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.