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freetype/include/freetype/ftoutln.h
Ben Wagner e34a860a07 Document FT_Outline_Decompose degenerate segments. 
`FT_Outline_Decompose` does not filter out and can return degenerate
segments in an outline. This can be surprising when attemping to
stroke such an outline. Clarify the existing documentation on this
matter to cover all forms of degeneracy (without specifying exactly
how they will be reported), why they might arise, and better explain in
what cases they may be an issue.

* include/freetype/ftoutlin.h (FT_Outline_Decompose): update
documentation.

Fixes #952.
2022-01-27 21:49:47 +01:00

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/****************************************************************************
*
* ftoutln.h
*
* Support for the FT_Outline type used to store glyph shapes of
* most scalable font formats (specification).
*
* Copyright (C) 1996-2022 by
* David Turner, Robert Wilhelm, and Werner Lemberg.
*
* This file is part of the FreeType project, and may only be used,
* modified, and distributed under the terms of the FreeType project
* license, LICENSE.TXT. By continuing to use, modify, or distribute
* this file you indicate that you have read the license and
* understand and accept it fully.
*
*/
#ifndef FTOUTLN_H_
#define FTOUTLN_H_
#include <freetype/freetype.h>
#ifdef FREETYPE_H
#error "freetype.h of FreeType 1 has been loaded!"
#error "Please fix the directory search order for header files"
#error "so that freetype.h of FreeType 2 is found first."
#endif
FT_BEGIN_HEADER
/**************************************************************************
*
* @section:
* outline_processing
*
* @title:
* Outline Processing
*
* @abstract:
* Functions to create, transform, and render vectorial glyph images.
*
* @description:
* 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.
*
* @order:
* FT_Outline
* FT_Outline_New
* FT_Outline_Done
* FT_Outline_Copy
* FT_Outline_Translate
* FT_Outline_Transform
* FT_Outline_Embolden
* FT_Outline_EmboldenXY
* FT_Outline_Reverse
* FT_Outline_Check
*
* FT_Outline_Get_CBox
* FT_Outline_Get_BBox
*
* FT_Outline_Get_Bitmap
* FT_Outline_Render
* FT_Outline_Decompose
* FT_Outline_Funcs
* FT_Outline_MoveToFunc
* FT_Outline_LineToFunc
* FT_Outline_ConicToFunc
* FT_Outline_CubicToFunc
*
* FT_Orientation
* FT_Outline_Get_Orientation
*
* FT_OUTLINE_XXX
*
*/
/**************************************************************************
*
* @function:
* FT_Outline_Decompose
*
* @description:
* 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-degenrate 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, this is, not calling any emitter); if necessary, you
* should filter this out, too.
*/
FT_EXPORT( FT_Error )
FT_Outline_Decompose( FT_Outline* outline,
const FT_Outline_Funcs* func_interface,
void* user );
/**************************************************************************
*
* @function:
* FT_Outline_New
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_New( FT_Library library,
FT_UInt numPoints,
FT_Int numContours,
FT_Outline *anoutline );
/**************************************************************************
*
* @function:
* FT_Outline_Done
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_Done( FT_Library library,
FT_Outline* outline );
/**************************************************************************
*
* @function:
* FT_Outline_Check
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_Check( FT_Outline* outline );
/**************************************************************************
*
* @function:
* FT_Outline_Get_CBox
*
* @description:
* 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_EXPORT( void )
FT_Outline_Get_CBox( const FT_Outline* outline,
FT_BBox *acbox );
/**************************************************************************
*
* @function:
* FT_Outline_Translate
*
* @description:
* 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_EXPORT( void )
FT_Outline_Translate( const FT_Outline* outline,
FT_Pos xOffset,
FT_Pos yOffset );
/**************************************************************************
*
* @function:
* FT_Outline_Copy
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_Copy( const FT_Outline* source,
FT_Outline *target );
/**************************************************************************
*
* @function:
* FT_Outline_Transform
*
* @description:
* 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_EXPORT( void )
FT_Outline_Transform( const FT_Outline* outline,
const FT_Matrix* matrix );
/**************************************************************************
*
* @function:
* FT_Outline_Embolden
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_Embolden( FT_Outline* outline,
FT_Pos strength );
/**************************************************************************
*
* @function:
* FT_Outline_EmboldenXY
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_EmboldenXY( FT_Outline* outline,
FT_Pos xstrength,
FT_Pos ystrength );
/**************************************************************************
*
* @function:
* FT_Outline_Reverse
*
* @description:
* 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_EXPORT( void )
FT_Outline_Reverse( FT_Outline* outline );
/**************************************************************************
*
* @function:
* FT_Outline_Get_Bitmap
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_Get_Bitmap( FT_Library library,
FT_Outline* outline,
const FT_Bitmap *abitmap );
/**************************************************************************
*
* @function:
* FT_Outline_Render
*
* @description:
* 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_EXPORT( FT_Error )
FT_Outline_Render( FT_Library library,
FT_Outline* outline,
FT_Raster_Params* params );
/**************************************************************************
*
* @enum:
* FT_Orientation
*
* @description:
* 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.
*
*/
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;
/**************************************************************************
*
* @function:
* FT_Outline_Get_Orientation
*
* @description:
* 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_EXPORT( FT_Orientation )
FT_Outline_Get_Orientation( FT_Outline* outline );
/* */
FT_END_HEADER
#endif /* FTOUTLN_H_ */
/* END */
/* Local Variables: */
/* coding: utf-8 */
/* End: */
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