[Bug 1435] New: OpenJDK 6/7 returns incorrect TrueType font metrics

[bugzilla-daemon at icedtea.classpath.org]

http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=1435

            Bug ID: 1435
           Summary: OpenJDK 6/7 returns incorrect TrueType font metrics
    Classification: Unclassified
           Product: IcedTea
           Version: 7-hg
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: IcedTea
          Assignee: gnu.andrew at redhat.com
          Reporter: drazzib at drazzib.com
                CC: unassigned at icedtea.classpath.org

Created attachment 873
  --> http://icedtea.classpath.org/bugzilla/attachment.cgi?id=873&action=edit
Patch from Nobuhiro Ban <ban.nobuhiro at gmail.com>

Problem
 =======
 On OpenJDK JRE and using some TrueType font, JasperReports does not
 display text element which height is just the same as the font size
 eg. { height="8", size="8", font="IPA mincho" }.
 JasperReports checks the text size before drawing the text elements.
 If (ascent + descent) of text is greater than the height of text
 element, this text is not drawn.
 In above case, Sun Java returns the same height (ascent + descent =
 fontsize), but OpenJDK returns the text height greater than font size,
 so not drawn.


 Sample code (includes Japanese char)
 ===================================
 import java.awt.Font;
 import java.awt.font.FontRenderContext;
 import java.awt.font.TextLayout;
 public class JavaApplication1 {
    public static void main(String[] args) throws Exception {
        Font f = new Font("IPA明朝",Font.PLAIN, 8);
        TextLayout layout = new TextLayout("IPA明朝", f,
                new FontRenderContext(null, true, true));
        System.out.println("Ascent:  " + layout.getAscent());
        System.out.println("Descent: " + layout.getDescent());
    }
 }


 Result of this code
 ===================
 Sun Java 6 (sun-java6-jre 6.26-0squeeze1)
 > Ascent:  7.0390625
 > Descent: 0.9609375

 OpenJDK 6 (openjdk-6-jre 6b24~pre2-1)
 > Ascent:  7.046875
 > Descent: 0.96875

 OpenJDK 7 (openjdk-7-jre 7~b147-2.0-1)
 > Ascent:  7.046875
 > Descent: 0.96875

 Sun Java returns correct height, but OpenJDK returns greater value
 than Sun's.


 Analysis of Source code
 =======================
 In OpenJDK6/7 native method
sun.font.FreetypeFontScaler.getFontMetricsNative():
 jdk/src/share/native/sun/font/freetypeScaler.c
 >JNIEXPORT jobject JNICALL
 >Java_sun_font_FreetypeFontScaler_getFontMetricsNative(
 >        JNIEnv *env, jobject scaler, jobject font2D,
 >        jlong pScalerContext, jlong pScaler) {
 (snip)
 >    /* ascent */
 >    ax = 0;
 >    ay = -(jfloat) FT26Dot6ToFloat(FT_MulFix(
 >                       ((jlong) scalerInfo->face->ascender + bmodifier/2),
 >                       (jlong) scalerInfo->face->size->metrics.y_scale));
 >    /* descent */
 >    dx = 0;
 >    dy = -(jfloat) FT26Dot6ToFloat(FT_MulFix(
 >                       ((jlong) scalerInfo->face->descender + bmodifier/2),
 >                       (jlong) scalerInfo->face->size->metrics.y_scale));
 (snip)
 >    metrics = (*env)->NewObject(env,
 >                                sunFontIDs.strikeMetricsClass,
 >                                sunFontIDs.strikeMetricsCtr,
 >                                ax, ay, dx, dy, bx, by, lx, ly, mx, my);
 >
 >    return metrics;
 >} 
 This code uses FT_MulFix to convert size. But FT_MulFix sometimes rounds
 up and loses precision.
 In FreeType2's FT_MulFix:
 freetype-2.4.8/src/base/ftcalc.c
 >   c = (FT_Long)( ( (FT_Int64)a * b + 0x8000L ) >> 16 );
 If both ascent and descent are rounded up, (ascent + descent) is greater
 than original height.
 In the sample case (IPA mincho font),
  bmodifier = 0
  scalerInfo->face->ascender = 1802L
  scalerInfo->face->descender = -246L
  scalerInfo->face->size->metrics.y_scale = 16384L
 In this case, 1802 mod 4 = 2 and 246 mod 4 = 2, so both are rounded up.
 This causes (ay + dy) > font-size.
 (Note: (Sun) 1802.0/256.0 = 7.0390625, (OpenJDK) 1804.0/256.0 = 7.046875)


 Suggested fix
 =============
 Fix to keep the precision in the font metrics conversion in
 Java_sun_font_FreetypeFontScaler_getFontMetricsNative().

 See attached patch from Nobuhiro Ban <ban.nobuhiro at gmail.com>

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