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<concept id="GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E" xml:lang="en"><title>GDI
Collection Overview</title><shortdesc>The Graphics Device Interface (GDI) collection is an important
collection within the Graphics subsystem. It provides a suite of abstract
base classes, interfaces and data structures. The suite represents and interacts
with physical graphics hardware such as display screens, off-screen memory
and printers. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section><title>Architectural Relationship</title> <p>The GDI collection provides
base classes and utility functions. Every graphics component ends up using
it. The components in this collection are closely linked and provide important
functionality. The GDI component is a low-level component in this collection,
BitGDI is based on this and DirectGDI is a potentially faster alternative
to BitGDI. The Colour Palette provides coloring indexing operations. </p> </section>
<section><title>Description</title> <p>GDI consists of a large number of diverse
APIs that cover many different aspects of graphics including graphics device
abstraction, color value handling, linear digital differential analysis, font
description and information, printer devices abstraction, line breaking, text
rendering, handling of bidirectional text, text wrapper and linked font creation. </p> <p>Graphics
Device Interface collection includes the following components: </p> <ul>
<li id="GUID-BA77A047-4A77-5527-9EE8-AE4302E759C9"><p><xref href="GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E.dita#GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E/GUID-25C920A3-EF7C-5E8E-B94E-531B56E36DB9">GDI</xref>  </p> </li>
<li id="GUID-08EB75C6-0E70-5F16-9CF1-BF43AFDB3CC7"><p><xref href="GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E.dita#GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E/GUID-4CDF56F2-E278-5488-AD9D-D42EFC145A40">BitGDI</xref>  </p> </li>
<li id="GUID-13396341-EBF4-54F4-BC5A-5223F269114C"><p><xref href="GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E.dita#GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E/GUID-81F903E9-F9BD-5141-8402-483D6E26F1F0">DirectGDI</xref>  </p> </li>
<li id="GUID-61FA0D95-AE43-50AB-B0BF-0BE9BF9D3C95"><p><xref href="GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E.dita#GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E/GUID-059D5B88-E69F-5898-8D87-161B349B7C0C">Colour Palette</xref>  </p> </li>
</ul> </section>
<section id="GUID-25C920A3-EF7C-5E8E-B94E-531B56E36DB9"><title>GDI</title> <p>GDI
is a low level component. Two key concepts related to GDI are Device (<xref href="GUID-500FC564-35E9-3B66-A0C2-1269371A2EA0.dita"><apiname>CGraphicsDevice</apiname></xref>)
and Context (<xref href="GUID-DAD09DCF-3123-38B4-99E9-91FB24B92138.dita"><apiname>CGraphicsContext</apiname></xref>). Throughout the Graphics
sub-system a graphics device provides a graphics context through which the
device may be drawn to. The context maintains 'settings', such as the pen
and brush colors and styles in use, and provides a rich API of drawing commands
for creating graphical and textual output. </p> <p>Most display devices use
bitmap graphics and the GDI provides specializations of both <xref href="GUID-500FC564-35E9-3B66-A0C2-1269371A2EA0.dita"><apiname>CGraphicsDevice</apiname></xref> and <xref href="GUID-DAD09DCF-3123-38B4-99E9-91FB24B92138.dita"><apiname>CGraphicsContext</apiname></xref> to
act as base classes for bitmap devices: namely <xref href="GUID-2DEFEC47-F36E-3133-A08D-55F7C2534CC0.dita"><apiname>CBitmapDevice</apiname></xref> and <xref href="GUID-FC746873-0570-3900-AD89-42B205FDC0D3.dita"><apiname>CBitmapContext</apiname></xref>.
These add, amongst other attributes, pixel awareness, blitting, scanlines
and fading. </p> <p>The GDI defines abstract base classes for storing and
manipulating fonts (<xref href="GUID-2A12FE3B-47F2-3016-8161-A971CA506491.dita"><apiname>CFont</apiname></xref>, <xref href="GUID-0CA79D2D-827F-3A20-A10C-1CC9C576E803.dita"><apiname>CTypefaceStore</apiname></xref>, <xref href="GUID-B68B7BC8-CFBA-32F4-A0BE-8378F0C105DA.dita"><apiname>CFontCache</apiname></xref>)
and additional supporting classes and structures. </p> <p>The GDI defines
the interface for mapping between device-specific pixels and real-world twips.
