right up to the very end. I use plain old MS Paint for this.
>
>If you have to add to add explanatory text or labels, try to avoid serif or decorative fonts...needless added detail compared to sans-serif. Labeling on a solid black or solid white background entails fewer pixel-to-pixel transitions than typing onto the noise background of a waterfall display, too, so the final compression can be slightly tighter.
>
>Judiciously crop and trim to include just the essential information you truly need in order to tell the story. This even includes the frequency scale. Do you really need every digit of every label down to the nearest half Hz (185,300.0, 185,300.5, 185,301.0 etc), or could you trim out the kHz and some or all of the remaining half-Hz labels (300, 301, 302 etc) and still convey enough?  (Unless you need long time ticks for measurement purposes, consider using short ticks to eliminate a bit of extra detail from the original image, too.)
>
>Like Gedas, I also selectively apply a bit of blur (easy to do in Irfanview) to redundant noise portions of the image in order to reduce the degree of detail that has to be compressed at the very end. That's only after I've gotten rid of everything else that I don't need in the picture.
>
>If humanly possible, I try to avoid having to shrink the resulting image at all. It definitely makes things harder to read, but rather surprisingly, it also does NOT shrink the file size by a corresponding amount! This is because the resizing process creates pixel-level artifacts, which themselves amount to data that has to be accounted for during compression.
>
>Very last step: Once I have a clean, artifact-free, trimmed original bitmap image in pristine form, then and only then do I save it to a JPEG file with Irfanview. If I start with a nice clean image that has been saved as a bitmap all the way through to this point, then I can compress the living heck out of it before it starts to look grungy!
>
>I start with as high a quality factor as I think I can get away with in the Save (original folder) dialog box of Irfanview, or Save As in some other programs. Without closing Irfanview, I check the resulting .jpg in File Manager. If it's over about 32 kB, I immediately return to the Save dialog and select a lower quality factor, and save over the first .jpg. If necessary, I repeat until I get under 32 kB.
>
>In Irfanview's scheme of things, 70 is considered average compression for a JPEG.  Starting with a good enough image, I can go as low as 40 without raising an eyebrow, 25 with some noticeable degradation, and have rarely used 15 before significant loss of quality.
>
>Never, ever attempt to further compress the saved .jpg image. Always reopen the archived .bmp and do it from there, if necessary
>
>Just some hints that may prove helpful here on the reflector.
>
>John D.
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