337 lines
10 KiB
C
337 lines
10 KiB
C
/****************************************************************************
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*
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* otsvg.h
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*
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* Interface for OT-SVG support related things (specification).
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*
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* Copyright (C) 2022 by
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* David Turner, Robert Wilhelm, Werner Lemberg, and Moazin Khatti.
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*
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* This file is part of the FreeType project, and may only be used,
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* modified, and distributed under the terms of the FreeType project
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* license, LICENSE.TXT. By continuing to use, modify, or distribute
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* this file you indicate that you have read the license and
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* understand and accept it fully.
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*
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*/
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#ifndef OTSVG_H_
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#define OTSVG_H_
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#include <freetype/freetype.h>
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#ifdef FREETYPE_H
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#error "freetype.h of FreeType 1 has been loaded!"
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#error "Please fix the directory search order for header files"
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#error "so that freetype.h of FreeType 2 is found first."
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#endif
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FT_BEGIN_HEADER
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/**************************************************************************
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*
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* @section:
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* svg_fonts
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*
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* @title:
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* OpenType SVG Fonts
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*
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* @abstract:
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* OT-SVG API between FreeType and an external SVG rendering library.
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*
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* @description:
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* This section describes the four hooks necessary to render SVG
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* 'documents' that are contained in an OpenType font's 'SVG~' table.
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*
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* For more information on the implementation, see our standard hooks
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* based on 'librsvg' in the [FreeType Demo
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* Programs](https://gitlab.freedesktop.org/freetype/freetype-demos)
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* repository.
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*
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*/
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/**************************************************************************
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*
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* @functype:
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* SVG_Lib_Init_Func
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*
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* @description:
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* A callback that is called when the first OT-SVG glyph is rendered in
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* the lifetime of an @FT_Library object. In a typical implementation,
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* one would want to allocate a structure and point the `data_pointer`
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* to it and perform any library initializations that might be needed.
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*
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* @inout:
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* data_pointer ::
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* The SVG rendering module stores a pointer variable that can be used
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* by clients to store any data that needs to be shared across
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* different hooks. `data_pointer` is essentially a pointer to that
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* pointer such that it can be written to as well as read from.
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*
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* @return:
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* FreeType error code. 0 means success.
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*
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* @since:
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* 2.12
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*/
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typedef FT_Error
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(*SVG_Lib_Init_Func)( FT_Pointer *data_pointer );
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/**************************************************************************
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*
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* @functype:
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* SVG_Lib_Free_Func
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*
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* @description:
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* A callback that is called when the `ot-svg` module is being freed.
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* It is only called if the init hook was called earlier. This means
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* that neither the init nor the free hook is called if no OT-SVG glyph
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* is rendered.
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*
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* In a typical implementation, one would want to free any state
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* structure that was allocated in the init hook and perform any
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* library-related closure that might be needed.
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*
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* @inout:
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* data_pointer ::
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* The SVG rendering module stores a pointer variable that can be used
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* by clients to store any data that needs to be shared across
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* different hooks. `data_pointer` is essentially a pointer to that
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* pointer such that it can be written to as well as read from.
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*
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* @since:
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* 2.12
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*/
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typedef void
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(*SVG_Lib_Free_Func)( FT_Pointer *data_pointer );
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/**************************************************************************
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*
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* @functype:
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* SVG_Lib_Render_Func
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*
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* @description:
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* A callback that is called to render an OT-SVG glyph. This callback
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* hook is called right after the preset hook @SVG_Lib_Preset_Slot_Func
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* has been called with `cache` set to `TRUE`. The data necessary to
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* render is available through the handle @FT_SVG_Document, which is set
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* in the `other` field of @FT_GlyphSlotRec.
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*
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* The render hook is expected to render the SVG glyph to the bitmap
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* buffer that is allocated already at `slot->bitmap.buffer`. It also
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* sets the `num_grays` value as well as `slot->format`.
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*
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* @input:
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* slot ::
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* The slot to render.
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*
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* @inout:
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* data_pointer ::
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* The SVG rendering module stores a pointer variable that can be used
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* by clients to store any data that needs to be shared across
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* different hooks. `data_pointer` is essentially a pointer to that
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* pointer such that it can be written to as well as read from.
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*
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* @return:
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* FreeType error code. 0 means success.
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*
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* @since:
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* 2.12
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*/
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typedef FT_Error
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(*SVG_Lib_Render_Func)( FT_GlyphSlot slot,
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FT_Pointer *data_pointer );
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/**************************************************************************
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*
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* @functype:
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* SVG_Lib_Preset_Slot_Func
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*
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* @description:
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* A callback that is called to preset the glyph slot. It is called from
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* two places.
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*
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* 1. When `FT_Load_Glyph` needs to preset the glyph slot.
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*
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* 2. Right before the `svg` module calls the render callback hook.
