glfont.c

/* This is a slightly modified
   https://github.com/ccxvii/snippets/blob/master/glfont.c by Tor Andersson
*/
/*
 * A very simple font cache and rasterizer that uses freetype
 * to draw fonts from a single OpenGL texture. The code uses
 * a linear-probe hashtable, and writes new glyphs into
 * the texture using glTexSubImage2D. When the texture fills
 * up, or the hash table gets too crowded, everything is wiped.
 *
 * This is designed to be used for horizontal text only,
 * and draws unhinted text with subpixel accurate metrics
 * and kerning. As such, you should always call the drawing
 * function with an identity transform that maps units
 * to pixels accurately.
 *
 * If you wish to use it to draw arbitrarily transformed
 * text, change the min and mag filters to GL_LINEAR and
 * add a pixel of padding between glyphs and rows, and
 * make sure to clear the texture when wiping the cache.
 */

#include FT_ADVANCES_H
typedef int Rune;               /* 32 bits */

#define PADDING 1               /* set to 0 to save some space but disallow arbitrary transforms */

#define MAXGLYPHS 4093  /* prime number for hash table goodness */
#define CACHESIZE 256
#define XPRECISION 4
#define YPRECISION 1

struct key
{
        FT_Face face;
        short size;
        short gid;
        short subx;
        short suby;
};

struct glyph
{
        signed char lsb, top, w, h;
        short s, t;
        float advance;
};

struct table
{
        struct key key;
        struct glyph glyph;
};

static FT_Library g_freetype_lib = NULL;
static struct table g_table[MAXGLYPHS];
static int g_table_load = 0;
static unsigned int g_cache_tex = 0;
static int g_cache_w = CACHESIZE;
static int g_cache_h = CACHESIZE;
static int g_cache_row_y = 0;
static int g_cache_row_x = 0;
static int g_cache_row_h = 0;
static int g_use_kern = 0;

static void init_font_cache(void)
{
        int code;

        code = FT_Init_FreeType(&g_freetype_lib);
        if (code)
                errx(1, "cannot initialize freetype");

        glGenTextures(1, &g_cache_tex);
        glBindTexture(GL_TEXTURE_2D, g_cache_tex);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, g_cache_w, g_cache_h, 0, GL_ALPHA, GL_UNSIGNED_BYTE, NULL);
}

static void clear_font_cache(void)
{
#if PADDING > 0
        unsigned char *zero = calloc(g_cache_w, g_cache_h);
        if (!zero)
                err(1, "malloc zero (%u bytes failed)", g_cache_w * g_cache_h);
        glBindTexture(GL_TEXTURE_2D, g_cache_tex);
        glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, g_cache_w, g_cache_h, GL_ALPHA, GL_UNSIGNED_BYTE, zero);
        free(zero);
#endif

        memset(g_table, 0, sizeof(g_table));
        g_table_load = 0;

        g_cache_row_y = PADDING;
        g_cache_row_x = PADDING;
        g_cache_row_h = 0;
}

static FT_Face load_font(char *fontname)
{
        FT_Face face;
        int code;

        if (g_freetype_lib == NULL)
        {
                init_font_cache();
                clear_font_cache();
        }

        code = FT_New_Face (g_freetype_lib, fontname, 0, &face);
        if (code) {
                fprintf (stderr, "failed to load font `%s'\n", fontname);
                return NULL;
        }

        FT_Select_Charmap(face, ft_encoding_unicode);
        return face;
}

static FT_Face UNUSED_ATTR load_builtin_font(const void *base, int len)
{
        FT_Face face;
        int code;

        if (g_freetype_lib == NULL)
        {
                init_font_cache();
                clear_font_cache();
        }

        code = FT_New_Memory_Face(g_freetype_lib, base, len, 0, &face);
        if (code) {
                fprintf (stderr, "failed to load builtin font\n");
                return NULL;
        }

        FT_Select_Charmap(face, ft_encoding_unicode);
        return face;
}

static void UNUSED_ATTR free_font(FT_Face face)
{
        clear_font_cache();
        FT_Done_Face(face);
}

static unsigned int hashfunc(struct key *key)
{
        unsigned char *buf = (unsigned char *)key;
        unsigned int len = sizeof(struct key);
        unsigned int h = 0;
        while (len--)
                h = *buf++ + (h << 6) + (h << 16) - h;
        return h;
}

static unsigned int lookup_table(struct key *key)
{
        unsigned int pos = hashfunc(key) % MAXGLYPHS;
        while (1)
        {
                if (!g_table[pos].key.face) /* empty slot */
                        return pos;
                if (!memcmp(key, &g_table[pos].key, sizeof(struct key))) /* matching slot */
                        return pos;
                pos = (pos + 1) % MAXGLYPHS;
        }
}

static struct glyph * lookup_glyph(FT_Face face, int size, int gid, int subx, int suby)
{
        FT_Vector subv;
        struct key key;
        unsigned int pos;
        int code;
        int w, h;

        /*
         * Look it up in the table
         */

        key.face = face;
        key.size = size;
        key.gid = gid;
        key.subx = subx;
        key.suby = suby;

        pos = lookup_table(&key);
        if (g_table[pos].key.face)
                return &g_table[pos].glyph;

        /*
         * Render the bitmap
         */
#ifdef FFP
        glEnd();
#endif

        subv.x = subx;
        subv.y = suby;

        FT_Set_Transform(face, NULL, &subv);

        code = FT_Load_Glyph(face, gid, FT_LOAD_NO_BITMAP | FT_LOAD_NO_HINTING);
        if (code < 0)
                return NULL;

        code = FT_Render_Glyph(face->glyph, FT_RENDER_MODE_LIGHT);
        if (code < 0)
                return NULL;

        w = face->glyph->bitmap.width;
        h = face->glyph->bitmap.rows;

