Dump Screen to PostScript File

This option may be used to produce a PostScript description of an image contained in one of the frame buffers. This description is written to a file which may then be printed on a suitable PostScript compatible printer. The main limitation of the "screen dumps" obtained in this manner is the low grey-scale resolution as PostScript requires a dithering technique. Nevertheless these screen dumps are useful, particularly as the dithering enables ready duplication on photocopying machines.

The file produced consists of a header containing procedures which control the plotting process and handle errors alongside the data in a run-length encoded form.

Screen dump of which buffer ?

This entry specifies which buffer is to be converted into PostScript. Any valid buffer may be used. The selected buffer will be made the current output buffer. Extended memory buffers will be vectored through buffer 0.

Dump Window or whole Screen ?

The screen dump may be made using either the contents of the entire buffer (<S>), or a window within the buffer (<W>).

Window Selection: Region to be dumped

If the screen dump is to be made of a window, then the window selection submenu will be produced to enable user specification of the window. For more information on window selection, refer to [H Help] within the submenu.

Number of bits for grey levels (0 => error diffusion) ?

This entry specifies the number of grey levels to be used in the printing process. It is specified in terms of the number of bits used to define the intensity. A value of 8 gives all 256 possibilities, 4 gives 16 levels, 2 four levels and 1 a black/white picture. Normally PostScript produces different grey levels in an ordered manner using a "spot function" to determine the order in which pixels are to be lightened with increasing intensity. However this process leads to a compromise between the graininess of the image and the dynamic range of the "grey". Generally the larger the spot function, the better the dynamic range, but the grainier the image. To overcome this DigImage includes the possibility of combining PostScript's normal greyscale rendering with an error diffusion algorithm to produce a wide dynamic range without undue graininess. This error diffusion algorithm is accessed by specifying 0 for the number of bit planes. In fact all eight bit planes of the raw image are used and compiled into two bit planes of PostScript grey scale dithered with error diffusion onto a virtual image with eight times as many "pixels". Overall the files are four times the size of conventional screen dumps and take correspondingly longer to print. Therefore use of option 0 should be confined to occassions when the highest quality images are required.

{If normal PostScript rendering}

Step size vertically (1 => full resolution) ?

This entry specifies the vertical resolution of the PostScript image. The lower the resolution (the larger the step size), the smaller the number of cells in the resultant image, the smaller the PostScript file and the faster the printing. It is seldom beneficial to have a vertical step size less than two pixels.

{If normal PostScript rendering}

Step size horizontally (1 => full resolution) ?

This entry specifies the horizontal resolution of the PostScript image. The lower the resolution (the larger the step size), the smaller the number of cells in the resultant image, the smaller the PostScript file and the faster the printing.

Render using Default, Greyscale, Negative greyscale, Colour (using DigImage colour translation) or colour using Printer colour translation ?

Normally the image will be plotted as greyscale using a laser printer. The image may be plotted with intensity 0 mapping to black and 255 to white (a positive image, <G>), or with 0 mapping to white and 255 to black (a negative image, <N>). If the PostScript printer is capable of rendering colour, then selecting <C> or <P> will plot the image using the current output look up table so the image will appear exactly as it does on screen. If you wish to use the default setting, simply select <D>.

If <C> or <P> is selected for a printer capable of producing only greyscale, then the result will depend on which level of PostScript the printer supports. For a Level 1 PostScript interperater, the result will be identical to that obtained if <G> had been chosen. For a Level 2 (or later) machine the current colour scheme will be converted to equivalent grey levels (i.e. bright bits will be printed lighter than dark bits).

Name of output PostScript file (blank to suppress; .PS for printable PostScript .EPS for encapsulated PostScript, or .DPS for data only PostScript) ?

Specifying a file in response to this prompt will cause a PostScript description of the output to be placed in the file. The file will subsequently need to be transfered to a suitable PostScript printer to enable the hard copy to be produced. Entering a blank line suppresses the production of the PostScript file.

Encapsulated PostScript (.EPS) and Data Only PostScript (.DPS) can not be printed directly, except using DigiFile. Normally they would be included within some other package (eg. a word processor) to be printed. The .DPS file is a special version of .EPS which does not contain the normal DigImage\PostScr\Header.PS and DigImage\PostScr\GraphVDU.PS header files. These may need to be added to the PostScript prolog used by the application into which the .DPS file is inserted.

If no extension is specified, then the default PostScript type specified in [;P Printers] in CONFIGUR.EXE will be produced, and the appropriate extension added.

This file may be sent to the printer immediately by specifying PRN as the file name.

Title for screen dump ?

The text specified at this prompt will be printed underneath the dumped image.

Number of copies ?

This option provides the possibility of printing multiple copies. The first copy of the screen dump always takes much longer than printing off additional copies at the same time. Depending on the amount of detail in an image and the printer being used, the first copy may take in the region of ten or twenty minutes to produce, while additional copies may be produced at four to ten per minute.

If the image contains large areas of black, then it is desirable to print at least two copies as it is not uncommon for laser printers to fail to produce correctly large areas of black.

The PostScript file will need to be downloaded to the printer...

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DigImage User Documentation

Stuart Dalziel, last page update: 19 February 1996