Skip to content

FreeType » Docs » Support API » Outline Processing

# Outline Processing

## Synopsis

This section contains routines used to create and destroy scalable glyph images known as ‘outlines’. These can also be measured, transformed, and converted into bitmaps and pixmaps.

## FT_Outline

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef struct  FT_Outline_
      {
        unsigned short   n_contours;  /* number of contours in glyph        */
        unsigned short   n_points;    /* number of points in the glyph      */

        FT_Vector*       points;      /* the outline's points               */
        unsigned char*   tags;        /* the points flags                   */
        unsigned short*  contours;    /* the contour end points             */

        int              flags;       /* outline masks                      */

      } FT_Outline;

This structure is used to describe an outline to the scan-line converter.

#### fields

n_contours | The number of contours in the outline.
n_points | The number of points in the outline.
points | A pointer to an array of `n_points` `FT_Vector` elements, giving the outline's point coordinates.
tags | A pointer to an array of `n_points` chars, giving each outline point's type. If bit 0 is unset, the point is ‘off’ the curve, i.e., a Bezier control point, while it is ‘on’ if set. Bit 1 is meaningful for ‘off’ points only. If set, it indicates a third-order Bezier arc control point; and a second-order control point if unset. If bit 2 is set, bits 5-7 contain the drop-out mode (as defined in the OpenType specification; the value is the same as the argument to the ‘SCANTYPE’ instruction). Bits 3 and 4 are reserved for internal purposes.
contours | An array of `n_contours` shorts, giving the end point of each contour within the outline. For example, the first contour is defined by the points ‘0’ to `contours[0]`, the second one is defined by the points `contours[0]+1` to `contours[1]`, etc.
flags | A set of bit flags used to characterize the outline and give hints to the scan-converter and hinter on how to convert/grid-fit it. See `FT_OUTLINE_XXX`.

#### note

The B/W rasterizer only checks bit 2 in the `tags` array for the first point of each contour. The drop-out mode as given with `FT_OUTLINE_IGNORE_DROPOUTS`, `FT_OUTLINE_SMART_DROPOUTS`, and `FT_OUTLINE_INCLUDE_STUBS` in `flags` is then overridden.

## FT_Outline_New

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_New( FT_Library   library,
                      FT_UInt      numPoints,
                      FT_Int       numContours,
                      FT_Outline  *anoutline );

Create a new outline of a given size.

#### input

library | A handle to the library object from where the outline is allocated. Note however that the new outline will not necessarily be freed, when destroying the library, by `FT_Done_FreeType`.
numPoints | The maximum number of points within the outline. Must be smaller than or equal to 0xFFFF (65535).
numContours | The maximum number of contours within the outline. This value must be in the range 0 to `numPoints`.

#### output

anoutline | A handle to the new outline.

#### return

FreeType error code. 0 means success.

#### note

The reason why this function takes a `library` parameter is simply to use the library's memory allocator.

## FT_Outline_Done

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Done( FT_Library   library,
                       FT_Outline*  outline );

Destroy an outline created with `FT_Outline_New`.

#### input

library | A handle of the library object used to allocate the outline.
outline | A pointer to the outline object to be discarded.

#### return

FreeType error code. 0 means success.

#### note

If the outline's ‘owner’ field is not set, only the outline descriptor will be released.

## FT_Outline_Copy

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Copy( const FT_Outline*  source,
                       FT_Outline        *target );

Copy an outline into another one. Both objects must have the same sizes (number of points & number of contours) when this function is called.

#### input

source | A handle to the source outline.

#### output

target | A handle to the target outline.

#### return

FreeType error code. 0 means success.

## FT_Outline_Translate

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( void )
      FT_Outline_Translate( const FT_Outline*  outline,
                            FT_Pos             xOffset,
                            FT_Pos             yOffset );

Apply a simple translation to the points of an outline.

#### inout

outline | A pointer to the target outline descriptor.

#### input

xOffset | The horizontal offset.
yOffset | The vertical offset.

## FT_Outline_Transform

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( void )
      FT_Outline_Transform( const FT_Outline*  outline,
                            const FT_Matrix*   matrix );

Apply a simple 2x2 matrix to all of an outline's points. Useful for applying rotations, slanting, flipping, etc.

