Transparent Images Problems

Some X-windows web browsers will have severe problems displaying complex transparent images on certain X-servers. Often the page takes a very long time to display (or redisplay), and while the image is being rendered the entire X-server is blocked (other windows don't operate either). Depending on the complexity of the image, the entire display can be hung up for many minutes (I've been blocked for half an hour). The effect is exacerbated if Netscape's progressive image display is turned on.

The problem is due to an inefficient implementation of clip masks in the X-server. Clip masks are used to select the region of the display where the background is to show through. The problem has been observed on the following DEC X-servers:-

DECWindows/Motif running on VAXstation (DECWindows/Alpha is OK).
DEC VXT2000 X-terminals.
DEC Multia (eXcursion) X Server V2.1.275 (Alpha), and probably all versions of DEC's eXcursion product.

There may also be other (DEC or non-DEC) X-servers with the problem. If you find any other X-servers with this problem, I'd be most interested to hear about it.

All X-windows versions of Netscape and Mosaic that support background images use clip masks when displaying a transparent image over a background image. Some versions of Netscape also use them (unnecessarily, I feel) with a solid (default or coloured) background: Unix Netscape 3.0 (initially beta 4) and VMS Netscape 2.02 (initially beta 1). This latter case is much more serious, since there are many many more web pages that could provoke it. I repeatedly reported the problem with Netscape 3.0b4, but have seen no response, fix, or acknowledgement from Netscape. Hopefully Netscape will incorporate DEC's fix (see below) in a future version of Unix Netscape.

The following implementations provide a workaround:-

VMS Mosiac 2.7-4 disables the transparency of complex images (of more than 2048 (or Mosaic*MaxClipTransitions, if specified) transitions between background and foreground) when it detects a DECWindows/VAX X-server. A patch, hopefully to be applied to future releases of both VAX and Unix Mosaic, allows Mosaic*MaxClipTransitions to be specified for non-VAX X-servers, as well as automatically enabling it for VXT2000 and eXcursion.
VMS Netscape 2.02 (initially beta 5) performs the clip-mask calculation (efficiently) on the client if it detects DECWindows/VAX. This behaviour can also be enabled explicitly (there seems little reason not always to do this -- we use it in our standard Netscape startup procedure).

Further discussions of this issue may be found in the VMS-MOSAIC mailing list archives.

Test Pages

W A R N I N G ! ! !

Do not select any of these links unless you are prepared to hang up your screen, perhaps for some time. If you want a safe example that shouldn't cause any problems have a look at this simple transparent image.

This page will display a more complex, but relatively small transparent image on the default background. Subsequent links allow you to try larger images (the problem gets dramatically worse as the image size increases) and background images. Alternatively, you can go to these pages directly using the following links, though with the danger of hanging your X-server even longer.

Default background: simple, small, medium, and large images.
Image background: simple, small, medium, and large images.

Other examples of web pages with complex transparent images (on the default background):-

Oxford University Speculative Fiction Group (includes images used in the above test pages).
~~Cornell Legal Information Institute~~ [transparent images no longer used].
Provincial Booksellers Fairs Association

Tim Adye, <T.J.Adye@rl.ac.uk>