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GDI requires all display drivers to provide an ExtTextOut function that
provides a minimum set of text-drawing capabilities. At the very least, a
driver must be able to draw a string of characters at a specified location
on the screen and clip any portion of a character that extends beyond the
bounding box for the string.
 
GDI calls the ExtTextOut function whenever an application calls a function
that draws text or computes text widths. The ExtTextOut function receives a
string of character values, a count of characters in the string, a starting
position, a physical font, and a DRAWMODE structure. The function uses these
values to create the individual glyph images on the screen.
 
GDI checks the RC_GDI20_OUTPUT value in the dpRaster member of the GDIINFO
structure to determine whether the driver supports this function. In earlier
versions of Windows, the StrBlt function supported text drawing, but StrBlt
is now obsolete. For compatibility with early versions of Windows
applications, however, the display driver must provide the StrBlt function.
In most drivers, StrBlt does nothing more than call or fall through to
ExtTextOut.
 
GDI checks the dpText member of the GDIINFO structure to determine which
text capabilities the display driver supports. Although Windows requires few
text capabilities for a display driver, all display drivers should support
as many capabilities as possible so that Windows applications have the
greatest flexibility when drawing text. The following are the text
capabilities.
 
Capability        Description
────────────────────────────────────────────────────────────────────────────
Clipping          Specifies whether the display driver can clip whole or
                  partial characters.
 
Cosmetics         Specifies whether the display driver can generate bold,
                  italic, underlined, or strikethrough characters from
                  existing characters.
 
Fonts             Specifies whether the display driver supports raster and
                  vector fonts.
 
Output precision  Specifies which font attributes the display driver uses
                  when drawing text.
 
Rotation          Specifies whether the display driver can rotate
                  characters.
 
Scaling           Specifies whether the display driver can generate new
                  sizes by scaling an existing size.
 
Scaling freedom   Specifies whether the display driver can scale
                  independently along the x- and y-axes.
 
Output Precision
 
GDI checks the TC_OP_CHARACTER and TC_OP_STROKE values in the dpText member
to determine the output precision of the driver. Output precision specifies
which font attributes the ExtTextOut function must use when it draws text.
Font attributes include width, height, intercharacter spacing, interword
spacing, escapement, orientation, and other attributes specified in the
TEXTXFORM and DRAWMODE structures. These structures are passed to the
ExtTextOut function.
 
A display driver sets the TC_OP_STROKE value if it can draw characters as a
set of line segments. When drawing with stroke precision, the driver must
adhere to all font attributes, and use the current transformations to
compute the starting point of the string.
 
A display driver sets the TC_OP_CHARACTER value if it can draw characters at
any given escapement. Character precision ensures the placement of
individual characters without guaranteeing the exact size requested. With
character precision, the driver must:
 
♦  Use the width and height to determine a "best-fit" character size. If no
   font matches exactly, the driver should use either the largest font that
   does not exceed the requested size or the smallest available font.
 
♦  Use the current transformations to compute the starting point of the
   string.
 
♦  Use the current intercharacter and interword spacing to position the
   individual characters in the string.
 
♦  Use the current escapement.
 
The driver can ignore all other attributes. If possible, the display driver
should use the display's character generation hardware to draw individual
characters.
 
If a display driver does not set the TC_OP_STROKE and TC_OP_CHARACTER
values, GDI requires the driver to support string precision. String
precision is identical to character precision except that the current
escapement can be ignored.
 
Output precision does not affect the bold, italic, underline, or
strikethrough capabilities. If a driver registers these abilities, it must
perform them when requested.
 
Clipping
 
GDI checks the TC_CP_STROKE value in the dpText member to determine the clip
precision of the driver. A display driver sets the TC_CP_STROKE value if it
can clip any portion of a character that is outside the clip region and draw
the rest of the character. If the driver does not set this value, GDI
requires that the driver clips the entire character if any portion of the
character is outside the clip region.
 
A display driver must support stroke clip precision.
 
Rotation
 
GDI checks the TC_CR_90 and TC_CR_ANY values in the dpText member to
determine whether the display driver can rotate characters. A display driver
sets the TC_CR_ANY value if it can rotate characters to any angle. The
driver sets the TC_CR_90 value it can rotate characters at 90 degree
increments only. If a display driver does not set either bit, GDI assumes
that the driver cannot rotate characters.
 
Scaling and Scaling Freedom
 
GDI checks the TC_SA_DOUBLE, TC_SA_INTEGER, and TC_SA_CONTIN values to
determine whether the display driver can scale characters. GDI also checks
the TC_SF_X_YINDEP value to determine whether the driver can scale
characters independently on the x- and y-axes.
 
A display driver sets the TC_SA_CONTIN value if it can scale existing
characters to any size, sets the TC_SA_INTEGER value if it can scale
characters by any integer multiple, or sets the TC_SA_DOUBLE value if it can
double the size of existing characters. If a display driver sets none of
these values, GDI assumes that the driver can not scale characters.
 
A display driver sets the TC_SF_X_YINDEP value if the driver can scale
characters independently in each direction. If this bit is not set but the
driver specifies that it can scale characters, GDI assumes that the driver
always scales characters the same amount in the each direction.
 
Whenever a display driver cannot match a requested size exactly, GDI
requires the driver to use the largest size available that will not exceed
the requested size in either direction.
 
Cosmetics
 
GDI checks the TC_EA_DOUBLE, TC_IA_ABLE, TC_UA_ABLE, and TC_SO_ABLE values
to determine whether the display driver can generate bold, italic,
underlined, or strikethrough characters from existing characters.
 
A display driver sets TC_EA_DOUBLE if it can generate a bold character by
doubling the existing character's weight. A typical method is to overstrike
the existing character after moving one device unit to the right.
 
A display driver sets the TC_IA_ABLE value if it can generate italic
characters. A typical method is to skew the gylph information, drawing the
the character as if it were contained by a parallelogram rather than a
rectangle.
 
A display driver sets the TC_UA_ABLE value if it can generate underlined
characters, and sets the TC_SO_ABLE value if it can generate strikethrough
characters.
 
Raster and Vector Fonts
 
GDI checks the TC_RA_ABLE and TC_VA_ABLE values in the dpText member to
determine whether the display driver supports raster and vector fonts,
respectively. A display driver must set at least one of these bits.
 
Orientation and Escapement
 
Whenever either the character orientation or the difference between the
character orientation and the escapement angle is a multiple of 90 degrees,
the intercharacter and interword spacing will be the standard intercharacter
spacing used for bounding boxes plus the spacing specified by the
CharacterExtra and BreakExtra members in the DRAWMODE structure.
 
The standard intercharacter spacing at a given escapement angle and
character orientation is defined as the minimum spacing along the escapement
vector, such that the character origins are on the escapement vector, and
the character bounding boxes touch. Variable pitch fonts are achieved by
using variable width bounding boxes. This model applies to all attribute
values. When the sides of the bounding boxes touch, extra space is added in
x and, when the tops touch, it is added in y.
 
In all other escapement and orientation cases, the standard intercharacter
spacing is device dependent. The preferred implementation is for the
90-degree cases. In all cases, it is required that all character origins lie
on the escapement vector.
 
It is assumed that arbitrary escapement angles can be achieved, if by no
other means than, by placing each character as a separate entity. Many
devices are able to do arbitrary character rotation only if the character
orientation matches the escapement angle. For such devices, it is assumed
that the driver will place each character individually at the proper
orientation and escapement, when escapement and character orientation do not
match.
 
 
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