VisibleGif - Capabilities

VisibleGif has two main capabilities. The first is to create an XML tagged file equivalent to a GIF image. The second is to create a GIF image from an XML file. Optimisations can be applied to the XML tagged file by manually editing the XML tagged file using any text processing software.

The steps in producing an efficient image

1. Analysis

   In this first step a GIF image file is converted to an XML tagged file. The elements of the image can be analysed to see if elements are present that can be deleted or amended to improve efficiency.

2. Optimisation

   In this second step optimisations are applied to the XML tagged file. Alternatively if the image was produced by an external agency they can be asked to improve their implementation of the image.

3. Generation

   In this third step the XML file is used to generate a new improved GIF image file.

Optimisations

The following are optimisations that can improve the implementation of images.

Single Images

• Remove the comment block.

• Remove duplicate entries in a colour table.

• Replace local colour tables by a global colour table.

• Replace the colour table (local or global) with a colour table that only contains colours used in the image (remove extraneous colours).

• For "single" colour images such as simple small icons or buttons, set colours to the minimum set of two colours one for the background colour and one for the foreground.

• Ensure transparancy is used to exploit background colours or background images as defined within the cascade style sheet (CSS).

• For images less than 10 KB in size remove interlacing.

• Remove anti-aliasing where image quality is not impacted. Check the <data-rows> view.

• Amend colours to the nearest colour in the set of 216 "safe" colours if consistency in old displays is required.

• Amend colours to the nearest colour in the set of 16 "safe" colours if consistency in old displays is required.

Animations

• Deconstruct GIF animation to component images.

• Apply optimisations as above.

Advanced

There are optimisations which can be applied in the areas of how the image data is packed into bits and how bits are packed into bytes.

It is important to keep the number of colours in the colour table as few as possible so that the "LZW minimum size" is kept as small as possible. A colour table can only have one of the following sizes 2, 4, 8, 16, 32, 64, 128, 256. Spare colour entries can be used to store "secret" security information such as a copyright check.

VisibleGif provides three views of the image data.

• <data-packed> shows the bytes containing the "packed" compression codes.

• <data-codes> shows the "unpacked" bytes containing compression bit sizes and compression codes.

• <data-rows> shows the "unpacked" bytes represented as editable characters where each character matches a colour in the colour table.

Only those interested in the mathematics of compression algorithms are interested in this view <data-packed>.

The <data-codes> view is used for checking the "LZW minimum size" is set correctly. A number of image editing software always set "LZW minimum size" to 8 irrespective of the colours used.