#### inout

outline | A pointer to the target outline descriptor.

#### input

matrix | A pointer to the transformation matrix.

#### note

You can use `FT_Outline_Translate` if you need to translate the outline's points.

## FT_Outline_Embolden

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Embolden( FT_Outline*  outline,
                           FT_Pos       strength );

Embolden an outline. The new outline will be at most 4 times `strength` pixels wider and higher. You may think of the left and bottom borders as unchanged.

Negative `strength` values to reduce the outline thickness are possible also.

#### inout

outline | A handle to the target outline.

#### input

strength | How strong the glyph is emboldened. Expressed in 26.6 pixel format.

#### return

FreeType error code. 0 means success.

#### note

The used algorithm to increase or decrease the thickness of the glyph doesn't change the number of points; this means that certain situations like acute angles or intersections are sometimes handled incorrectly.

If you need ‘better’ metrics values you should call `FT_Outline_Get_CBox` or `FT_Outline_Get_BBox`.

To get meaningful results, font scaling values must be set with functions like `FT_Set_Char_Size` before calling FT_Render_Glyph.

#### example

      FT_Load_Glyph( face, index, FT_LOAD_DEFAULT );

      if ( face->glyph->format == FT_GLYPH_FORMAT_OUTLINE )
        FT_Outline_Embolden( &face->glyph->outline, strength );

## FT_Outline_EmboldenXY

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_EmboldenXY( FT_Outline*  outline,
                             FT_Pos       xstrength,
                             FT_Pos       ystrength );

Embolden an outline. The new outline will be `xstrength` pixels wider and `ystrength` pixels higher. Otherwise, it is similar to `FT_Outline_Embolden`, which uses the same strength in both directions.

#### since

2.4.10

## FT_Outline_Reverse

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( void )
      FT_Outline_Reverse( FT_Outline*  outline );

Reverse the drawing direction of an outline. This is used to ensure consistent fill conventions for mirrored glyphs.

#### inout

outline | A pointer to the target outline descriptor.

#### note

This function toggles the bit flag `FT_OUTLINE_REVERSE_FILL` in the outline's `flags` field.

It shouldn't be used by a normal client application, unless it knows what it is doing.

## FT_Outline_Check

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Check( FT_Outline*  outline );

Check the contents of an outline descriptor.

#### input

outline | A handle to a source outline.

#### return

FreeType error code. 0 means success.

#### note

An empty outline, or an outline with a single point only is also valid.

## FT_Outline_Get_CBox

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( void )
      FT_Outline_Get_CBox( const FT_Outline*  outline,
                           FT_BBox           *acbox );

Return an outline's ‘control box’. The control box encloses all the outline's points, including Bezier control points. Though it coincides with the exact bounding box for most glyphs, it can be slightly larger in some situations (like when rotating an outline that contains Bezier outside arcs).

Computing the control box is very fast, while getting the bounding box can take much more time as it needs to walk over all segments and arcs in the outline. To get the latter, you can use the ‘ftbbox’ component, which is dedicated to this single task.

#### input

outline | A pointer to the source outline descriptor.

#### output

acbox | The outline's control box.

#### note

See `FT_Glyph_Get_CBox` for a discussion of tricky fonts.

## FT_Outline_Get_BBox

Defined in FT_BBOX_H (freetype/ftbbox.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Get_BBox( FT_Outline*  outline,
                           FT_BBox     *abbox );

Compute the exact bounding box of an outline. This is slower than computing the control box. However, it uses an advanced algorithm that returns very quickly when the two boxes coincide. Otherwise, the outline Bezier arcs are traversed to extract their extrema.

#### input

outline | A pointer to the source outline.

#### output

abbox | The outline's exact bounding box.

#### return

FreeType error code. 0 means success.

#### note

If the font is tricky and the glyph has been loaded with `FT_LOAD_NO_SCALE`, the resulting BBox is meaningless. To get reasonable values for the BBox it is necessary to load the glyph at a large ppem value (so that the hinting instructions can properly shift and scale the subglyphs), then extracting the BBox, which can be eventually converted back to font units.

