Topic       : The ATARI Compendium
Author      : Scott Sanders / JAY Software
Version     : 1.25 (20/6/2003)
Subject     : Documentation
Nodes       : 1117
Index Size  : 32614
HCP-Version : 6
Compiled on : Atari
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@lang       : en
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                              VDI  Raster Forms


The VDI handles raster forms using three commands, vro_cpyfm(),
vrt_cpyfm(), and vr_trnfm(). vro_cpyfm() and vrt_cpyfm()  are responsible
for 'blitting' raster images between memory and a workstation. These
functions may also be used to copy images from one location on
a workstation to another. 'Blitting' is the process of copying memory from
one location to another. Atari computers use the BLiTTER chip (when one
is installed) or a software bit blit algorithm to quickly move memory.
While these calls are designed to transfer screen memory, if carefully
used, they may also be used to transfer other types of memory as well.

vr_trnfm() is responsible for the transformation of images between
device-specific and VDI standard format, the two raster image formats
recognized by the VDI. Device-specific format is limited to images in the
format of the source device whereas the second is a generic format
recommended for transporting images to non-standard displays.

VDI Device-Specific Format

Device-specific format simply mimics the layout of pixels and planes on
the source device. When using vro_cpyfm() and vrt_cpyfm() the source form
will be transferred to the destination form in device-specific format∙The definitions of vro_cpyfm() and vrt_cpyfm()
allow for the specification of the format of
the source and destination form, however,
this feature is not currently supported by any
version of the operating system. Any call which
specifies either the source or destination form
to be in device-independent format will fail.
.

If you intend to save images to disk you should first utilize vr_trnfm()
to transform the image into a VDI standard format so that the image can be
successfully ported to any display.

VDI Standard Format

VDI standard format is designed to provide a portable method of
specifying raster images which may be displayed on any device. Images
stored in VDI standard format must be transformed with vr_trnfm() before
copying them to a workstation.

Images in VDI standard format appear in memory in a plane-by-plane
fashion. All of the bits for plane #0 appear first followed by the bits
for plane #1, and so on for as many planes as exist in the image.

Images may be easily transferred to devices with a higher number of
planes by simply inserting empty bytes to account for planes not present
in the source image. This method will only work, however, with palette
based devices.