[Lowfer] Hints for Compressing Graphic Files

[J D]

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.