## FT_Outline_Get_Bitmap

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Get_Bitmap( FT_Library        library,
                             FT_Outline*       outline,
                             const FT_Bitmap  *abitmap );

Render an outline within a bitmap. The outline's image is simply OR-ed to the target bitmap.

#### input

library | A handle to a FreeType library object.
outline | A pointer to the source outline descriptor.

#### inout

abitmap | A pointer to the target bitmap descriptor.

#### return

FreeType error code. 0 means success.

#### note

This function does not create the bitmap, it only renders an outline image within the one you pass to it! Consequently, the various fields in `abitmap` should be set accordingly.

It will use the raster corresponding to the default glyph format.

The value of the `num_grays` field in `abitmap` is ignored. If you select the gray-level rasterizer, and you want less than 256 gray levels, you have to use `FT_Outline_Render` directly.

## FT_Outline_Render

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Render( FT_Library         library,
                         FT_Outline*        outline,
                         FT_Raster_Params*  params );

Render an outline within a bitmap using the current scan-convert.

#### input

library | A handle to a FreeType library object.
outline | A pointer to the source outline descriptor.

#### inout

params | A pointer to an `FT_Raster_Params` structure used to describe the rendering operation.

#### return

FreeType error code. 0 means success.

#### note

This advanced function uses `FT_Raster_Params` as an argument. The field `params.source` will be set to `outline` before the scan converter is called, which means that the value you give to it is actually ignored. Either `params.target` must point to preallocated bitmap, or `FT_RASTER_FLAG_DIRECT` must be set in `params.flags` allowing FreeType rasterizer to be used for direct composition, translucency, etc. See `FT_Raster_Params` for more details.

## FT_Outline_Decompose

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Error )
      FT_Outline_Decompose( FT_Outline*              outline,
                            const FT_Outline_Funcs*  func_interface,
                            void*                    user );

Walk over an outline's structure to decompose it into individual segments and Bezier arcs. This function also emits ‘move to’ operations to indicate the start of new contours in the outline.

#### input

outline | A pointer to the source target.
func_interface | A table of ‘emitters’, i.e., function pointers called during decomposition to indicate path operations.

#### inout

user | A typeless pointer that is passed to each emitter during the decomposition. It can be used to store the state during the decomposition.

#### return

FreeType error code. 0 means success.

#### note

Degenerate contours, segments, and Bezier arcs may be reported. In most cases, it is best to filter these out before using the outline for stroking or other path modification purposes (which may cause degenerate segments to become non-degenerate and visible, like when stroke caps are used or the path is otherwise outset). Some glyph outlines may contain deliberate degenerate single points for mark attachement.

Similarly, the function returns success for an empty outline also (doing nothing, that is, not calling any emitter); if necessary, you should filter this out, too.

## FT_Outline_Funcs

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef struct  FT_Outline_Funcs_
      {
        FT_Outline_MoveToFunc   move_to;
        FT_Outline_LineToFunc   line_to;
        FT_Outline_ConicToFunc  conic_to;
        FT_Outline_CubicToFunc  cubic_to;

        int                     shift;
        FT_Pos                  delta;

      } FT_Outline_Funcs;

A structure to hold various function pointers used during outline decomposition in order to emit segments, conic, and cubic Beziers.

#### fields

move_to | The ‘move to’ emitter.
line_to | The segment emitter.
conic_to | The second-order Bezier arc emitter.
cubic_to | The third-order Bezier arc emitter.
shift | The shift that is applied to coordinates before they are sent to the emitter.
delta | The delta that is applied to coordinates before they are sent to the emitter, but after the shift.

#### note

The point coordinates sent to the emitters are the transformed version of the original coordinates (this is important for high accuracy during scan-conversion). The transformation is simple:

      x' = (x << shift) - delta
      y' = (y << shift) - delta

Set the values of `shift` and `delta` to 0 to get the original point coordinates.

## FT_Outline_MoveToFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Outline_MoveToFunc)( const FT_Vector*  to,
                                void*             user );

    #define FT_Outline_MoveTo_Func  FT_Outline_MoveToFunc

A function pointer type used to describe the signature of a ‘move to’ function during outline walking/decomposition.

A ‘move to’ is emitted to start a new contour in an outline.

#### input

to | A pointer to the target point of the ‘move to’.
user | A typeless pointer, which is passed from the caller of the decomposition function.

