mkdir build
cd build
cmake -G "Unix Makefiles" ..
make
make test

The basic model for running graphite is to pass text, font and face information to create a segment. A segment consists of a linked list of slots which each correspond to an output glyph. In addition a segment holds charinfo for each character in the input text.

#include <graphite2/Segment.h>
#include <stdio.h>

/* usage: ./simple fontfile.ttf string */
int main(int argc, char **argv)
{
    int rtl = 0;                /* are we rendering right to left? probably not */
    int pointsize = 12;         /* point size in points */
    int dpi = 96;               /* work with this many dots per inch */

    char *pError;               /* location of faulty utf-8 */
    gr_font *font = NULL;
    size_t numCodePoints = 0;
    gr_segment * seg = NULL;
    const gr_slot *s;
    gr_face *face = gr_make_file_face(argv[1], 0);                              /**/
    if (!face) return 1;
    font = gr_make_font(pointsize * dpi / 72.0f, face);                         /**/
    if (!font) return 2;
    numCodePoints = gr_count_unicode_characters(gr_utf8, argv[2], NULL,
                (const void **)(&pError));                                      /**/
    if (pError) return 3;
    seg = gr_make_seg(font, face, 0, 0, gr_utf8, argv[2], numCodePoints, rtl);  /**/
    if (!seg) return 3;

    for (s = gr_seg_first_slot(seg); s; s = gr_slot_next_in_segment(s))         /**/
        printf("%d(%f,%f) ", gr_slot_gid(s), gr_slot_origin_X(s), gr_slot_origin_Y(s));
    gr_seg_destroy(seg);
    gr_font_destroy(font);
    gr_face_destroy(face);
    return 0;
}

Source for this program may be found in tests/examples/simple.c

Assuming that graphite2 has been built and installed, this example can be built and run on linux using:

gcc -o simple -lgraphiteng simple.c
LD_LIBRARY_PATH=/usr/local/lib ./simple ../fonts/Padauk.ttf 'Hello World!'

Running simple gives the results:

43(0.000000,0.000000) 72(9.859375,0.000000) 79(17.609375,0.000000) 79(20.796875,0.000000) 82(23.984375,0.000000) 3(32.203125,0.000000) 58(38.109375,0.000000) 82(51.625000,0.000000) 85(59.843750,0.000000) 79(64.875000,0.000000) 71(68.062500,0.000000) 4(76.281250,0.000000)

Not very pretty, but reassuring! Graphite isn’t a graphical rendering engine, it merely calculates which glyphs should render where and leaves the actual process of displaying those glyphs to other libraries.

The primary contents of a segment is slots. These slots are organised into a doubly linked list and each corresponds to a glyph to be rendered. The linked list is terminated at each end by a NULL. There are also functions to get the first and last slot in a segment.

In addition to the main slot list, slots may be attached to each other. This means that two glyphs have been attached to each other in the GDL. Again, attached slots are held in a separate singly linked list associated with the slot to which they attach. Thus slots will be in the main linked list and may be in an attachment linked list. Each slot in an attachment linked list has the same attachment parent accessed via gr_slot_attached_to(). To get the start of the linked list of all the slots directly attached to a parent, one calls gr_slot_first_attachment() and then gr_slot_next_attachment() to walk forwards through that linked list. Given that a diacritic may attach to another diacritic, an attached slot may in its turn have a linked list of attached slots. In all cases, linked lists terminate with a NULL.

The core information held by a slot is the glyph id of the glyph the slot corresponds to (gr_slot_gid()); the position relative to the start of the segment that the glyph is to be rendered at (gr_slot_origin_X() and gr_slot_origin_Y()); the advance for the glyph which corresponds to the glyph metric advance as adjusted by kerning. In addition a slot indicates whether the font designer wants to allow a cursor to be placed before this glyph or not. This information is accessible via gr_slot_can_insert_before().

For each unicode character in the input, there is a CharInfo structure that can be queried for such information as the code unit position in the input string, the before slot index (if we are before this character, which is the earliest slot we are before) and the corresponding after slot index.

The gr_face type is the memory correspondance of a font. It holds the data structures corresponding to those in a font file as required to process text using that font. In creating a gr_face it is necessary to pass a function by which graphite can get hold of font tables. The tables that graphite queries for must be available for the lifetime of the gr_face, except when a gr_face is created with the faceOptions of gr_face_preloadAll. This then loads everything from the font at gr_face construction time, leaving nothing further to be read from the font when the gr_face is used. This reduces the required lifetime of the in memory font tables to the gr_make_face call. In situations where the tables are only stored for the purposes of creating a gr_face, it can save memory to preload everything and delete the tables.

Graphite2 has the capability to make use of a subsegmental cache. What this does is to chop each run of characters at a word break, as defined by the linebreaking pass. Each sub run is then looked up in the cache rather than calculating the values from scratch. The cache is most effective when similar runs of text are processed. For raw benchmark testing against wordlists, the cache can be slightly slower than uncached processing. But most people use real text in their documents and that has a much higher level of redundancy.

