Save bitmap data in JPEG format if you no longer have to edit the bitmap files. Depending on the compression level, different quantities of pixel are compiled to form a single unit of information. This causes information to be lost that cannot be recovered.This is normally not visible to the human eye if the medium compression level is used. The greater the compression, the more defects in the image are visible, which appears mostly as highly contrasting edges and color transitions.

The major advantage of JPEG format is the enormous reduction in file sizes it offers.The file size depends on various factors, including the resolution, dimension and image depth that the file was saved in.

The resolution
defines how many pixels are present per inch (dpi = dots per inch). The higher the resolution, the better the quality. However, this also rapidly increases the size of the file. A resolution of up to 300 dpi is normal for color images required for offset printing.

A lower resolution is sufficient for other printers, e.g. laser or inkjet printers. To retain flawless results, the resolution of a scanned image of say 300 dpi is halved to 150 dpi. No interpolation is employed in the image, as would be the case at a resolution of 200 dpi. Interpolation involves the generation of new pixels and can cause the image to lose its sharpness. 

The dimension
is the extent that the image is saved in. In IsoDraw, there is a choice between the sheet size and extent of the graphic. Extent refers to the outermost coordinates of a graphic, i.e. the bounding box.

You can work with a user-defined bounding box if you plan to subsequently position the graphic in a DTP program. To do this, you create an invisible frame behind the graphic that corresponds to the frame size in the DTP program. The advantage is that the graphic no longer has to be scaled, rendering text and line thickness scaling unnecessary.

Image depth in a JPEG file
defines the amount of different colors that can be contained in a JPEG file. A line-art illustration can only contain black or white pixels, and a grayscale image can contain up to 256 different levels of gray.

Color images are limited to 256 colors (8 bits) or any of the settings up to and including 16.7 million colors (24 bits). A color image can be saved as a 24 bit color image or 8 bit grayscale image in JPEG format.

24 bit color image compression 6	24 bit color image compression 2	8 bit grayscale image compression 6	8 bit grayscale image compression 2

Increased blurring between the pixels can be seen in compression 2. In this case, the file size can be greatly reduced, however this has a detrimental effect on quality.

The compression
The compression option allows you to markedly reduce the size of the JPEG file without affecting the quality of the image. You should therefore only compress an image after you have finished your work or instead use another format, e.g. Tiff format. PackBits or group 3 or group 4 fax compressions are suitable for black/white images. Group 4 compression in particular produces remarkable results. Unfortunately, not all programs support this compression.

Sample grayscale image:

Compression of a grayscale image	File size
JPEG uncompressed	264 KB
JPEG medium compression at level 6	60 KB
JPEG low compression at level 2	30 KB

Use the medium setting for grayscale or color images.

Color image compression	File size
JPEG uncompressed	121 KB
JPEG medium compression at level 6	30 KB
JPEG low compression at level 2	18 KB