gfxFT2Utils.cpp

/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "gfxFT2FontBase.h"
#include "gfxFT2Utils.h"
#include "mozilla/Likely.h"
#include FT_TRUETYPE_TAGS_H
#include FT_TRUETYPE_TABLES_H
#include <algorithm>

#ifdef HAVE_FONTCONFIG_FCFREETYPE_H
#include <fontconfig/fcfreetype.h>
#endif

#include "prlink.h"

// aScale is intended for a 16.16 x/y_scale of an FT_Size_Metrics
static inline FT_Long
ScaleRoundDesignUnits(FT_Short aDesignMetric, FT_Fixed aScale)
{
    FT_Long fixed26dot6 = FT_MulFix(aDesignMetric, aScale);
    return ROUND_26_6_TO_INT(fixed26dot6);
}

// Snap a line to pixels while keeping the center and size of the line as
// close to the original position as possible.
//
// Pango does similar snapping for underline and strikethrough when fonts are
// hinted, but nsCSSRendering::GetTextDecorationRectInternal always snaps the
// top and size of lines.  Optimizing the distance between the line and
// baseline is probably good for the gap between text and underline, but
// optimizing the center of the line is better for positioning strikethough.
static void
SnapLineToPixels(gfxFloat& aOffset, gfxFloat& aSize)
{
    gfxFloat snappedSize = std::max(floor(aSize + 0.5), 1.0);
    // Correct offset for change in size
    gfxFloat offset = aOffset - 0.5 * (aSize - snappedSize);
    // Snap offset
    aOffset = floor(offset + 0.5);
    aSize = snappedSize;
}

void
gfxFT2LockedFace::GetMetrics(gfxFont::Metrics* aMetrics,
                             uint32_t* aSpaceGlyph)
{
    NS_PRECONDITION(aMetrics != nullptr, "aMetrics must not be NULL");
    NS_PRECONDITION(aSpaceGlyph != nullptr, "aSpaceGlyph must not be NULL");

    if (MOZ_UNLIKELY(!mFace)) {
        // No face.  This unfortunate situation might happen if the font
        // file is (re)moved at the wrong time.
        const gfxFloat emHeight = mGfxFont->GetStyle()->size;
        aMetrics->emHeight = emHeight;
        aMetrics->maxAscent = aMetrics->emAscent = 0.8 * emHeight;
        aMetrics->maxDescent = aMetrics->emDescent = 0.2 * emHeight;
        aMetrics->maxHeight = emHeight;
        aMetrics->internalLeading = 0.0;
        aMetrics->externalLeading = 0.2 * emHeight;
        const gfxFloat spaceWidth = 0.5 * emHeight;
        aMetrics->spaceWidth = spaceWidth;
        aMetrics->maxAdvance = spaceWidth;
        aMetrics->aveCharWidth = spaceWidth;
        aMetrics->zeroOrAveCharWidth = spaceWidth;
        const gfxFloat xHeight = 0.5 * emHeight;
        aMetrics->xHeight = xHeight;
        aMetrics->capHeight = aMetrics->maxAscent;
        const gfxFloat underlineSize = emHeight / 14.0;
        aMetrics->underlineSize = underlineSize;
        aMetrics->underlineOffset = -underlineSize;
        aMetrics->strikeoutOffset = 0.25 * emHeight;
        aMetrics->strikeoutSize = underlineSize;

        *aSpaceGlyph = 0;
        return;
    }

    const FT_Size_Metrics& ftMetrics = mFace->size->metrics;

    gfxFloat emHeight;
    // Scale for vertical design metric conversion: pixels per design unit.
    // If this remains at 0.0, we can't use metrics from OS/2 etc.
    gfxFloat yScale = 0.0;
    if (FT_IS_SCALABLE(mFace)) {
        // Prefer FT_Size_Metrics::x_scale to x_ppem as x_ppem does not
        // have subpixel accuracy.
        //
        // FT_Size_Metrics::y_scale is in 16.16 fixed point format.  Its
        // (fractional) value is a factor that converts vertical metrics from
        // design units to units of 1/64 pixels, so that the result may be
        // interpreted as pixels in 26.6 fixed point format.
        yScale = FLOAT_FROM_26_6(FLOAT_FROM_16_16(ftMetrics.y_scale));
        emHeight = mFace->units_per_EM * yScale;
    } else { // Not scalable.
        emHeight = ftMetrics.y_ppem;
        // FT_Face doc says units_per_EM and a bunch of following fields
        // are "only relevant to scalable outlines". If it's an sfnt,
        // we can get units_per_EM from the 'head' table instead; otherwise,
        // we don't have a unitsPerEm value so we can't compute/use yScale.
        const TT_Header* head =
            static_cast<TT_Header*>(FT_Get_Sfnt_Table(mFace, ft_sfnt_head));
        if (head) {
            gfxFloat emUnit = head->Units_Per_EM;
            yScale = emHeight / emUnit;
        }
    }

