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.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. |
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.
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.
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.
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.
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.
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.
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.