SVG text smaller than 0.5px not displayed properly

[Jeff Schiller]

http://codedread.com/browser-tests/subpixel.svg

or

<svg xmlns='http://www.w3.org/2000/svg'>
  <text x='10' y='30'>The two words should be the same size:</text>
  <g transform='scale(15)'>
    <text y='12' style='font-size:0.5px'>TINY</text>
    <text y='24' style='font-size:0.499999px'>TINY</text>
  </g>
</svg>

Both words should be small on the page.  The latter one at font-size smaller than 0.5px is not displayed appropriately.

Also, you can see the effect more dramatically here: http://upload.wikimedia.org/wikipedia/commons/f/f9/PI.svg

[Jeff Schiller]

Created attachment 75617 [details]
Test case showing the problem

[Nikolas Zimmermann]

Thanks for the testcase, Jeff!
I've traced it down:

#1  0x0426c654 in WebCore::FontCache::createFontPlatformData (this=0x1a5965c0, fontDescription=@0x1d4adda0, family=@0x1d4adda0) at /Users/nikolaszimmermann/Coding/WebKit/WebCore/platform/graphics/mac/FontCacheMac.mm:214

(gdb) p fontDescription
$61 = (const class WebCore::FontDescription &) @0x1d4adda0: {
  m_familyList = {
    m_family = {
      m_string = {
        m_impl = {
          <WTF::FastAllocBase> = {<No data fields>}, 
          members of WTF::RefPtr<WTF::StringImpl>: 
          m_ptr = 0x1a56e650
        }
      }
    }, 
    m_next = {
      <WTF::RefPtr<WebCore::SharedFontFamily>> = {
        <WTF::FastAllocBase> = {<No data fields>}, 
        members of WTF::RefPtr<WebCore::SharedFontFamily>: 
        m_ptr = 0x0
      }, <No data fields>}
  }, 
  m_specifiedSize = 0.49000001, 
  m_computedSize = 0.49000001, 

The font size is stored correctly in the FontDescription, but then "computedPixelSize()" is used to determine the actual font size.

193	
194	FontPlatformData* FontCache::createFontPlatformData(const FontDescription& fontDescription, const AtomicString& family)
195	{
196	    NSFontTraitMask traits = fontDescription.italic() ? NSFontItalicTrait : 0;
197	    NSInteger weight = toAppKitFontWeight(fontDescription.weight());
198	    float size = fontDescription.computedPixelSize();

(gdb) p size
$63 = 0

FontDescription.h:
    int computedPixelSize() const { return int(m_computedSize + 0.5f); }

This is the root of the problem. Just keeping my analysis here for the record.

[Nikolas Zimmermann]

The patch on bug 12448 fixes this problem. Thanks a lot for the testcase, I've integrated it in my patch.

*** This bug has been marked as a duplicate of bug 12448 ***

[Simon Fraser (smfr)]

Is it really the same thing? In this testcase the text is just a line of pixels (even if it's not sharp). In Firefox, the text is much bigger.

[Nikolas Zimmermann]

(In reply to comment #4)
> Is it really the same thing? In this testcase the text is just a line of pixels (even if it's not sharp). In Firefox, the text is much bigger.

Which testcase are you referring to?

[Simon Fraser (smfr)]

One in a bug that I can't find right now, but was probably marked as a duplicate of this.

[Dirk Schulze]

I guess Simon thought about bug 52892.

[Simon Fraser (smfr)]

Yep, that's the one. Sorry, I thought this was that bug.