    TT_OS2 *os2 =
        static_cast<TT_OS2*>(FT_Get_Sfnt_Table(mFace, ft_sfnt_os2));

    aMetrics->maxAscent = FLOAT_FROM_26_6(ftMetrics.ascender);
    aMetrics->maxDescent = -FLOAT_FROM_26_6(ftMetrics.descender);
    aMetrics->maxAdvance = FLOAT_FROM_26_6(ftMetrics.max_advance);

    gfxFloat lineHeight;
    if (os2 && os2->sTypoAscender && yScale > 0.0) {
        aMetrics->emAscent = os2->sTypoAscender * yScale;
        aMetrics->emDescent = -os2->sTypoDescender * yScale;
        FT_Short typoHeight =
            os2->sTypoAscender - os2->sTypoDescender + os2->sTypoLineGap;
        lineHeight = typoHeight * yScale;

        // If the OS/2 fsSelection USE_TYPO_METRICS bit is set,
        // set maxAscent/Descent from the sTypo* fields instead of hhea.
        const uint16_t kUseTypoMetricsMask = 1 << 7;
        if (os2->fsSelection & kUseTypoMetricsMask) {
            aMetrics->maxAscent = NS_round(aMetrics->emAscent);
            aMetrics->maxDescent = NS_round(aMetrics->emDescent);
        } else {
            // maxAscent/maxDescent get used for frame heights, and some fonts
            // don't have the HHEA table ascent/descent set (bug 279032).
            // We use NS_round here to parallel the pixel-rounded values that
            // freetype gives us for ftMetrics.ascender/descender.
            aMetrics->maxAscent =
                std::max(aMetrics->maxAscent, NS_round(aMetrics->emAscent));
            aMetrics->maxDescent =
                std::max(aMetrics->maxDescent, NS_round(aMetrics->emDescent));
        }
    } else {
        aMetrics->emAscent = aMetrics->maxAscent;
        aMetrics->emDescent = aMetrics->maxDescent;
        lineHeight = FLOAT_FROM_26_6(ftMetrics.height);
    }

    cairo_text_extents_t extents;
    *aSpaceGlyph = GetCharExtents(' ', &extents);
    if (*aSpaceGlyph) {
        aMetrics->spaceWidth = extents.x_advance;
    } else {
        aMetrics->spaceWidth = aMetrics->maxAdvance; // guess
    }

    aMetrics->zeroOrAveCharWidth = 0.0;
    if (GetCharExtents('0', &extents)) {
        aMetrics->zeroOrAveCharWidth = extents.x_advance;
    }

    // Prefering a measured x over sxHeight because sxHeight doesn't consider
    // hinting, but maybe the x extents are not quite right in some fancy
    // script fonts.  CSS 2.1 suggests possibly using the height of an "o",
    // which would have a more consistent glyph across fonts.
    if (GetCharExtents('x', &extents) && extents.y_bearing < 0.0) {
        aMetrics->xHeight = -extents.y_bearing;
        aMetrics->aveCharWidth = extents.x_advance;
    } else {
        if (os2 && os2->sxHeight && yScale > 0.0) {
            aMetrics->xHeight = os2->sxHeight * yScale;
        } else {
            // CSS 2.1, section 4.3.2 Lengths: "In the cases where it is
            // impossible or impractical to determine the x-height, a value of
            // 0.5em should be used."
            aMetrics->xHeight = 0.5 * emHeight;
        }
        aMetrics->aveCharWidth = 0.0; // updated below
    }

    if (GetCharExtents('H', &extents) && extents.y_bearing < 0.0) {
        aMetrics->capHeight = -extents.y_bearing;
    } else {
        if (os2 && os2->sCapHeight && yScale > 0.0) {
            aMetrics->capHeight = os2->sCapHeight * yScale;
        } else {
            aMetrics->capHeight = aMetrics->maxAscent;
        }
    }

