[Webkit-unassigned] [Bug 50619] [GTK] Glyphs in vertical text tests are rotated 90 degrees clockwise

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=50619





--- Comment #37 from Denis Nomiyama (dnomi) <d.nomiyama at samsung.com>  2013-10-03 07:33:40 PST ---
(In reply to comment #36)
> (From update of attachment 212584 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=212584&action=review

> > Source/WebCore/platform/graphics/freetype/FontPlatformDataFreeType.cpp:323
> > +        cairo_matrix_translate(&fontMatrix, 0.0, 1.0);
> 
> Could you explain why this makes the translation in the y-axis, while in FontPlatformData::setOrientation you translate in the x-axis?

Sure. Here is my understanding, please correct me if I'm wrong.
In the Cairo coordinate system, glyphs are drawn in the quadrant where x and y are positive. For example, in the figure (a) below, the glyph is drawn in quadrant 4.
Assuming texts are rotated 90 degrees in the user space because the text orientation is vertical, then the coordinates are rotated as in (b) and the glyph is located at quadrant 1.
So the idea behind the fix is to keep the glyph in quadrant 1 (b) but rotated anti-clockwise 90 degrees. Therefore, rotation angle = -90 degrees, and translation = (0,1). Where (0,1) means a translation of font size towards y-axis. Assuming the system is scaled by font size.

(a) HORIZONTAL ORIENTATION   (b) VERTICAL ORIENTATION
          |                            |
       3  |  4                      2  |  3
          |     x                y     |
     ----------->                <-----------
          |                            |
       2  |  1                      1  |  4
          |                            |
          v y                          v x

    Translation: (0,0)           Translation: (0,1)
    Rotation: 0                  Rotation: -90 degrees

     | 1  0  0 |               | 1  0  0 |   | 1  0  0 |   | 0 -1  0 |   | 0 -1  0 |
     | 0  1  0 |               | 0  1  0 | x | 0  1  0 | x | 1  0  0 | = | 1  0  0 |
     | 0  0  1 |               | 0  0  1 |   | 0  1  1 |   | 0  0  1 |   | 1  0  1 |
  Horizontal Matrix            Horizontal    Translation    Rotation      Vertical
     (Identity)                  Matrix        Matrix        Matrix        Matrix

Where translation matrix = |  cos  sin  0 |   cos - cosine of the rotation angle
                           | -sin  cos  0 |   sin - sine of the rotation angle
                           |  tx   ty   1 |

         rotation matrix = |  1    0    0 |   tx,ty - translation in x and y,
                           |  0    1    0 |           respectively
                           |  tx   ty   1 |

At FontPlatformData::setOrientation(), there are two situations:

1) From vertical orientation to horizontal orientation
When changing the orientation from vertical to horizontal, the coordinate system changes from (c) to (d) below, and the glyph is located in quadrant 1.
So this time, we want to keep the glyph in quadrant 1 (d) but rotated clockwise 90 degrees.
Therefore, rotation angle = 90 degrees and translation = (-1,0). The peculiar Cairo coordinate system is the reason why translation is done in x-axis and not in y-axis as in (b).
The resulting rotation angle and translation in (d) should be the same as in (a).

(c) VERTICAL ORIENTATION     (d) HORIZONTAL ORIENTATION
          |                            |
       2  |  3                      3  |  4
    y     |                            |     x
    <-----------                  ----------->
          |                            |
       1  |  4                      2  |  1
          |                            |
          v x                          v y

    Translation: (0,1)           Translation: (-1,0)
    Rotation: -90 degrees        Rotation: 90 degrees

     | 0 -1  0 |               | 0 -1  0 |   | 1  0  0 |   | 0  1  0 |   | 1  0  0 |
     | 1  0  0 |               | 1  0  0 | x | 0  1  0 | x |-1  0  0 | = | 0  1  0 |
     | 1  0  1 |               | 1  0  1 |   |-1  0  1 |   | 0  0  1 |   | 0  0  1 |
      Vertical                  Vertical     Translation    Rotation     Horizontal
       Matrix                    Matrix        Matrix        Matrix        Matrix

2) From horizontal orientation to vertical orientation
This is the same case as in (a) to (b).
I realised this last case is incorrect in my patch, which incorrectly translates in the x-axis. This case is not currently used in the code and that explains why this bug wasn't spotted before.
I will fix it.

> > Source/WebCore/platform/graphics/freetype/FontPlatformDataFreeType.cpp:363
> > +    uint32_t tag = (table >> 24) | ((table >> 8) & 0xff00) | ((table & 0xff00) << 8) | ((table & 0xff) << 24);
> 
> I am not sure, but maybe you could try to use http://www.freetype.org/freetype2/docs/reference/ft2-basic_types.html#FT_MAKE_TAG for this, and it would increase readability.

Nice one :) That line can be simplified a bit:

uint32_t tag = FT_MAKE_TAG(table & 0xff,  table & 0xff00, table & 0xff0000, table & 0xff000000);

> > Source/WebCore/platform/graphics/freetype/FontPlatformDataFreeType.cpp:388
> > +    cairo_matrix_t ctm;
> 
> Maybe you could use a more verbose name here?

Sure. I'll do it in the next patch.

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