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*
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* When it is the former, the argument `cache` is set to `FALSE`. When
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* it is the latter, the argument `cache` is set to `TRUE`. This
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* distinction has been made because many calculations that are necessary
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* for presetting a glyph slot are the same needed later for the render
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* callback hook. Thus, if `cache` is `TRUE`, the hook can _cache_ those
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* calculations in a memory block referenced by the state pointer.
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*
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* This hook is expected to preset the slot by setting parameters such as
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* `bitmap_left`, `bitmap_top`, `width`, `rows`, `pitch`, and
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* `pixel_mode`. It is also expected to set all the metrics for the slot
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* including the vertical advance if it is not already set. Typically,
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* fonts have horizontal advances but not vertical ones. If those are
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* available, they had already been set, otherwise they have to be
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* estimated and set manually. The hook must take into account the
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* transformations that have been set, and translate the transformation
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* matrices into the SVG coordinate system, as the original matrix is
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* intended for the TTF/CFF coordinate system.
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*
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* @input:
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* slot ::
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* The glyph slot that has the SVG document loaded.
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*
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* cache ::
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* See description.
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*
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* @inout:
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* data_pointer ::
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* The SVG rendering module stores a pointer variable that can be used
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* by clients to store any data that needs to be shared across
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* different hooks. `data_pointer` is essentially a pointer to that
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* pointer such that it can be written to as well as read from.
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*
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* @return:
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* FreeType error code. 0 means success.
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*
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* @since:
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* 2.12
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*/
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typedef FT_Error
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(*SVG_Lib_Preset_Slot_Func)( FT_GlyphSlot slot,
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FT_Bool cache,
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FT_Pointer *state );
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/**************************************************************************
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*
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* @struct:
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* SVG_RendererHooks
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*
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* @description:
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* A structure that stores the four hooks needed to render OT-SVG glyphs
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* properly. The structure is publicly used to set the hooks via the
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* @svg-hooks driver property.
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*
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* The behavior of each hook is described in its documentation. One
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* thing to note is that the preset hook and the render hook often need
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* to do the same operations; therefore, it's better to cache the
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* intermediate data in a state structure to avoid calculating it twice.
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* For example, in the preset hook one can draw the glyph on a recorder
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* surface and later create a bitmap surface from it in the render hook.
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*
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* All four hooks must be non-NULL.
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*
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* @fields:
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* init_svg ::
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* The initialization hook.
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*
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* free_svg ::
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* The cleanup hook.
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*
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* render_hook ::
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* The render hook.
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*
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* preset_slot ::
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* The preset hook.
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*
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* @since:
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* 2.12
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*/
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typedef struct SVG_RendererHooks_
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{
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SVG_Lib_Init_Func init_svg;
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SVG_Lib_Free_Func free_svg;
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SVG_Lib_Render_Func render_svg;
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SVG_Lib_Preset_Slot_Func preset_slot;
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} SVG_RendererHooks;
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/**************************************************************************
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*
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* @struct:
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* FT_SVG_DocumentRec
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*
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* @description:
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* A structure that models one SVG document.
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*
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* @fields:
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* svg_document ::
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* A pointer to the SVG document.
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*
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* svg_document_length ::
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* The length of `svg_document`.
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*
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* metrics ::
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* A metrics object storing the size information.
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*
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* units_per_EM ::
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* The size of the EM square.
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*
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* start_glyph_id ::
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* The first glyph ID in the glyph range covered by this document.
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*
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* end_glyph_id ::
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* The last glyph ID in the glyph range covered by this document.
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*
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* transform ::
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* A 2x2 transformation matrix to apply to the glyph while rendering
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* it.
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*
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* delta ::
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* The translation to apply to the glyph while rendering.
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*
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* @note:
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* When an @FT_GlyphSlot object `slot` is passed down to a renderer, the
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* renderer can only access the `metrics` and `units_per_EM` fields via
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* `slot->face`. However, when @FT_Glyph_To_Bitmap sets up a dummy
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* object, it has no way to set a `face` object. Thus, metrics
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* information and `units_per_EM` (which is necessary for OT-SVG) has to
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* be stored separately.
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*
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* @since:
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* 2.12
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*/
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typedef struct FT_SVG_DocumentRec_
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{
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FT_Byte* svg_document;
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FT_ULong svg_document_length;
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FT_Size_Metrics metrics;
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FT_UShort units_per_EM;
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FT_UShort start_glyph_id;
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FT_UShort end_glyph_id;
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FT_Matrix transform;
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FT_Vector delta;
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} FT_SVG_DocumentRec;
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/**************************************************************************
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*
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* @type:
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* FT_SVG_Document
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*
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* @description:
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* A handle to an @FT_SVG_DocumentRec object.
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*
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* @since:
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* 2.12
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*/
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typedef struct FT_SVG_DocumentRec_* FT_SVG_Document;
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FT_END_HEADER
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#endif /* OTSVG_H_ */
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/* END */
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