# Document Properties: Images

The Images properties page allows you to set options for the handling of images.

## Alternative Text of Images

In an HTML file, references to images often also include alternative text that can be used in place of the image. If you choose not to include images, you are recommended to check the Include alternative text of images checkbox so that if the image was a hyperlink, the hyperlink can still be utilized.

## Including Images

Check the Include images checkbox to have images included in the document. This setting applies to root source file images, images referenced through HTML <img> tags, and images that are hyperlink targets. If you include images, you can also set various options for them.

### Resizing large images

Check the Resize large images: checkbox to have iSiloX resize large images. You specify the threshold for resizing by specifying the Maximum width and Maximum height in pixels. iSiloX resizes images proportionately so that both dimensions fall within the limits.

### Improving contrast

Check the Improve contrast checkbox to have iSiloX improve the contrast for black and white and grayscale images.

### Dithering

Dithering improves the look of an image when converting it to a lower color bit depth by diffusing the error introduced in lowering the number of colors available for use. Check the Dither checkbox to have iSiloX dither images. Dithering has no effect when the image only consists of colors from the target color bit depth.

### Compression

Compressing an image reduces the space required to store the image. Check the Compress checkbox to have iSiloX apply compression to images. In some cases, the compression algorithm can increase the size of an image, but iSiloX checks for this and does not apply compression if this would be the case.

#### Lossy compression

You can optionally enable lossy compression to significantly reduce the size of the size of converted documents containing images. This type of compression results in the loss of fine detail from the image and so is best applied only to continuous-tone images (i.e., photographs of the real world). Lossy compression has the following options:

- None: Do not apply lossy compression to any images
- Lossy images only: Only apply lossy compression to lossy source images only (i.e., JPEG source images)
- 200+color images: Only apply lossy compression to images with at least 200 unique colors
- 100+color images: Only apply lossy compression to images with at least 100 unique colors
- 50+color images: Only apply lossy compression to images with at least 50 unique colors
- 20+color images: Only apply lossy compression to images with at least 20 unique colors
- 10+color images: Only apply lossy compression to images with at least 10 unique colors
- All images: Apply lossy compression to all images

You can adjust the quality of the resulting images to which lossy compression is applied by setting the Lossy image quality field to any value from 1 to 100. The higher the value, the better the image quality, but the lower the compression. The lower the value, the lower the image quality, but the higher the compression.

Note that regardless of the image quality setting, lossy compression applied to an image always results in a reduction in the fidelity of the image. So even an image quality setting of 100 results in a reduction in image fidelity.

### Color bit depth

If you are creating a document for use on different devices with different types of displays and different versions of Palm OS®, then you may need to include images at more than one color bit depth. If you are creating a document for use on only one device, then it is usually sufficient for you to include images at only one color bit depth. You can include images at the following bit depths:

- 1-bit (black and white)
- 2-bit (four shades of gray)
- 4-bit (16 shades of gray)
- 8-bit (the websafe palette, 256 colors)
- 16-bit (64K colors)

Each additional bit depth you include generally increases the size of the resulting document, but not always, because iSiloX always tries to use the lowest possible maximum color bit depth possible for a given image. So even if you include 16-bit images but all your images are pure black and white 1-bit images, iSiloX will only store 1-bit versions of those images.

The following table lists the Palm OS® versions and the maximum image color bit depths supported.

Palm OS® version | Maximum depth | Notes
--- | --- | ---
3.0/3.1/3.2 | 2-bit | None
3.3 | 4-bit | None
3.5 | 4-bit/8-bit/16-bit | Non-color devices display images at a maximum color depth of 4-bit. The only color device running Palm OS® 3.5 that can display images with 16-bit color is the Visor® Prism.
4.0 | 4-bit/16-bit | Non-color devices display images at a maximum color depth of 4-bit.

The above table may help you decide which color bit depths to include in a document if your audience is wide. One thing you can be sure of is that 1-bit images work across all devices.

On any supported device, regardless of the color depths supported and regardless of what bit depths are included for the images, iSilo™ will still display each image by either choosing the highest supported bit depth of the image or using the closest available colors to display each color of the image. No dithering is applied in the latter case, so the image may not appear as good, but the effect is usually usable.