gecko-dev/gfx/thebes/gfxFT2FontBase.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 "harfbuzz/hb.h"
#include "mozilla/Likely.h"
#include "gfxFontConstants.h"
#include "gfxFontUtils.h"
#include <algorithm>
#include <dlfcn.h>

#include FT_TRUETYPE_TAGS_H
#include FT_TRUETYPE_TABLES_H
#include FT_ADVANCES_H
#include FT_MULTIPLE_MASTERS_H

#ifndef FT_FACE_FLAG_COLOR
#define FT_FACE_FLAG_COLOR ( 1L << 14 )
#endif

using namespace mozilla::gfx;

gfxFT2FontBase::gfxFT2FontBase(const RefPtr<UnscaledFontFreeType>& aUnscaledFont,
                               cairo_scaled_font_t *aScaledFont,
                               gfxFontEntry *aFontEntry,
                               const gfxFontStyle *aFontStyle,
                               bool aEmbolden)
    : gfxFont(aUnscaledFont, aFontEntry, aFontStyle, kAntialiasDefault, aScaledFont)
    , mSpaceGlyph(0)
    , mEmbolden(aEmbolden)
{
    cairo_scaled_font_reference(mScaledFont);

    InitMetrics();
}

gfxFT2FontBase::~gfxFT2FontBase()
{
    cairo_scaled_font_destroy(mScaledFont);
}

uint32_t
gfxFT2FontBase::GetGlyph(uint32_t aCharCode)
{
    // FcFreeTypeCharIndex needs to lock the FT_Face and can end up searching
    // through all the postscript glyph names in the font.  Therefore use a
    // lightweight cache, which is stored on the cairo_font_face_t.

    cairo_font_face_t *face =
        cairo_scaled_font_get_font_face(GetCairoScaledFont());

    if (cairo_font_face_status(face) != CAIRO_STATUS_SUCCESS)
        return 0;

    // This cache algorithm and size is based on what is done in
    // cairo_scaled_font_text_to_glyphs and pango_fc_font_real_get_glyph.  I
    // think the concept is that adjacent characters probably come mostly from
    // one Unicode block.  This assumption is probably not so valid with
    // scripts with large character sets as used for East Asian languages.

    struct CmapCacheSlot {
        uint32_t mCharCode;
        uint32_t mGlyphIndex;
    };
    const uint32_t kNumSlots = 256;
    static cairo_user_data_key_t sCmapCacheKey;

    CmapCacheSlot *slots = static_cast<CmapCacheSlot*>
        (cairo_font_face_get_user_data(face, &sCmapCacheKey));

    if (!slots) {
        // cairo's caches can keep some cairo_font_faces alive past our last
        // destroy, so the destroy function (free) for the cache must be
        // callable from cairo without any assumptions about what other
        // modules have not been shutdown.
        slots = static_cast<CmapCacheSlot*>
            (calloc(kNumSlots, sizeof(CmapCacheSlot)));
        if (!slots)
            return 0;

        cairo_status_t status =
            cairo_font_face_set_user_data(face, &sCmapCacheKey, slots, free);
        if (status != CAIRO_STATUS_SUCCESS) { // OOM
            free(slots);
            return 0;
        }

        // Invalidate slot 0 by setting its char code to something that would
        // never end up in slot 0.  All other slots are already invalid
        // because they have mCharCode = 0 and a glyph for char code 0 will
        // always be in the slot 0.
        slots[0].mCharCode = 1;
    }

    CmapCacheSlot *slot = &slots[aCharCode % kNumSlots];
    if (slot->mCharCode != aCharCode) {
        slot->mCharCode = aCharCode;
        slot->mGlyphIndex = gfxFT2LockedFace(this).GetGlyph(aCharCode);
    }

    return slot->mGlyphIndex;
}

void
gfxFT2FontBase::GetGlyphExtents(uint32_t aGlyph, cairo_text_extents_t* aExtents)
{
    NS_PRECONDITION(aExtents != nullptr, "aExtents must not be NULL");

    cairo_glyph_t glyphs[1];
    glyphs[0].index = aGlyph;
    glyphs[0].x = 0.0;
    glyphs[0].y = 0.0;
    // cairo does some caching for us here but perhaps a small gain could be
    // made by caching more.  It is usually only the advance that is needed,
    // so caching only the advance could allow many requests to be cached with
    // little memory use.  Ideally this cache would be merged with
    // gfxGlyphExtents.
    cairo_scaled_font_glyph_extents(GetCairoScaledFont(), glyphs, 1, aExtents);
}