#### return

Error code. 0 means success.

## FT_Outline_LineToFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Outline_LineToFunc)( const FT_Vector*  to,
                                void*             user );

    #define FT_Outline_LineTo_Func  FT_Outline_LineToFunc

A function pointer type used to describe the signature of a ‘line to’ function during outline walking/decomposition.

A ‘line to’ is emitted to indicate a segment in the outline.

#### input

to | A pointer to the target point of the ‘line to’.
user | A typeless pointer, which is passed from the caller of the decomposition function.

#### return

Error code. 0 means success.

## FT_Outline_ConicToFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Outline_ConicToFunc)( const FT_Vector*  control,
                                 const FT_Vector*  to,
                                 void*             user );

    #define FT_Outline_ConicTo_Func  FT_Outline_ConicToFunc

A function pointer type used to describe the signature of a ‘conic to’ function during outline walking or decomposition.

A ‘conic to’ is emitted to indicate a second-order Bezier arc in the outline.

#### input

control | An intermediate control point between the last position and the new target in `to`.
to | A pointer to the target end point of the conic arc.
user | A typeless pointer, which is passed from the caller of the decomposition function.

#### return

Error code. 0 means success.

## FT_Outline_CubicToFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Outline_CubicToFunc)( const FT_Vector*  control1,
                                 const FT_Vector*  control2,
                                 const FT_Vector*  to,
                                 void*             user );

    #define FT_Outline_CubicTo_Func  FT_Outline_CubicToFunc

A function pointer type used to describe the signature of a ‘cubic to’ function during outline walking or decomposition.

A ‘cubic to’ is emitted to indicate a third-order Bezier arc.

#### input

control1 | A pointer to the first Bezier control point.
control2 | A pointer to the second Bezier control point.
to | A pointer to the target end point.
user | A typeless pointer, which is passed from the caller of the decomposition function.

#### return

Error code. 0 means success.

## FT_Orientation

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      typedef enum  FT_Orientation_
      {
        FT_ORIENTATION_TRUETYPE   = 0,
        FT_ORIENTATION_POSTSCRIPT = 1,
        FT_ORIENTATION_FILL_RIGHT = FT_ORIENTATION_TRUETYPE,
        FT_ORIENTATION_FILL_LEFT  = FT_ORIENTATION_POSTSCRIPT,
        FT_ORIENTATION_NONE

      } FT_Orientation;

A list of values used to describe an outline's contour orientation.

The TrueType and PostScript specifications use different conventions to determine whether outline contours should be filled or unfilled.

#### values

FT_ORIENTATION_TRUETYPE | According to the TrueType specification, clockwise contours must be filled, and counter-clockwise ones must be unfilled.
FT_ORIENTATION_POSTSCRIPT | According to the PostScript specification, counter-clockwise contours must be filled, and clockwise ones must be unfilled.
FT_ORIENTATION_FILL_RIGHT | This is identical to `FT_ORIENTATION_TRUETYPE`, but is used to remember that in TrueType, everything that is to the right of the drawing direction of a contour must be filled.
FT_ORIENTATION_FILL_LEFT | This is identical to `FT_ORIENTATION_POSTSCRIPT`, but is used to remember that in PostScript, everything that is to the left of the drawing direction of a contour must be filled.
FT_ORIENTATION_NONE | The orientation cannot be determined. That is, different parts of the glyph have different orientation.

## FT_Outline_Get_Orientation

Defined in FT_OUTLINE_H (freetype/ftoutln.h).

      FT_EXPORT( FT_Orientation )
      FT_Outline_Get_Orientation( FT_Outline*  outline );

This function analyzes a glyph outline and tries to compute its fill orientation (see `FT_Orientation`). This is done by integrating the total area covered by the outline. The positive integral corresponds to the clockwise orientation and `FT_ORIENTATION_POSTSCRIPT` is returned. The negative integral corresponds to the counter-clockwise orientation and `FT_ORIENTATION_TRUETYPE` is returned.

Note that this will return `FT_ORIENTATION_TRUETYPE` for empty outlines.

#### input

outline | A handle to the source outline.

#### return

The orientation.

