Re: The Challenge! was: optimisation of images - help?

Angus Manwaring wrote:
On 17-Mar-07 02:30:05, frederick said
Angus Manwaring wrote:
On 15-Mar-07 20:39:04, frederick said
Angus Manwaring wrote:
I felt sure somebody here would know, said Angus provocatively. :)


The sample screenshot you posted is RGB and has more than 256 colours.
Was that the original size - or did you resize in PS?

I believe that's the original grab. Simply saved into Paint in BMP format.

Was that the image your friend saved as 10kb png?

Exactly that, yes.

Convert to 16 colours indexed (no dithering), save as png level 9
compression. File size 11.7kb.
http://i17.tinypic.com/35lt83k.png

I can't use this as the colours are not representative of the original.

Leave as RGB, save as jpeg. Quality 25 (out of 100), DCT method -
floating. File size 11.7kb.
http://i18.tinypic.com/29o4y81.jpg

The compression here has caused pretty serious degradation of the image.

For saving in *png(using The Gimp):
Converting colour mode to 256 colour indexed. Save as png, level 9
compression. File size 36.4 kb
http://i14.tinypic.com/4hlas0k.png

This is entirely acceptable from a quality point of view, but
unfortunately it is nearly 4 times the file size of the version my friend
produced.

I have put his version on my site so it can be seen. I think he#s gone a
bit over the top with the sharpness, but he has produced a high quality
image (note the intact graduation of the sky for instance) with a
comparatively tiny file size. How? :)


Frederick, thanks for your time on this. If anybody think's they can
approach the optimised image in terms of file size and quality, I would be
very eager for them to follow-up with advice.

Cheers.

I don't see a link to the optimised png you mention above.
The original bitmap has 64,000 pixels. Colorcube analysis shows that it has 18215 unique colours, so the start position shows that to compress to the sort of file size you want, then before compressing and saving as png a lot of the original image information has to be discarded beforehand. It does reduce down to indexed 256 colours (without dithering) quite well (that's a subjective call), but even optimising using a special tool like PNGOut on extreme settings, to get a reasonable result then file size is still much larger than 10kb. You can reduce the number of colours further and retain some "smoothness" by different dithering methods, but that means that (much) less compression is possible. In photoshop, you can preview the effect in a dialog as you change image mode from rgb to indexed, and play with the number of indexed colours, dithering method etc (No dithering will always give lowest file size)
I feel that the *trick* - if there is one - is back further in the workflow. Are you producing the bitmaps from an emulator? Are there settings to reduce colours or change video settings, so that you get an optimum appearance with minimum colours?
Perhaps has some trick like perhaps using a known, saved, reduced colour pallette that he has produced from other images from the emulator to index your bmp to before saving as a png. Unless it's something like that, then I think I am am stumped.
.

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