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    12 <concept id="GUID-C1B080D9-9C6C-520B-B73E-4EB344B1FC5E" xml:lang="en"><title>GDI

    13 Collection Overview</title><shortdesc>The Graphics Device Interface (GDI) collection is an important

    14 collection within the Graphics subsystem. It provides a suite of abstract

    15 base classes, interfaces and data structures. The suite represents and interacts

    16 with physical graphics hardware such as display screens, off-screen memory

    17 and printers. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    18 <section><title>Architectural Relationship</title> <p>The GDI collection provides

    19 base classes and utility functions. Every graphics component ends up using

    20 it. The components in this collection are closely linked and provide important

    21 functionality. The GDI component is a low-level component in this collection,

    22 BitGDI is based on this and DirectGDI is a potentially faster alternative

    23 to BitGDI. The Colour Palette provides coloring indexing operations. </p> </section>

    24 <section><title>Description</title> <p>GDI consists of a large number of diverse

    25 APIs that cover many different aspects of graphics including graphics device

    26 abstraction, color value handling, linear digital differential analysis, font

    27 description and information, printer devices abstraction, line breaking, text

    28 rendering, handling of bidirectional text, text wrapper and linked font creation. </p> <p>Graphics

    29 Device Interface collection includes the following components: </p> <ul>

    30 <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>

    31 <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>

    32 <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>

    33 <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>

    34 </ul> </section>

    35 <section id="GUID-25C920A3-EF7C-5E8E-B94E-531B56E36DB9"><title>GDI</title> <p>GDI

    36 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>)

    37 and Context (<xref href="GUID-DAD09DCF-3123-38B4-99E9-91FB24B92138.dita"><apiname>CGraphicsContext</apiname></xref>). Throughout the Graphics

    38 sub-system a graphics device provides a graphics context through which the

    39 device may be drawn to. The context maintains 'settings', such as the pen

    40 and brush colors and styles in use, and provides a rich API of drawing commands

    41 for creating graphical and textual output. </p> <p>Most display devices use

    42 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

    43 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>.

    44 These add, amongst other attributes, pixel awareness, blitting, scanlines

    45 and fading. </p> <p>The GDI defines abstract base classes for storing and

    46 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>)

    47 and additional supporting classes and structures. </p> <p>The GDI defines

    48 the interface for mapping between device-specific pixels and real-world twips.

    49 A twip is a twentieth of a point and a point is 1/72 of an inch. That makes

    50 a twip 1/1440 of an inch, 1/567 of a centimeter or about 0.018mm. </p> <p>The

    51 GDI defines a base class for representing embedded pictures, <xref href="GUID-829D57CD-9659-347B-AAE9-5F0A0ECD555C.dita"><apiname>CPicture</apiname></xref>,

    52 and supporting classes. It also provides base classes, interfaces and structures

    53 for printing. </p> </section>

    54 <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

    55 used by legacy applications and the Window Server in the <xref href="GUID-F64E6551-670E-5E12-8103-DE504D3EC94F.dita">non-ScreenPlay

    56 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,

    57 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

    58 bitmap-based devices. This component enables: </p> <ul>

    59 <li id="GUID-989D5F4A-3DB5-55DA-91FE-5E70D3DFF57E"><p>hardware-independent

    60 access to screen attributes </p> </li>

    61 <li id="GUID-7E420A37-E7A9-5583-8FE0-393677696319"><p>bitmapped drawing which

    62 holds settings and supports drawing to bitmapped devices and screens independent

    63 of the display. </p> </li>

    64 </ul> <fig id="GUID-C75462F4-7DB3-57BF-ABDE-C500669F45C5">

    65 <image href="GUID-71E64F3E-40CB-560B-A482-CA92C7FD1AA2_d0e205815_href.png" placement="inline"/>

    66 </fig> </section>

    67 <section id="GUID-059D5B88-E69F-5898-8D87-161B349B7C0C"><title>Colour

    68 Palette</title> <p>The <xref href="GUID-E82FABA9-9CCC-548F-99DB-EF2B9036B405.dita">Colour

    69 Palette component</xref> provides coloring functionality. A user can specify

    70 the use of a particular color, as described by a <xref href="GUID-F84C7F40-6DEB-39D1-B172-CB0CC3918E27.dita"><apiname>TRgb</apiname></xref>,

    71 and have that color rendered on a machine’s display device as closely as possible

    72 to the color specified. A related functionality allows arrays of colors to

    73 be stored as bitmaps, and displayed at a later date in the same manner. </p> <p>Color

    74 palettes also provide user-definable palette support to the GDI. A palette

    75 is a user-defined set of colors, which is a subset of the full range of 24-bit

    76 colors. This allows users the advantages of having a low bpp color mode whilst

    77 being able to specify the colors available in that mode. Only a palette of

    78 16 colors enables changing the palette. Palettes are also used to allow 24-bit

    79 bitmaps to be stored in a more compressed form by finding the actual number

    80 of different colors used in the bitmap. This creates a palette that allows

    81 mapping of colors to a smaller index space. The pixels of the bitmaps are

    82 encoded using indexes to this new index space. A palette has a size which

    83 is set at its creation and cannot alter the number of entries in the palette.

    84 Each entry in a palette is a mapping between that entry's index and a <codeph>TRgb</codeph> value.

    85 Palette entries can be got and set at any time between the palette's creation

    86 and destruction. The GDI's palette support also provides functions to find

    87 the nearest palette color to a requested <codeph>TRgb</codeph> color. </p> </section>

    88 </conbody><related-links>

    89 <link href="GUID-E89F034F-C807-5FF9-B06B-F7CCD2441041.dita"><linktext>Graphics

    90 Device Interface (GDI) Collection</linktext></link>

    91 </related-links></concept>