Some advice you should ignore

I have an interest in graphic formats – I wrote the perl package Image::Pngslimmer a while back when I was hacking some perl database code that delivered (via Ajax) graphs and photographs.

I built the whole website for fun and learning purposes and so therefore used PNG graphics when JPEGs would have (for the photographs at least) been more appropriate.

So, therefore an article that begins:

Virtually every developer will use bitmaps at times in their programming. Or if not in their programming, then in a website, blog, or family photos. Yet many of us don’t know the trade-offs between a GIF, JPEG, or PNG file – and there are some major differences there. This is a short post on the basics which will be sufficient for most, and a good start for the rest. Most of this I learned as a game developer (inc. Enemy Nations) where you do need a deep understanding of graphics.

has my attention.

But what a load of rubbish it turns out to be. The author plainly doesn’t know what the “trade-offs” are either as he states:

PNG – 32-bit (or less), lossless compression, small file sizes – what’s not to like. Older versions of some browsers (like Internet Explorer) would display the transparent pixels with an off-white color but the newer versions handle it properly. Use this (in 32-bit mode using 8 bits for transparency) for everything.

…and…

JPEG – 24-bit only (i.e. no transparency), lossy (can be lossless but compressions drops a lot), small file sizes. There is no reason to use this format unless you have significant numbers of people using old browsers. It’s not a bad format, but it is inferior to PNG with no advantages.

This is seriously bad advice and it is also technically wrong.

First, PNG is not a lossless format. Many PNGs are indeed lossless, but they do not have to be.  A substantial part of the  code in the Image::Pngslimmer package is about using the median cut algorithm to produce a lossy PNG out of a lossless one by producing a best match set of colours for the image and then converting pixels to the colour in the set which is closest to them in RGB space (the median cut algorithm ensures that there are more members of the set in the bits of RGB space where there are more colours in the original image).

Second, JPEG remains by far the best way of representing photographs in the real world internet, where the trade-off between size and quality is important.  Look closely at the colour boundaries in a JPEG, especially a highly compressed one, and you can see how the edges are blurred and colours cross the boundaries – that is an artefact of the compression and indeed represents a loss (once compressed in this way the quality of the original image cannot be recovered). But it is also a highly efficient algorithm and can produce much smaller image files than the lossless PNG equivalent without any noticeable dimunition in quality for the average viewer.

As a parallel think of MP3s and FLACs for audio. Like FLACs, PNGs (can be) lossless and compressed – so delivering a much smaller file than the source WAV or similar file. But MP3s can be much smaller again without any noticeable dimunition in quality for most people on most equipment.

Yes, more and more people are connecting at 25Mb/s, but then more and more people are also connecting using GPRS, EDGE and low bandwidth 3G connections. Size still matters.

PNG is absolutely superior for representing line drawings (including artefacts such as graphs) as no one wants to see the lines start to fuzz. But that is a pretty limited subset of images on the internet.

So here is some proper advice: use JPEGs (at about 85% compression) for almost all photographs you want to put on the internet. If you have a line drawing or similar, prefer PNGs. If you have a truly lossless image (remember someone may have compressed it before it gets to you) and you need to keep it that way, then use the PNG format: but expect your file size to be very large.

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