// 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;
}

/**
 * Get extents for a simple character representable by a single glyph.
 * The return value is the glyph id of that glyph or zero if no such glyph
 * exists.  aExtents is only set when this returns a non-zero glyph id.
 */
uint32_t
gfxFT2FontBase::GetCharExtents(char aChar, cairo_text_extents_t* aExtents)
{
    FT_UInt gid = GetGlyph(aChar);
    if (gid) {
        GetGlyphExtents(gid, aExtents);
    }
    return gid;
}

void
gfxFT2FontBase::InitMetrics()
{
    mFUnitsConvFactor = 0.0;

    if (MOZ_UNLIKELY(GetStyle()->size <= 0.0) ||
        MOZ_UNLIKELY(GetStyle()->sizeAdjust == 0.0)) {
        memset(&mMetrics, 0, sizeof(mMetrics)); // zero initialize
        mSpaceGlyph = GetGlyph(' ');
        return;
    }

    // Explicitly lock the face so we can release it early before calling
    // back into Cairo below.
    FT_Face face = cairo_ft_scaled_font_lock_face(GetCairoScaledFont());

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

        SanitizeMetrics(&mMetrics, false);
        return;
    }

    // For variation fonts, figure out the variation coordinates to be applied
    // for each axis, in freetype's order (which may not match the order of
    // axes in mStyle.variationSettings, so we need to search by axis tag).
    if (face->face_flags & FT_FACE_FLAG_MULTIPLE_MASTERS) {
        typedef FT_UInt (*GetVarFunc)(FT_Face, FT_MM_Var**);
        typedef FT_UInt (*SetCoordsFunc)(FT_Face, FT_UInt, FT_Fixed*);
        static GetVarFunc getVar =
            (GetVarFunc)dlsym(RTLD_DEFAULT, "FT_Get_MM_Var");
        static SetCoordsFunc setCoords =
            (SetCoordsFunc)dlsym(RTLD_DEFAULT, "FT_Set_Var_Design_Coordinates");
        FT_MM_Var* ftVar;
        if (getVar && setCoords && FT_Err_Ok == (*getVar)(face, &ftVar)) {
            for (unsigned i = 0; i < ftVar->num_axis; ++i) {
                mCoords.AppendElement(ftVar->axis[i].def);
                for (const auto& v : mStyle.variationSettings) {
                    if (ftVar->axis[i].tag == v.mTag) {
                        FT_Fixed val = v.mValue * 0x10000;
                        val = std::min(val, ftVar->axis[i].maximum);
                        val = std::max(val, ftVar->axis[i].minimum);
                        mCoords[i] = val;
                        break;
                    }
                }
            }
            free(ftVar);
            if (!mCoords.IsEmpty()) {
                (*setCoords)(face, mCoords.Length(), mCoords.Elements());
            }
        }
    }

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

    mMetrics.maxAscent = FLOAT_FROM_26_6(ftMetrics.ascender);
    mMetrics.maxDescent = -FLOAT_FROM_26_6(ftMetrics.descender);
    mMetrics.maxAdvance = FLOAT_FROM_26_6(ftMetrics.max_advance);
    gfxFloat lineHeight = FLOAT_FROM_26_6(ftMetrics.height);

    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(face)) {
        // 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.
        mFUnitsConvFactor = FLOAT_FROM_26_6(FLOAT_FROM_16_16(ftMetrics.x_scale));
        yScale = FLOAT_FROM_26_6(FLOAT_FROM_16_16(ftMetrics.y_scale));
        emHeight = face->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 or
        // mFUnitsConvFactor (x scale).
        const TT_Header* head =
            static_cast<TT_Header*>(FT_Get_Sfnt_Table(face, ft_sfnt_head));
        if (head) {
            // Bug 1267909 - Even if the font is not explicitly scalable,
            // if the face has color bitmaps, it should be treated as scalable
            // and scaled to the desired size. Metrics based on y_ppem need
            // to be rescaled for the adjusted size. This makes metrics agree
            // with the scales we pass to Cairo for Fontconfig fonts.
            if (face->face_flags & FT_FACE_FLAG_COLOR) {
                emHeight = GetAdjustedSize();
                gfxFloat adjustScale = emHeight / ftMetrics.y_ppem;
                mMetrics.maxAscent *= adjustScale;
                mMetrics.maxDescent *= adjustScale;
                mMetrics.maxAdvance *= adjustScale;
                lineHeight *= adjustScale;
            }
            gfxFloat emUnit = head->Units_Per_EM;
            mFUnitsConvFactor = ftMetrics.x_ppem / emUnit;
            yScale = emHeight / emUnit;
        }
    }

