PNG vs JPEG Image Quality and File Size Comparision
by Craig A. Smith, 1999-12-24
Amazing but True
PNG-24 files are smaller than the smallest JPEG (Lowest Quality), but look
better than the largeest JPEG (High Quality). PNG-8 is limited to
256 colors, so not a good choice for continious tones (photographs).
Like the veritible GIF format, images saved as PNG are compressed - but
without loss. Before PNG, the only fault I could find with JPEG was
that they aren't a good intermediate format. Repeated Saves will
degrade the image since a "lossy" algorythm is used. But unless
you were editing the same image over-and-over, you could avoid this in
practive by using High Quality until your final Save. But now PNG
offers smaller files with lossless compression and 24-bit color!
What will they think of next?
Strangness and Multi-Scan Images
Making a PNG "interlaced" will increases its size. But making a JPEG
"progressive" will decrease the size slightly. Adding more
scans shrinks it a tiny bit more. In both cases, a blury proxy of
these images load quickly, then shapens with progressive scans. Phillip
Greenspun (photo.net)
is not a fan of progressive JPEGS, but Derick MeCulland (The
PhotoShop Bible) likes the idea. I've noted some problems saving
these with earlier versions of PhotoShop, so I guess I've avoided them
for practical reasons until now. But the advent of PNG forces me
to reasses these image formats.
HowTo
To create PNG files with PhotoShop5.5, be sure to "Save for Web."
If you "Save As" or "Save a Copy" into the PNG format then, although PhotoShop
can still open them and browsers display in-line images fine, the latest
browsers render a very "blackened" image when you to open just a PNG image
(click to test). Besides "Save for
Web" yields slightly smaller files.
