AUTHOR lessons are standard HTML and are run in a browser but have a preferred pixel size. The lesson may be zoomed in the browser to suit the current display.
The target platform will usually determine the size of your lesson screen. A display size of 800 x 480 will give a widescreen format that will fit smartphones and tablets and can be zoomed up on higher resolution displays with a slight loss of image clarity. If you are targeting tablets then 960x640 or with computers a resolution of 1024x768 or 1300x740 for a resolution of 1366x768 (the minimum for Windows 8) may be more appropriate.
Resolution should always be a compromise between adequate detail and storage space. Remember the more pixels you use in a image, the greater the storage requirements and the longer it tkes to download.
A single True Colour 1024 x 768 pixel picture on a computer screen requires around 2304MB to store uncompressed - which explains why graphics are generally stored in compressed format on a disk. A variety of formats exist - some trading quality for small size, others speed of decompression over size.
PNG
Portable Network Graphics (a non-proprietary replacement for the limited 256 colour GIF format) format offer medium compression and fast decompression with PNG usually producing smaller file sizes. Can produce small files for images that have large areas of the same colour. The PNG format should be preferred over JPEG for these types of images. E.g. screen captures, line drawings, cartoons etc. It is also preferred if you require image transparency.
JPG
JPEG files offer the best compression and also allows you trade off quality for small size. It is possible to produce large full colour photographs from files as small as 4-8K. Recommended format.
BMP
These are native graphic files, usually without compression. They are usually used for small images.
GIF
The GIF format is restricted to 256 colours, but provides animation.
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