writing truecolor PNG

[herr_k.]

of what i read so far, it seems to be the best lossless format to use for my needs (better compression than tiff/faster than jp2).
but how can i save a file with this format?
iin the write dialog there is only png.
to get a lossless png -> set compression level to "0" ?

[Clo]

Hello

- According to the daily practice, I can say you that XnView saves the PNGs like the current colour-depth used by the System display.
- 16 & 24 BPP are saved as 24 BPP, but 32 BPP is not supported at saving; I tested also under that colour-depth, I got only 24 BPP.
- However, 32 BPP can be read (I checked only with BMP, I've not any else 32 BPP image here…)

Kind regards,
Claude
Clo

[herr_k.]

ah ok. seems to work.
compression level -> 0: file = 5.81mb
compression level -> 9: file = 3.61mb (this is even like jp2!(3.58mb)) and 20 times faster than jp2.
good format. i will use this defenitely for archiving.
thx.

[mk.2]

If you use PNG for archiving, you probably interest in PNGOut.
http://advsys.net/ken/utils.htm
http://advsys.net/ken/util/pngout.htm

Most of the time, it can dramatically reduce size of your PNG files. However, it's pretty slow.

[Drahken]

PNGout is a great tool for 256 or less color PNGs, but is a waste of time for true color PNGs. The author's site claims it doesn't work at all with truecolor ones, but it doesw work somewhat (just not well) and may wind up converting your truecolor PNGs into 256 color ones.

There is never 2 seperate PNG options in any program. In most programs, there is some option on the PNG options page to decide wheter to save it as truecolor, indexed (256 color), or greyscale. In some progs (such as xnview), the PNG will be saved depending on how many colors you currently have it set to within the program itself. If you have decreased the colors to 256 or less, it will save as indexed PNG. If you have set the image to greyscale or bitonal, it will save as greyscale. If you have not decreased the colors, it will save as truecolor.

Never save a PNG with a compression of 0, it's just a waste of time. PNG compression is similar to zip, rar, etc in that the compression ratio has nothing to do with image quality, it is merely a tradeoff between filesize and compress/decompress time.
With indexed PNGs, you should always use a level of 9 (unless you have a really old comp, like a 386 or older). With truecolor PNGs, you will often get a better filesize at a level of 3 or 4 than you do at 9. For some reason, photographic images make a filesize jump at compression level 4 or 5.