    // aveCharWidth is used for the width of text input elements so be
    // liberal rather than conservative in the estimate.
    if (os2 && os2->xAvgCharWidth) {
        // Round to pixels as this is compared with maxAdvance to guess
        // whether this is a fixed width font.
        gfxFloat avgCharWidth =
            ScaleRoundDesignUnits(os2->xAvgCharWidth, ftMetrics.x_scale);
        aMetrics->aveCharWidth =
            std::max(aMetrics->aveCharWidth, avgCharWidth);
    }
    aMetrics->aveCharWidth =
        std::max(aMetrics->aveCharWidth, aMetrics->zeroOrAveCharWidth);
    if (aMetrics->aveCharWidth == 0.0) {
        aMetrics->aveCharWidth = aMetrics->spaceWidth;
    }
    if (aMetrics->zeroOrAveCharWidth == 0.0) {
        aMetrics->zeroOrAveCharWidth = aMetrics->aveCharWidth;
    }
    // Apparently hinting can mean that max_advance is not always accurate.
    aMetrics->maxAdvance =
        std::max(aMetrics->maxAdvance, aMetrics->aveCharWidth);

    // gfxFont::Metrics::underlineOffset is the position of the top of the
    // underline.
    //
    // FT_FaceRec documentation describes underline_position as "the
    // center of the underlining stem".  This was the original definition
    // of the PostScript metric, but in the PostScript table of OpenType
    // fonts the metric is "the top of the underline"
    // (http://www.microsoft.com/typography/otspec/post.htm), and FreeType
    // (up to version 2.3.7) doesn't make any adjustment.
    //
    // Therefore get the underline position directly from the table
    // ourselves when this table exists.  Use FreeType's metrics for
    // other (including older PostScript) fonts.
    if (mFace->underline_position && mFace->underline_thickness && yScale > 0.0) {
        aMetrics->underlineSize = mFace->underline_thickness * yScale;
        TT_Postscript *post = static_cast<TT_Postscript*>
            (FT_Get_Sfnt_Table(mFace, ft_sfnt_post));
        if (post && post->underlinePosition) {
            aMetrics->underlineOffset = post->underlinePosition * yScale;
        } else {
            aMetrics->underlineOffset = mFace->underline_position * yScale
                + 0.5 * aMetrics->underlineSize;
        }
    } else { // No underline info.
        // Imitate Pango.
        aMetrics->underlineSize = emHeight / 14.0;
        aMetrics->underlineOffset = -aMetrics->underlineSize;
    }

    if (os2 && os2->yStrikeoutSize && os2->yStrikeoutPosition && yScale > 0.0) {
        aMetrics->strikeoutSize = os2->yStrikeoutSize * yScale;
        aMetrics->strikeoutOffset = os2->yStrikeoutPosition * yScale;
    } else { // No strikeout info.
        aMetrics->strikeoutSize = aMetrics->underlineSize;
        // Use OpenType spec's suggested position for Roman font.
        aMetrics->strikeoutOffset = emHeight * 409.0 / 2048.0
            + 0.5 * aMetrics->strikeoutSize;
    }
    SnapLineToPixels(aMetrics->strikeoutOffset, aMetrics->strikeoutSize);

    aMetrics->maxHeight = aMetrics->maxAscent + aMetrics->maxDescent;

    // Make the line height an integer number of pixels so that lines will be
    // equally spaced (rather than just being snapped to pixels, some up and
    // some down).  Layout calculates line height from the emHeight +
    // internalLeading + externalLeading, but first each of these is rounded
    // to layout units.  To ensure that the result is an integer number of
    // pixels, round each of the components to pixels.
    aMetrics->emHeight = floor(emHeight + 0.5);

    // maxHeight will normally be an integer, but round anyway in case
    // FreeType is configured differently.
    aMetrics->internalLeading =
        floor(aMetrics->maxHeight - aMetrics->emHeight + 0.5);

    // Text input boxes currently don't work well with lineHeight
    // significantly less than maxHeight (with Verdana, for example).
    lineHeight = floor(std::max(lineHeight, aMetrics->maxHeight) + 0.5);
    aMetrics->externalLeading =
        lineHeight - aMetrics->internalLeading - aMetrics->emHeight;