## FT_OUTLINE_XXX

Defined in FT_IMAGE_H (freetype/ftimage.h).

    #define FT_OUTLINE_NONE             0x0
    #define FT_OUTLINE_OWNER            0x1
    #define FT_OUTLINE_EVEN_ODD_FILL    0x2
    #define FT_OUTLINE_REVERSE_FILL     0x4
    #define FT_OUTLINE_IGNORE_DROPOUTS  0x8
    #define FT_OUTLINE_SMART_DROPOUTS   0x10
    #define FT_OUTLINE_INCLUDE_STUBS    0x20
    #define FT_OUTLINE_OVERLAP          0x40

    #define FT_OUTLINE_HIGH_PRECISION   0x100
    #define FT_OUTLINE_SINGLE_PASS      0x200

      /* these constants are deprecated; use the corresponding */
      /* `FT_OUTLINE_XXX` values instead                       */
    #define ft_outline_none             FT_OUTLINE_NONE
    #define ft_outline_owner            FT_OUTLINE_OWNER
    #define ft_outline_even_odd_fill    FT_OUTLINE_EVEN_ODD_FILL
    #define ft_outline_reverse_fill     FT_OUTLINE_REVERSE_FILL
    #define ft_outline_ignore_dropouts  FT_OUTLINE_IGNORE_DROPOUTS
    #define ft_outline_high_precision   FT_OUTLINE_HIGH_PRECISION
    #define ft_outline_single_pass      FT_OUTLINE_SINGLE_PASS

A list of bit-field constants used for the flags in an outline's `flags` field.

#### values

FT_OUTLINE_NONE | Value 0 is reserved.
FT_OUTLINE_OWNER | If set, this flag indicates that the outline's field arrays (i.e., `points`, `flags`, and `contours`) are ‘owned’ by the outline object, and should thus be freed when it is destroyed.
FT_OUTLINE_EVEN_ODD_FILL | By default, outlines are filled using the non-zero winding rule. If set to 1, the outline will be filled using the even-odd fill rule (only works with the smooth rasterizer).
FT_OUTLINE_REVERSE_FILL | By default, outside contours of an outline are oriented in clock-wise direction, as defined in the TrueType specification. This flag is set if the outline uses the opposite direction (typically for Type 1 fonts). This flag is ignored by the scan converter.
FT_OUTLINE_IGNORE_DROPOUTS | By default, the scan converter will try to detect drop-outs in an outline and correct the glyph bitmap to ensure consistent shape continuity. If set, this flag hints the scan-line converter to ignore such cases. See below for more information.
FT_OUTLINE_SMART_DROPOUTS | Select smart dropout control. If unset, use simple dropout control. Ignored if `FT_OUTLINE_IGNORE_DROPOUTS` is set. See below for more information.
FT_OUTLINE_INCLUDE_STUBS | If set, turn pixels on for ‘stubs’, otherwise exclude them. Ignored if `FT_OUTLINE_IGNORE_DROPOUTS` is set. See below for more information.
FT_OUTLINE_OVERLAP | [Since 2.10.3] This flag indicates that this outline contains overlapping contours and the anti-aliased renderer should perform oversampling to mitigate possible artifacts. This flag should not be set for well designed glyphs without overlaps because it quadruples the rendering time.
FT_OUTLINE_HIGH_PRECISION | This flag indicates that the scan-line converter should try to convert this outline to bitmaps with the highest possible quality. It is typically set for small character sizes. Note that this is only a hint that might be completely ignored by a given scan-converter.
FT_OUTLINE_SINGLE_PASS | This flag is set to force a given scan-converter to only use a single pass over the outline to render a bitmap glyph image. Normally, it is set for very large character sizes. It is only a hint that might be completely ignored by a given scan-converter.

#### note

The flags `FT_OUTLINE_IGNORE_DROPOUTS`, `FT_OUTLINE_SMART_DROPOUTS`, and `FT_OUTLINE_INCLUDE_STUBS` are ignored by the smooth rasterizer.

There exists a second mechanism to pass the drop-out mode to the B/W rasterizer; see the `tags` field in `FT_Outline`.

Please refer to the description of the ‘SCANTYPE’ instruction in the OpenType specification how simple drop-outs, smart drop-outs, and stubs are defined.