I was in a meeting today where we ended up discussing file formats and what’s appropriate depending on the intended usage. This wasn’t a web-specific conversation but instead was focused around using a digital asset management system to allow repurposing files of various formats for use in PowerPoint presentations, web sites, print or other media.
There’s a definite lack of understanding of the multitude of graphic file formats out there by the general populace. I find that I take for granted the years of experience I’ve had with many of those formats including some of the more obscure ones like Scitex CT and forget that not everyone has seen or used these formats. I get a little dumbfounded when people don’t know what a TIFF or an EPS file is. Don’t even bother trying to explain the different levels of PostScript… Then there’s the whole issue of dpi, lpi and things appropriate for print not being suitable for on-screen use and so on.
When it comes to the web, although image formats in use have been largely dominated by JPEG and GIF, most modern browsers (Windows IE I’m looking in your general direction…) also support rendering the PNG format. Despite providing some support for PNG, its use is generally more limited due to IE not completely supporting the alpha transparency channel in 24 bit PNG images.
The single largest problem with PNG images, aside from a current lack of alpha support in IE6 and below is file size. PNGs can get big. For users with a broadband connection this is less of an issue, but making sites accessible for users on slower connections should still be a concern (within reason). The colour depth and file dimensions have an effect as does any embedded metadata. PNGs can potentially be 5 to 6 times the size of an equivalent JPEG.
At the same time as PNG is beginning to gain more widespread use, the new JPEG 2000 format (lossless) has appeared on the landscape. Although I don’t have any practical experience with the file format, it sounds as though it provides all the benefits of normal JPEG but without the compression artifacts. At the moment I’m unsure of browser support for the JPEG 2000 format, so perhaps someone out there cares to comment on that.
Do you have any preference for web file formats? Why one over the other? Are you using PNGs, and if not, why?