Raster (Bitmap) Files
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Dialog Box - called from: File menu

--Windows and Linux 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 Clipboard/PICT option (File menu) can also write raster or bitmap files, in the PICT format.

--End Windows/Macintosh/Linux 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.

Using colors other than white, black or gray for background may result in increased size of raster files or PostScript files in 3D Drawing Modes. In some cases a simple run-length-encoding method is used for compression of these files (see below), and this fails for 24-bit RGB (true color) files when the three components are not identical. That is, for a row of white pixels the sequence of bytes is 255, 255, 255, 255... whereas for a row of red pixels it is 255, 0, 0, 255, 0, 0...

The images will look much the same as a screen image, except that for black-and-white files gray shades are always rendered as dot patterns, and there is a choice of coarse or fine patterns, as for printing (see the Print dialog in the File menu). Fine patterns are appropriate for small images which will not be reproduced photographically, or for images which will be shown on the screen only; coarse patterns mimic halftone screens used in printing, and generally reproduce better photographically, especially when the image is reduced in size. Note that this coarse/fine setting is the same as that in the Print and Metafiles or PICT files dialogs in the File menu.

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 pretends 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.

Full-page output (8 by 11 inches) with 32-bit color requires about 30 megabyes at 300 dots per inch, and about 500 megabytes at 1200 dpi. As of 2010, most computers have enough memory - several gigabytes - to support the entire image at once. It should therefore not be necessary to have the maximum memory set to other than zero. However, it may be necessary to close other programs to make all the memory available. The following applies mainly to older systems.

ATOMS is capable of imaging and writing raster files in segments or bands to avoid the problem of using virtual memory. 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. 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, ATOMS 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), ATOMS 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, ATOMS 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 ATOMS 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 ATOMS (although 3D images do not compress as well as standard-mode images). 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 ATOMS does not use this type of compression because of the copyright problem. The run-length-encoding which ATOMS 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 ATOMS to support all of them. ATOMS 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.