Dialog Box: Raster (Bitmap) Files [File menu]
--Windows Only ----------
This command can write files in four different formats (File type): Windows device-independent format (.BMP); PC-Paintbrush (.PCX); Tag-image-format (.TIF); and Portable Network Graphics (.PNG) files.
--Macintosh only---------
This command can write files in the Portable Network Graphics (.PNG) format. The PICT/Clipboard option (File menu) can also write raster or bitmap files, in the PICT format.
--End Windows/Macintosh only----
See Selection of File Format below for information on choosing the format.
The drawing may be either black and white or color. If black and white, the pixel size is 1-bit. For color files the pixel size may in some cases be selected. In Windows, 16-bit is only available for .bmp files. In Macintosh, 8-bit (256-color) is only available if the screen is currently set at this color depth.
If the Compress box is checked, .TIF files are compressed with run-length encoding, which is one of the required formats under the TIFF 6.0 standard. However, some applications may not support this type of compression, or may reject .TIF files for other reasons. Also, if the Compress box is checked, 4- and 8-bit .BMP files are compressed. 1-bit and 24- bit .BMP files are never compressed. Owing to a Windows system bug, compression of 4-bit .BMP files may not work when banding is used (see below). .PCX files are always compressed. with their own format. 24-bit .PCX files can compress well, whereas 24-bit .TIF files are only partially compressed (and then only if there are large areas of black, white or grey), and 24-bit .BMP files are not compressed at all. PNG files also are always compressed.
See Frames for details of the frame and frame units.
Maximum memory. Windows and Macintosh both use virtual memory , which means that they pretend that a hard disk can be used in place of actual hardware memory chips. For some applications this is useful, but when writing raster files it usually results a form of lingering death involving "disk thrashing", or constant reading and writing of data to the hard disk.
CRYSCON is capable of imaging and writing raster files in segments or bands to avoid this problem. This requires repeating the entire drawing process for each band, but this usually is much faster than the reading/writing to disk involved in using virtual memory. To do this requires knowing how much actual chip memory is available. Unfortunately, at present Windows does not provide this information in a way that can be used for both 16- (standard Windows 3.1) and 32-bit (Windows 95/98/NT/XP) systems. Thus the user may need to determine this by trial and error using the Maximum memory parameter.
When the Maximum memory parameter is set to zero, CRYSCON will simply ask the system for a block of memory large enough to image the entire drawing at once. This amount of memory is printed out in the dialog which appears after starting the raster imaging: "nnnn bytes required, mmmm bytes available - will require 1 bands". If this amount of memory is greater than the amount of chip memory available, Windows will use virtual memory, which is typically manifested by constant running of the hard disk for long periods (disk thrashing). If you change the Maximum memory to some amount smaller than nnnn (say half), CRYSCON will only request this amount of memory from the system, and virtual memory may not come into play. A Maximum memory setting should be good for a given session, regardless of the size of the raster file; larger files will use more bands. However, the amount of memory available may depend on whether other applications are active.
The amount of virtual memory is determined by the available space on the hard disk. If this is insufficient, CRYSCON will automatically use bands, but since virtual memory is being used, it may still be advantageous to use a Maximum memory value which will increase the number of bands.
Selection of File Format. The choice of format depends first of all on what formats are supported by the target software (paint program; publishing program; etc.). Beyond this the first choice should be PNG, second choice PCX, and then PICT, TIF or BMP depending of what is supported and how much memory or disk space is available.
As of 1997, the preferred graphics file types for the internet were GIF and JPEG. However, GIF uses the LZW compression scheme copyrighted by Unisys Corp., which in principle charges a license fee for every application using it. JPEG files are compact but involve losses and degradation of the image and are thus not suitable for CRYSCON files.
The PNG format is intended to be a replacement for GIF, and in fact is superior to most other formats, being lossless and giving very good compression for the type of images generated by CRYSCON. Essentially all major software vendors have subscribed to the PNG convention and releases from 1998 onward should support it.
Most TIF files also use LZW compression, but CRYSCON does not use this type of compression because of the copyright problem. The run-length-encoding which CRYSCON does use is supposed to be supported by all TIF readers, but in fact some do not support it, and this algorithm does not work well for 24-bit (RGB) images.
PICT bitmap files on the Macintosh are compressed by the system software and the algorithm may vary. PNG files are usually smaller.
There are dozens of raster graphics formats and it will never be possible for CRYSCON to support all of them. CRYSCON users who work with raster files should consider investing in a conversion program - there are many such programs which are not expensive. The PNG home page (www.cdrom.com/pub/pnp) has links to many shareware and conventional commercial conversion packages.