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Speed

The time taken by QuickSlideshow to display an image is dependent on a number of factors:

Size of the file on disk

The larger the file, the longer it takes to read from disk or CDROM. Also, the larger the file the longer it takes to be decoded. GIF pictures (and some TIFF pictures) are stored internally in a compressed form using the LZW (Lempel, Ziv, Welch) technique. Before being displayed the image must be decompressed.

The decompression routine used by QuickSlideshow was written (or should I say "hand-crafted"?) in assembler code to make it as fast as possible.

Scaling factor

Copying the decompressed picture from the area of memory into which it has been decompressed onto the screen takes a single Mac Toolbox instruction (CopyBits), except for interlaced images, discussed below. CopyBits works fastest when it does not have to scale images, and particularly slowly if the scaling is not an integral amount. In other words, scaled pictures will display at the following rates:

You can control whether or not pictures are scaled from the "slideshow options" dialog box. The slideshow options dialog box is available from the "Options" menu, or you can press Command-J.

Dithering

If the picture needs to be dithered, it takes longer to draw. You can optionally dither your pictures if:

You can control whether or not pictures are dithered from the "slideshow options" dialog box. The slideshow options dialog box is available from the "Options" menu, or you can press Command-J.

Number of screen colours

QuickSlideshow decodes images internally into 8-bit colour mode (i.e. 256 colours). Thus they will be copied to the screen fastest by CopyBits if the screen is also set to 256 colours, as this effectively means just a simple memory-to-memory copy (unless the image is scaled).

If your monitor is set to 16 colours or less you will notice a significant performance degradation.

Also, if the monitor is set to less than 256 colours, QuickSlideshow optionally "dithers" the image to make it more readable. Dithering also slows the display.

Interlaced pictures

Some GIF pictures are stored in "interlaced" form. You can easily detect these because they are drawn in four phases, starting with a coarse image and gradually becoming finer.

Unfortunately these take somewhat longer to draw because rather than a single "CopyBits" instruction to copy them to the screen, the application has to do a few hundred, in order to insert the correct parts of the image into the correct position on the screen. There is nothing you can do to speed up these images, particularly if they are on CDROM and cannot be changed. If the image is on a hard disk you could open it with a program that edits GIF images and re-save it as non-interlaced.

Time taken to scan disks or folders

This topic is covered earlier in the section "Starting a Slideshow". The most important point covered was the fact that reading in a "tag file" produced by QuickSlideshow is about 40 times as fast as scanning a disk looking for images. You are strongly advised to produce tag files of your favourite CDROMs or disks. For more details, see the section "Saving a picture sequence as a tag file".


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Written by Nick Gammon - 5K

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Page updated on Wednesday, 15 December 2004