    // Ensure emAscent + emDescent == emHeight
    gfxFloat sum = aMetrics->emAscent + aMetrics->emDescent;
    aMetrics->emAscent = sum > 0.0 ?
        aMetrics->emAscent * aMetrics->emHeight / sum : 0.0;
    aMetrics->emDescent = aMetrics->emHeight - aMetrics->emAscent;
}

uint32_t
gfxFT2LockedFace::GetGlyph(uint32_t aCharCode)
{
    if (MOZ_UNLIKELY(!mFace))
        return 0;

#ifdef HAVE_FONTCONFIG_FCFREETYPE_H
    // FcFreeTypeCharIndex will search starting from the most recently
    // selected charmap.  This can cause non-determistic behavior when more
    // than one charmap supports a character but with different glyphs, as
    // with older versions of MS Gothic, for example.  Always prefer a Unicode
    // charmap, if there is one.  (FcFreeTypeCharIndex usually does the
    // appropriate Unicode conversion, but some fonts have non-Roman glyphs
    // for FT_ENCODING_APPLE_ROMAN characters.)
    if (!mFace->charmap || mFace->charmap->encoding != FT_ENCODING_UNICODE) {
        FT_Select_Charmap(mFace, FT_ENCODING_UNICODE);
    }

    return FcFreeTypeCharIndex(mFace, aCharCode);
#else
    return FT_Get_Char_Index(mFace, aCharCode);
#endif
}

typedef FT_UInt (*GetCharVariantFunction)(FT_Face  face,
                                          FT_ULong charcode,
                                          FT_ULong variantSelector);

uint32_t
gfxFT2LockedFace::GetUVSGlyph(uint32_t aCharCode, uint32_t aVariantSelector)
{
    NS_PRECONDITION(aVariantSelector, "aVariantSelector should not be NULL");

    if (MOZ_UNLIKELY(!mFace))
        return 0;

    // This function is available from FreeType 2.3.6 (June 2008).
    static CharVariantFunction sGetCharVariantPtr = FindCharVariantFunction();
    if (!sGetCharVariantPtr)
        return 0;

#ifdef HAVE_FONTCONFIG_FCFREETYPE_H
    // FcFreeTypeCharIndex may have changed the selected charmap.
    // FT_Face_GetCharVariantIndex needs a unicode charmap.
    if (!mFace->charmap || mFace->charmap->encoding != FT_ENCODING_UNICODE) {
        FT_Select_Charmap(mFace, FT_ENCODING_UNICODE);
    }
#endif

    return (*sGetCharVariantPtr)(mFace, aCharCode, aVariantSelector);
}

uint32_t
gfxFT2LockedFace::GetCharExtents(char aChar, cairo_text_extents_t* aExtents)
{
    NS_PRECONDITION(aExtents != nullptr, "aExtents must not be NULL");

    if (!mFace)
        return 0;

    FT_UInt gid = mGfxFont->GetGlyph(aChar);
    if (gid) {
        mGfxFont->GetGlyphExtents(gid, aExtents);
    }

    return gid;
}

gfxFT2LockedFace::CharVariantFunction
gfxFT2LockedFace::FindCharVariantFunction()
{
    // This function is available from FreeType 2.3.6 (June 2008).
    PRLibrary *lib = nullptr;
    CharVariantFunction function =
        reinterpret_cast<CharVariantFunction>
        (PR_FindFunctionSymbolAndLibrary("FT_Face_GetCharVariantIndex", &lib));
    if (!lib) {
        return nullptr;
    }

    FT_Int major;
    FT_Int minor;
    FT_Int patch;
    FT_Library_Version(mFace->glyph->library, &major, &minor, &patch);

    // Versions 2.4.0 to 2.4.3 crash if configured with
    // FT_CONFIG_OPTION_OLD_INTERNALS.  Presence of the symbol FT_Alloc
    // indicates FT_CONFIG_OPTION_OLD_INTERNALS.
    if (major == 2 && minor == 4 && patch < 4 &&
        PR_FindFunctionSymbol(lib, "FT_Alloc")) {
        function = nullptr;
    }

    // Decrement the reference count incremented in
    // PR_FindFunctionSymbolAndLibrary.
    PR_UnloadLibrary(lib);

    return function;
}