To use the cache, one simply creates a cached face, specifying the size of the cache in elements. A cache size of 5,000 to 10,000 has produced a good compromise between time and space.

In the example above, point 1 becomes:

    gr_face *face = make_file_face_with_seg_cache(argv[1]);

It is common for applications to work with simplified clusters, these are sequences of glyphs associated with a sequence of characters, such that these simplified clusters are as small as possible and are never reordered or split in relation to each other. In addition, a cursor may occur between simplified clusters.

The following code gives an example algorithm for calculating such clusters:

#include <graphite2/Segment.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct cluster_t {
    unsigned int base_char;
    unsigned int num_chars;
    unsigned int base_glyph;
    unsigned int num_glyphs;
} cluster_t;

/* usage: ./cluster fontfile.ttf string */
int main(int argc, char **argv)
{
    int rtl = 0;                /* are we rendering right to left? probably not */
    int pointsize = 12;         /* point size in points */
    int dpi = 96;               /* work with this many dots per inch */

    char *pError;               /* location of faulty utf-8 */
    gr_font *font = NULL;
    size_t numCodePoints = 0;
    gr_segment * seg = NULL;
    cluster_t *clusters;
    int ic, ci = 0;
    const gr_slot *s, *is;
    FILE *log;
    gr_face *face = gr_make_file_face(argv[1], 0);
    if (!face) return 1;
    font = gr_make_font(pointsize * dpi / 72.0f, face);
    if (!font) return 2;
    numCodePoints = gr_count_unicode_characters(gr_utf8, argv[2], NULL,
                (const void **)(&pError));
    if (pError || !numCodePoints) return 3;
    seg = gr_make_seg(font, face, 0, 0, gr_utf8, argv[2], numCodePoints, rtl);      /**/
    if (!seg) return 3;

    clusters = (cluster_t *)malloc(numCodePoints * sizeof(cluster_t));
    memset(clusters, 0, numCodePoints * sizeof(cluster_t));
    for (is = gr_seg_first_slot(seg), ic = 0; is; is = gr_slot_next_in_segment(is), ic++)
    {
        unsigned int before = gr_slot_before(is);
        unsigned int after = gr_slot_after(is);
        while (clusters[ci].base_char > before && ci)                               /**/
        {
            clusters[ci-1].num_chars += clusters[ci].num_chars;
            clusters[ci-1].num_glyphs += clusters[ci].num_glyphs;
            --ci;
        }

        if (gr_slot_can_insert_before(is) && clusters[ci].num_chars
                && before >= clusters[ci].base_char + clusters[ci].num_chars)       /**/
        {
            cluster_t *c = clusters + ci + 1;
            c->base_char = clusters[ci].base_char + clusters[ci].num_chars;
            c->num_chars = before - c->base_char;
            c->base_glyph = ic;
            c->num_glyphs = 0;
            ++ci;
        }
        ++clusters[ci].num_glyphs;

        if (clusters[ci].base_char + clusters[ci].num_chars < after + 1)            /**/
            clusters[ci].num_chars = after + 1 - clusters[ci].base_char;
    }

    ci = 0;
    log = fopen("cluster.log", "w");
    for (s = gr_seg_first_slot(seg); s; s = gr_slot_next_in_segment(s))
    {
        fprintf(log, "%d(%f,%f) ", gr_slot_gid(s), gr_slot_origin_X(s),
                                   gr_slot_origin_Y(s));
        if (--clusters[ci].num_glyphs == 0)                                         /**/
        {
            fprintf(log, "\n");
            ++ci;
        }
    }
    fclose(log);
    free(clusters);
    gr_seg_destroy(seg);
    gr_font_destroy(font);
    gr_face_destroy(face);
    return 0;
}

Whilst most applications will convert glyphs and positions out of the gr_slot structure into some internal structure, if graphite is to be used for justification, then it is necessary to line break the text and justify it within graphite’s data structures. Graphite provides two functions to help with this. The first is gr_slot_linebreak_before() which will chop the slot linked list before a given slot. The application needs to keep track of the start of each of the subsequent linked lists itself, since graphite does not do that. After line breaking, the application may call gr_seg_justify() on each line linked list. The following example shows how this might be done in an application.

Notice that this example does not take into considering whitespace hanging outside the right margin.