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

    if (os2 && os2->sTypoAscender && yScale > 0.0) {
        mMetrics.emAscent = os2->sTypoAscender * yScale;
        mMetrics.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) {
            mMetrics.maxAscent = NS_round(mMetrics.emAscent);
            mMetrics.maxDescent = NS_round(mMetrics.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.
            mMetrics.maxAscent =
                std::max(mMetrics.maxAscent, NS_round(mMetrics.emAscent));
            mMetrics.maxDescent =
                std::max(mMetrics.maxDescent, NS_round(mMetrics.emDescent));
        }
    } else {
        mMetrics.emAscent = mMetrics.maxAscent;
        mMetrics.emDescent = mMetrics.maxDescent;
    }

    // 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 (face->underline_position && face->underline_thickness && yScale > 0.0) {
        mMetrics.underlineSize = face->underline_thickness * yScale;
        TT_Postscript *post = static_cast<TT_Postscript*>
            (FT_Get_Sfnt_Table(face, ft_sfnt_post));
        if (post && post->underlinePosition) {
            mMetrics.underlineOffset = post->underlinePosition * yScale;
        } else {
            mMetrics.underlineOffset = face->underline_position * yScale
                + 0.5 * mMetrics.underlineSize;
        }
    } else { // No underline info.
        // Imitate Pango.
        mMetrics.underlineSize = emHeight / 14.0;
        mMetrics.underlineOffset = -mMetrics.underlineSize;
    }

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

    if (os2 && os2->sxHeight && yScale > 0.0) {
        mMetrics.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."
        mMetrics.xHeight = 0.5 * emHeight;
    }

    // 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.
        mMetrics.aveCharWidth =
            ScaleRoundDesignUnits(os2->xAvgCharWidth, ftMetrics.x_scale);
    } else {
        mMetrics.aveCharWidth = 0.0; // updated below
    }

    if (os2 && os2->sCapHeight && yScale > 0.0) {
        mMetrics.capHeight = os2->sCapHeight * yScale;
    } else {
        mMetrics.capHeight = mMetrics.maxAscent;
    }

    // Release the face lock to safely load glyphs with GetCharExtents if
    // necessary without recursively locking.
    cairo_ft_scaled_font_unlock_face(GetCairoScaledFont());

    cairo_text_extents_t extents;
    mSpaceGlyph = GetCharExtents(' ', &extents);
    if (mSpaceGlyph) {
        mMetrics.spaceWidth = extents.x_advance;
    } else {
        mMetrics.spaceWidth = mMetrics.maxAdvance; // guess
    }

    if (GetCharExtents('0', &extents)) {
        mMetrics.zeroOrAveCharWidth = extents.x_advance;
    } else {
        mMetrics.zeroOrAveCharWidth = 0.0;
    }

    // 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) {
        mMetrics.xHeight = -extents.y_bearing;
        mMetrics.aveCharWidth =
            std::max(mMetrics.aveCharWidth, extents.x_advance);
    }

    if (GetCharExtents('H', &extents) && extents.y_bearing < 0.0) {
        mMetrics.capHeight = -extents.y_bearing;
    }

    mMetrics.aveCharWidth =
        std::max(mMetrics.aveCharWidth, mMetrics.zeroOrAveCharWidth);
    if (mMetrics.aveCharWidth == 0.0) {
        mMetrics.aveCharWidth = mMetrics.spaceWidth;
    }
    if (mMetrics.zeroOrAveCharWidth == 0.0) {
        mMetrics.zeroOrAveCharWidth = mMetrics.aveCharWidth;
    }
    // Apparently hinting can mean that max_advance is not always accurate.
    mMetrics.maxAdvance =
        std::max(mMetrics.maxAdvance, mMetrics.aveCharWidth);

    mMetrics.maxHeight = mMetrics.maxAscent + mMetrics.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.
    mMetrics.emHeight = floor(emHeight + 0.5);

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

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

    SanitizeMetrics(&mMetrics, false);