A twip is a twentieth of a point and a point is 1/72 of an inch. That makes
a twip 1/1440 of an inch, 1/567 of a centimeter or about 0.018mm. </p> <p>The
GDI defines a base class for representing embedded pictures, <xref href="GUID-829D57CD-9659-347B-AAE9-5F0A0ECD555C.dita"><apiname>CPicture</apiname></xref>,
and supporting classes. It also provides base classes, interfaces and structures
for printing. </p> </section>
<section id="GUID-4CDF56F2-E278-5488-AD9D-D42EFC145A40"><title> BitGDI</title> <p>The <xref href="GUID-EAAD1719-C02C-5705-A5C3-993E36441BE6.dita">BitGDI Component</xref> is
used by legacy applications and the Window Server in the <xref href="GUID-F64E6551-670E-5E12-8103-DE504D3EC94F.dita">non-ScreenPlay
variant</xref>. The BitGDI component provides the <xref href="GUID-B229156F-2344-3F46-8542-AC65882D80DE.dita"><apiname>CFbsScreenDevice</apiname></xref> and <xref href="GUID-4A501086-7EFF-376D-8901-6D9B2EB4EFF2.dita"><apiname>CFbsBitGc</apiname></xref> APIs,
which are concrete implementations of <xref href="GUID-500FC564-35E9-3B66-A0C2-1269371A2EA0.dita"><apiname>CGraphicsDevice</apiname></xref> and <xref href="GUID-DAD09DCF-3123-38B4-99E9-91FB24B92138.dita"><apiname>CGraphicsContext</apiname></xref> for
bitmap-based devices. This component enables: </p> <ul>
<li id="GUID-989D5F4A-3DB5-55DA-91FE-5E70D3DFF57E"><p>hardware-independent
access to screen attributes </p> </li>
<li id="GUID-7E420A37-E7A9-5583-8FE0-393677696319"><p>bitmapped drawing which
holds settings and supports drawing to bitmapped devices and screens independent
of the display. </p> </li>
</ul> <fig id="GUID-C75462F4-7DB3-57BF-ABDE-C500669F45C5">
<image href="GUID-71E64F3E-40CB-560B-A482-CA92C7FD1AA2_d0e200807_href.png" placement="inline"/>
</fig> </section>
<section id="GUID-059D5B88-E69F-5898-8D87-161B349B7C0C"><title>Colour
Palette</title> <p>The <xref href="GUID-E82FABA9-9CCC-548F-99DB-EF2B9036B405.dita">Colour
Palette component</xref> provides coloring functionality. A user can specify
the use of a particular color, as described by a <xref href="GUID-F84C7F40-6DEB-39D1-B172-CB0CC3918E27.dita"><apiname>TRgb</apiname></xref>,
and have that color rendered on a machine’s display device as closely as possible
to the color specified. A related functionality allows arrays of colors to
be stored as bitmaps, and displayed at a later date in the same manner. </p> <p>Color
palettes also provide user-definable palette support to the GDI. A palette
is a user-defined set of colors, which is a subset of the full range of 24-bit
colors. This allows users the advantages of having a low bpp color mode whilst
being able to specify the colors available in that mode. Only a palette of
16 colors enables changing the palette. Palettes are also used to allow 24-bit
bitmaps to be stored in a more compressed form by finding the actual number
of different colors used in the bitmap. This creates a palette that allows
mapping of colors to a smaller index space. The pixels of the bitmaps are
encoded using indexes to this new index space. A palette has a size which
is set at its creation and cannot alter the number of entries in the palette.
Each entry in a palette is a mapping between that entry's index and a <codeph>TRgb</codeph> value.
Palette entries can be got and set at any time between the palette's creation
and destruction. The GDI's palette support also provides functions to find
the nearest palette color to a requested <codeph>TRgb</codeph> color. </p> </section>
</conbody><related-links>
<link href="GUID-E89F034F-C807-5FF9-B06B-F7CCD2441041.dita"><linktext>Graphics
Device Interface (GDI) Collection</linktext></link>
</related-links></concept>