#include <graphite2/Segment.h>
#include <stdio.h>
#include <stdlib.h>

/* usage: ./linebreak fontfile.ttf width string */
int main(int argc, char **argv)
{
    int rtl = 0;                /* are we rendering right to left? probably not */
    int pointsize = 12;         /* point size in points */
    int dpi = 96;               /* work with this many dots per inch */
    int width = atoi(argv[2]) * dpi / 72;  /* linewidth in points */

    char *pError;               /* location of faulty utf-8 */
    gr_font *font = NULL;
    size_t numCodePoints = 0;
    gr_segment * seg = NULL;
    const gr_slot *s, *sprev;
    int i;
    int lineend = width;
    int numlines = 0;
    const gr_slot **lineslots;
    gr_face *face = gr_make_file_face(argv[1], 0);
    if (!face) return 1;
    font = gr_make_font(pointsize * dpi / 72.0f, face);
    if (!font) return 2;
    numCodePoints = gr_count_unicode_characters(gr_utf8, argv[3], NULL,
                (const void **)(&pError));
    if (pError) return 3;
    seg = gr_make_seg(font, face, 0, 0, gr_utf8, argv[3], numCodePoints, rtl);  /**/
    if (!seg) return 3;

    lineslots = (const gr_slot **)malloc(numCodePoints * sizeof(gr_slot *));
    lineslots[numlines++] = gr_seg_first_slot(seg);                             /**/
    for (s = lineslots[0]; s; s = gr_slot_next_in_segment(s))
    {
        sprev = NULL;
        if (gr_slot_origin_X(s) > lineend)                                      /**/
        {
            for (sprev = gr_slot_prev_in_segment(s); sprev;                     /**/
                                    s = sprev, sprev = gr_slot_prev_in_segment(sprev))
            {
                int bw = gr_cinfo_break_weight(gr_seg_cinfo(seg, gr_slot_before(s)));
                if (bw < -15)                                                   /**/
                    continue;
                bw  = gr_cinfo_break_weight(gr_seg_cinfo(seg, gr_slot_after(sprev)));
                if (bw > 15)
                    continue;
                break;
            }
            lineslots[numlines++] = s;
            gr_slot_linebreak_before((gr_slot *)s);                             /**/
            lineend = gr_slot_origin_X(s) + width;                              /**/
        }
    }

    printf("%d:", width);
    for (i = 0; i < numlines; i++)
    {
        gr_seg_justify(seg, (gr_slot *)lineslots[i], font, width, 0, NULL, NULL); /**/
        for (s = lineslots[i]; s; s = gr_slot_next_in_segment(s))               /**/
            printf("%d(%.2f,%.2f@%d) ", gr_slot_gid(s), gr_slot_origin_X(s), gr_slot_origin_Y(s), gr_slot_attr(s, seg, gr_slatJWidth, 0));
        printf("\n");
    }
    free(lineslots);
    gr_font_destroy(font);
    gr_face_destroy(face);
    return 0;
}

Bidirectional processing is complex not so much because of any algorithms involved, but because of the tendency for applications to address bidi text processing differently. Some try to do everything themselves, inverting the text order, etc. While others do nothing, expecting the shaper to resolve all the orders. In addition, there is the question of mirroring characters and where that is done. Graphite2 adds the complexity that it tries to enable extensions to the bidi algorithm by giving PUA characters directionality. To facilitate all these different ways of working, Graphite2 uses the rtl attribute to pass various bits to control bidi processing within the Graphite engine.

To compile the graphite2 library for integration into another application framework, there are some useful utilities and also various definitions that need to be defined. The file src/files.mk will create three make variables containing lists of files that someone will want to build in order to build graphite2 from source as part of an application build. The variable names are controlled by setting _NS to a prefix that is then applied to the variable names _SOURCES, _PRIVATE_HEADERS and _PUBLIC_HEADERS. All files are listed relative to the variable _BASE (with its _NS expansion prefix). The _MACHINE variable with its _NS expansion prefix, must be set to call or direct depending on which kind of virtual machine to use inside the engine. gcc supports direct call, while all other compilers without a computed goto should use the call style virtual machine. See src/direct_machine.cpp and src/call_machine.cpp for details.

Various C preprocessor definitions are also used as part of compiling graphite2:

While GraphiteNG is written in C++, it is targetted at environments where libstdc++ is not present. While this can be problematic since functions used in C++ may be arbitrarily placed in libc and libstdc++, there are general approaches that can help. To this end we use a mixed memory allocation model. For graphiteng classes, we declare our own new() methods and friends, allowing the use of C++ new() and all that it gives us in terms of constructors. For types that are not classes we use malloc() or the type safe version gralloc().

There are various facilities that silgraphite provides that graphite2 as yet does not. The primary motivation in developing graphite2 is that it be use case driven. Thus only those facilities that have a proven use case in the target applications into which graphite is to be integrated will be implemented.

Graphite2 is written based on experience of using SilGraphite. SilGraphite was written primarily to be feature complete in terms of all features that may be needed. Graphite2 takes the experience of using SilGraphite and starts from a use case requirement before a feature is added to the engine. Thus a number of features that have not been used in SilGraphite have been removed, although the design is such that they can be replaced in future.

In addition, a number of techniques were used to speed up the engine. Some of these are discussed here.