The program begins with a splash screen. You can dismiss the splash screen by pressing a key or button; itsagif then asks for a PICT file to open. As an example, we'll open the “Small Frog” file, which contains an image of a small frog. The “Small Frog” image is in the public domain, and was graciously placed on the web by Public Domain Images.

Source Controls

Size: 150x107 (62.6K)

This control displays the size of the original image. Our example image is 150 pixels wide and 107 pixels high. When placing the GIF file in some HTML the size should be included. For our example, the appropriate HTML would be <img src=“Small Frog.gif” width=150 height=107>.

The memory requirements are listed after the image dimensions. The small frog image uses 62.6K when stored as a 24-bit image without any compression.

10397 colours

This control displays the number of colours present in the original image; our example image has 10397 distinct colours.

Destination Controls

Size: 150x107 (110K)

This control displays the size of the produced image; the dimensions of the produced image match those of the original image. However, the produced image will be a 110K GIF file. The GIF file will be identical to the preview image if you are previewing the image with a 24-bit video card. If you are previewing the image with a 1,2,4,8, or 16-bit video card the produced GIF file will be as good or better than the preview image.

The remaining destination controls impact the quality and memory requirements of the produced GIF file. After changing any of the controls, itsagif will automatically recompute the preview image and memory requirements. While computing the memory requirements itsagif may display lower estimates of the size (for example “>36K”...“>44K”...“>60K”...“110K”).

10397 colours

This control displays the number of colours used in the produced image. You may specify the number of colours by entering a value into this control. Using fewer colours will usually reduce the size of the produced GIF file.

Here is the small frog image, rendered using 256 colours:

Using 4 colours, the small frog image appears as follows:

41 passes

This control displays the number of passes needed to render the image. Each pass can paint at most 256 colours. You may specify the number of passes by entering a value into this control. Using fewer passes will usually reduce the size of the produced GIF file.

First pass: 256 colours

This control displays the number of colours used to paint the first pass. Using a small number of colours on the first pass allows viewers to get a quick preview of image. To see what the image will look like after the first pass, set this control as desired and subsequently specify that one pass is to be used to render the image (using the # of passes control described above). You will also see the memory size of the first pass; 14,400 baud modems transfer approximately 1K per second.

Later passes: 255 colours

This control displays the number of colours used to paint all passes after the first pass. With 2 colours on the first pass, and 1 colour on later passes, the image would appear as follows:

1 pass	2 passes	3 passes	4 passes
2 colours	3 colours	4 colours	5 colours
602 bytes	1.1K	1.9K	2.6K

Using 3 colours on later passes, but still 2 colours on the first pass, the image would appear as follows:

1 pass	2 passes	3 passes	4 passes
2 colours	5 colours	8 colours	11 colours
602 bytes	2.2K	3.9K	5.4K

Although the example image renderings only use a few colours per pass (for clarity), the general trends illustrated hold for more colourful renderings.

The next rendering example uses 4 colours on the first pass and 7 colours on later passes. Notice that only four passes are needed for a reasonable reproduction of the original image.

1 pass	2 passes	3 passes	4 passes
4 colours	11 colours	18 colours	25 colours
1.4K	4.6K	7.2K	9.2K

Background (FFFFFF)

This control displays a small swatch of colour that represents the image's intended background colour along with the HTML description of that colour. Clicking on the colour allows you to choose a new colour; option-clicking gives you finer control over the colour chosen. Our example image has a white background (the default) which is specified in HTML as bgcolor=#FFFFFF in the body tag, as follows: <body bgcolor=#FFFFFF>.

Itsagif will display the background colour around the preview image as well as over portions of the image that have been marked as transparent.

Transparency

When this control is checked, portions of the image are transparent.

• To specify a transparent colour, move the pointer over the desired colour on the preview image and click once.
• Click over other colours to specify multiple transparency colours.
• Clicking over a transparent region causes the underlying colour to become opaque again.
• The delete key removes the most recently added transparency colour.

Unchecking this control will disable transparency.

Interlace first pass

When this control is checked, the first pass of the image will be stored in an interlaced manner. Interlacing the first pass allows viewers to get a very quick idea of what the image will look like as it is being downloaded. Interlacing usually increases the memory size, although only slightly.

Interlace later passes

When this control is checked, all passes after the first pass are interlaced. Most browsers will present the image to the viewer in less time if later passes are not interlaced.

Use global colour table

When this control is checked, a single colour table is used for all passes instead of one colour table per pass. Images with a global colour table are compatible wuth more (older) browsers and utilities. Multiple passes may still be used to provide sharp previews.

Optional Windows

Quality vs Number of Colours

This window displays the quality (faithfulness of the new image to the original image) as a function of the number of colours used. The quality is computed by going over every pixel in the new image and comparing it to the corresponding pixel in the original image. 100% quality ensures that there is no loss - the new image is identical to the original image. If the original image was pure white and the new image pure black (or vice-versa) the new image would have 0% quality.


The quality scale defaults to a logarithmetic scale while the number of colours defaults to a linear scale. You can override this by using the controls in the lower left corner of the window. You can use the cursor keys or the mouse to select points on the curve. Itsagif will display the quality for the selected number of colours.


Press the space bar or mouse button to preview the image at the selected number of colours.

Maximum Error vs Number of Colours

This window displays the maximum error between the new and original images. The maximum error is given by the pixel from the new image that deviates the most from the corresponding pixel in the original image. An error of 100% indicates a black pixel representing a white pixel (or vice-versa) while an error of 0% indicates the pixels are identical.


As with the “Quality vs Number of Colours” window, the mouse and keyboard may be used to explore the trade-offs between image quality and the number of colours used.

File Size vs Number of Colours

This windows displays the size of the produced GIF file as a function of the number of colours used.

Clipboard Support

Many thanks to Harold Poskanzer for kindly providing the textures used in the itsagif splash screen and web site.