#if 0
    //    printf("font name: %s %f\n", NS_ConvertUTF16toUTF8(GetName()).get(), GetStyle()->size);
    //    printf ("pango font %s\n", pango_font_description_to_string (pango_font_describe (font)));

    fprintf (stderr, "Font: %s\n", NS_ConvertUTF16toUTF8(GetName()).get());
    fprintf (stderr, "    emHeight: %f emAscent: %f emDescent: %f\n", mMetrics.emHeight, mMetrics.emAscent, mMetrics.emDescent);
    fprintf (stderr, "    maxAscent: %f maxDescent: %f\n", mMetrics.maxAscent, mMetrics.maxDescent);
    fprintf (stderr, "    internalLeading: %f externalLeading: %f\n", mMetrics.externalLeading, mMetrics.internalLeading);
    fprintf (stderr, "    spaceWidth: %f aveCharWidth: %f xHeight: %f\n", mMetrics.spaceWidth, mMetrics.aveCharWidth, mMetrics.xHeight);
    fprintf (stderr, "    uOff: %f uSize: %f stOff: %f stSize: %f\n", mMetrics.underlineOffset, mMetrics.underlineSize, mMetrics.strikeoutOffset, mMetrics.strikeoutSize);
#endif
}

const gfxFont::Metrics&
gfxFT2FontBase::GetHorizontalMetrics()
{
    return mMetrics;
}

// Get the glyphID of a space
uint32_t
gfxFT2FontBase::GetSpaceGlyph()
{
    return mSpaceGlyph;
}

uint32_t
gfxFT2FontBase::GetGlyph(uint32_t unicode, uint32_t variation_selector)
{
    if (variation_selector) {
        uint32_t id =
            gfxFT2LockedFace(this).GetUVSGlyph(unicode, variation_selector);
        if (id) {
            return id;
        }
        unicode = gfxFontUtils::GetUVSFallback(unicode, variation_selector);
        if (unicode) {
            return GetGlyph(unicode);
        }
        return 0;
    }

    return GetGlyph(unicode);
}

int32_t
gfxFT2FontBase::GetGlyphWidth(DrawTarget& aDrawTarget, uint16_t aGID)
{
    if (!mGlyphWidths) {
        mGlyphWidths =
            mozilla::MakeUnique<nsDataHashtable<nsUint32HashKey,int32_t>>(128);
    }

    int32_t width;
    if (mGlyphWidths->Get(aGID, &width)) {
        return width;
    }

    gfxFT2LockedFace face(this);
    int32_t flags = gfxPlatform::GetPlatform()->FontHintingEnabled()
                    ? FT_LOAD_DEFAULT
                    : FT_LOAD_NO_AUTOHINT | FT_LOAD_NO_HINTING;
    FT_Fixed advance = 0;
    mozilla::DebugOnly<FT_Error> ftError =
        FT_Get_Advance(face.get(), aGID, flags, &advance);
    MOZ_ASSERT(!ftError);

    // If freetype emboldening is being used, and it's not a zero-width glyph,
    // adjust the advance to account for the increased width.
    if (mEmbolden && advance > 0) {
        // This is the embolden "strength" used by FT_GlyphSlot_Embolden,
        // converted from 26.6 to 16.16
        FT_Fixed strength = 1024 *
            FT_MulFix(face.get()->units_per_EM,
                      face.get()->size->metrics.y_scale) / 24;
        advance += strength;
    }

    width = advance;
    mGlyphWidths->Put(aGID, width);

    return width;
}

bool
gfxFT2FontBase::SetupCairoFont(DrawTarget* aDrawTarget)
{
    // The scaled font ctm is not relevant right here because
    // cairo_set_scaled_font does not record the scaled font itself, but
    // merely the font_face, font_matrix, font_options.  The scaled_font used
    // for the target can be different from the scaled_font passed to
    // cairo_set_scaled_font.  (Unfortunately we have measured only for an
    // identity ctm.)
    cairo_scaled_font_t *cairoFont = GetCairoScaledFont();

    if (cairo_scaled_font_status(cairoFont) != CAIRO_STATUS_SUCCESS) {
        // Don't cairo_set_scaled_font as that would propagate the error to
        // the cairo_t, precluding any further drawing.
        return false;
    }
    // Thoughts on which font_options to set on the context:
    //
    // cairoFont has been created for screen rendering.
    //
    // When the context is being used for screen rendering, we should set
    // font_options such that the same scaled_font gets used (when the ctm is
    // the same).  The use of explicit font_options recorded in
    // CreateScaledFont ensures that this will happen.
    //
    // XXXkt: For pdf and ps surfaces, I don't know whether it's better to
    // remove surface-specific options, or try to draw with the same
    // scaled_font that was used to measure.  As the same font_face is being
    // used, its font_options will often override some values anyway (unless
    // perhaps we remove those from the FcPattern at face creation).
    //
    // I can't see any significant difference in printing, irrespective of
    // what is set here.  It's too late to change things here as measuring has
    // already taken place.  We should really be measuring with a different
    // font for pdf and ps surfaces (bug 403513).
    cairo_set_scaled_font(gfxFont::RefCairo(aDrawTarget), cairoFont);
    return true;
}