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Appendix A — The geometry in SWF — Coordinates

The most common and simple geometric information are the object coordinates on the output screen. These are defined in TWIPs. There are 20 twips per pixels. Note that an embedded SWF file can be enlarged and/or reduced thus changing this basic scaling factor. To have exactly 20 twips per pixel you must ensure that the EMBED and/or OBJECT tags use a WIDTH and HEIGHT with exactly the same value as in the rectangle defined in the SWF header file divided by 20. The coordinates are defined from the top-left of the screen area to the bottom-right (x increases from the left to the right as expected; y increases from top to bottom as on most graphical devices on computers). The following shows you the coordinates system.


Fig 1. Coordinates

Because one can use scaling and translations, the coordinates can easilly be inverted to have the y coordinates grow from bottom to top. However, to create your own player or generate proper SWF files, you need to know how the raw coordinates system works.

There are limits to everything including coordinates. In order to enable all sorts of objects to be drawn, one should look at the result in a pixel environment (ie. as it will be drawn on the final screen). The idea of using TWIPs is to enable some interested people to zoom in (up to 20 times!) and still keep a high quality (this isn't true for images though).

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