        /*
         * Find an empty slot in the texture
         */

        if (g_table_load == (MAXGLYPHS * 3) / 4)
        {
                lprintf("font cache table full, clearing cache");
                clear_font_cache();
                pos = lookup_table(&key);
        }

        if (h + PADDING > g_cache_h || w + PADDING > g_cache_w)
                errx(1, "rendered glyph exceeds cache dimensions");

        if (g_cache_row_x + w + PADDING > g_cache_w)
        {
                g_cache_row_y += g_cache_row_h + PADDING;
                g_cache_row_x = PADDING;
                g_cache_row_h = 0;
        }
        if (g_cache_row_y + h + PADDING > g_cache_h)
        {
                lprintf("font cache texture full, clearing cache");
                clear_font_cache();
                pos = lookup_table(&key);
        }

        /*
         * Copy bitmap into texture
         */

        memcpy(&g_table[pos].key, &key, sizeof(struct key));
        g_table[pos].glyph.w = face->glyph->bitmap.width;
        g_table[pos].glyph.h = face->glyph->bitmap.rows;
        g_table[pos].glyph.lsb = face->glyph->bitmap_left;
        g_table[pos].glyph.top = face->glyph->bitmap_top;
        g_table[pos].glyph.s = g_cache_row_x;
        g_table[pos].glyph.t = g_cache_row_y;
        g_table[pos].glyph.advance = face->glyph->advance.x / 64.0;
        g_table_load ++;

        glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
        glPixelStorei(GL_UNPACK_ROW_LENGTH, face->glyph->bitmap.pitch);
        glTexSubImage2D(GL_TEXTURE_2D, 0, g_cache_row_x, g_cache_row_y, w, h,
                        GL_ALPHA, GL_UNSIGNED_BYTE, face->glyph->bitmap.buffer);
        glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);

#ifdef FFP
        glBegin(GL_QUADS);
#endif
        g_cache_row_x += w + PADDING;
        if (g_cache_row_h < h + PADDING)
                g_cache_row_h = h + PADDING;

        return &g_table[pos].glyph;
}

static float draw_glyph(FT_Face face, int size, int gid, float x, float y)
{
        struct glyph *glyph;
        int subx = (x - floor(x)) * XPRECISION;
        int suby = (y - floor(y)) * YPRECISION;
#ifndef FFP
        GLfloat *t = state.texcoords;
        GLfloat *v = state.vertices;
#endif
        float s0, t0, s1, t1, xc, yc;

        subx = (subx * 64) / XPRECISION;
        suby = (suby * 64) / YPRECISION;

        glyph = lookup_glyph(face, size, gid, subx, suby);
        if (!glyph)
                return 0.0;

        s0 = (float) glyph->s / g_cache_w;
        t0 = (float) glyph->t / g_cache_h;
        s1 = (float) (glyph->s + glyph->w) / g_cache_w;
        t1 = (float) (glyph->t + glyph->h) / g_cache_h;
        xc = floor(x) + glyph->lsb;
        yc = floor(y) - glyph->top + glyph->h;

#ifndef FFP
        t[0] = s0; t[1] = t0; v[0] = xc;            v[1] = yc - glyph->h;
        t[2] = s1; t[3] = t0; v[2] = xc + glyph->w; v[3] = yc - glyph->h;
        t[4] = s0; t[5] = t1; v[4] = xc;            v[5] = yc;
        t[6] = s1; t[7] = t1; v[6] = xc + glyph->w; v[7] = yc;

        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
#else
        glTexCoord2f(s0, t0); glVertex2f(xc, yc - glyph->h);
        glTexCoord2f(s1, t0); glVertex2f(xc + glyph->w, yc - glyph->h);
        glTexCoord2f(s1, t1); glVertex2f(xc + glyph->w, yc);
        glTexCoord2f(s0, t1); glVertex2f(xc, yc);
#endif

        return glyph->advance;
}

static float measure_string(FT_Face face, float fsize, char *str)
{
        int size = fsize * 64;
        FT_Fixed advance;
        Rune ucs, gid;
        float w = 0.0;
        int left = 0;

        FT_Set_Char_Size(face, size, size, 72, 72);

        while (*str)
        {
                str += fz_chartorune(&ucs, str);
                gid = FT_Get_Char_Index(face, ucs);
                FT_Get_Advance(face, gid, FT_LOAD_NO_BITMAP | FT_LOAD_NO_HINTING, &advance);
                w += advance / 65536.0;
                if (g_use_kern) {
                    FT_Vector kern;

                    FT_Get_Kerning(face, left, gid, FT_KERNING_UNFITTED, &kern);
                    w += kern.x / 64.0;
                }
                left = gid;
        }

        return w;
}

static float draw_string(FT_Face face, float fsize, float x, float y, char *str)
{
        int size = fsize * 64;
        Rune ucs, gid;
        int left = 0;

        FT_Set_Char_Size(face, size, size, 72, 72);

        glBindTexture(GL_TEXTURE_2D, g_cache_tex);
#ifdef FFP
        glBegin(GL_QUADS);
#else
        glVertexPointer(2, GL_FLOAT, 0, state.vertices);
        glTexCoordPointer(2, GL_FLOAT, 0, state.texcoords);
#endif
        while (*str)
        {
                str += fz_chartorune(&ucs, str);
                gid = FT_Get_Char_Index(face, ucs);
                x += draw_glyph(face, size, gid, x, y);
                if (g_use_kern) {
                    FT_Vector kern;

                    FT_Get_Kerning(face, left, gid, FT_KERNING_UNFITTED, &kern);
                    x += kern.x / 64.0;
                }
                left = gid;
        }

#ifdef FFP
        glEnd();
#endif

        return x;
}