Impress/Performance

pngperf cws

The slide sorter of Impress is quite slow in some situations. Many of these performance problems have to do with the load time of huge image files and with the work to downscale those images to the much smaller sizes needed for the slide previews. The CWS pngperf utilizes the fact that some image formats were designed with this use case in mind. It also benefits from the much smaller memory footprint of preview images compared to the original size images. The CWS was started by hdu few months ago.

Previews

PNG images can be saved in interlaced mode. This increases the file size a little but it allows to load preview image up to 64 times faster. Without it you can for example end with loading few megabytes image - only to compute a small preview containing 64x64 pixels. (see http://www.libpng.org/pub/png/pngintro.html for interlaced example)

The JPEG image format contains data stored hierarchicaly and thus can even provide higher quality preview images than a naive downsampling of the original size image. For JPEGs that have been saved in "progressive mode" only a small part of the image file has to be read for previews.

The pngperf cws adds necessary API for loading images in preview mode.

API

So far there is API for previews, but OOo doesn't use it yet.

There exist slidesorter-demo-* patches in ooo-build http://cvs.gnome.org/viewcvs/ooo-build/patches/src680/ to use pngperf API, but they have still some problems (such as impress sometimes showing preview quality images in slideshow presentation). There is problem with using that API, as Sun needs to keep API/ABI compatability, and thus it is now only possible to set parameters to png/jpeg loaders by static functions.

CWS status:

The CWS pngperf will be in OOo 2.1. of course it will be integrated into an earlier developer build (probably SRC680m184).

Things TODO:

• provide hints for more situations where big images are previewed
• many more optimizations are possible...
• make an extension to downsample large images (5megapixels is common from digital camera) to screen resolutions such as 1024x768 or 800x600
• add image downsampling to support to [Impress Photo Album Creator extension]