[cairo] Intermittent issues using cairo_rotate to rotate an array of text glyphs

[Matthew Conway]

I have modified my code to set the font after doing the transform 
(previously was setting before doing the transform) but doing this 
doesn't seem to make any difference.

Here's an edited version of the code I've implemented outlining key 
points for cairo and font interactions...


void draw_glyphs( cairo_t *cr, FT_Face face, unsigned int const x, 
unsigned int const y, paint_t paint )
{
     cairo_save( cr );

     cairo_translate( cr, x, y );
     cairo_rotate( cr, 1.5708 );

     /* The freetype {x,y}_scale is a 16.16 value to convert from font 
units 26.6 pixels.  The cairo scaling factor is the size of the em 
square in pixels.  */
     cairo_matrix_t font_scale;
     float units_per_EM_scaled = (float)face->units_per_EM / 64.0f;
     double sx = (double)( (float)face->size->metrics.x_scale / 65536.0f 
* units_per_EM_scaled );
     double sy = (double)( (float)face->size->metrics.y_scale / 65536.0f 
* units_per_EM_scaled );
     cairo_matrix_init_scale( &font_scale, sx, sy );
     cairo_set_font_matrix( cr, &font_scale);

     cairo_font_face_t *font_face;
     font_face = cairo_ft_font_face_create_for_ft_face( face, 0 );
     cairo_set_font_face( cr, font_face );

     cairo_set_source_rgba( cr, paint->red, paint->green, paint->blue, 
paint->alpha );
     cairo_show_glyphs( cr, glyphs, num_glyphs );

     cairo_identity_matrix( cr );
     cairo_set_font_face( cr, NULL);
     cairo_font_face_destroy( font_face );

     cairo_restore( cr );
}


I re-ran the code after commenting out the line:

/* cairo_set_font_face( cr, font_face ); */

This resulted in a default font face being used (looks like a cursive 
font?), but the rotation worked perfectly. I never saw any mixed 
horizontal and vertical characters, which to me, indicates there's an 
issue somewhere in Cairo when drawing a "non-default" font face after 
being rotated.


BTW, I also re-ran the code using a scaled font and locking / unlocking 
it, but doing that also shows the issue.


Thanks,
Matt.


On 06/01/16 18:38, Bill Spitzak wrote:
> You have to set the font after you do the transform.
>
> However I did not expect this mess, I thought what Cairo did was draw 
> the font as though the transform it was using before was still in 
> effect (ie "font lock", similar to the proposed "line width lock").
>
> On Wed, Jan 6, 2016 at 7:45 AM, Matthew Conway <mconway at espial.com 
> <mailto:mconway at espial.com>> wrote:
>
>     I'm trying to rotate an array of glyphs (where each glyph
>     represents a textual character).
>
>     Consider the following:
>
>     ----------------
>     | A TEXT LABEL |
>     ----------------
>
>     Where the rectangle is a solid coloured background and the text is
>     drawn (centered) on top of the rectangle (think subtitles being
>     displayed on a TV screen).
>
>     For some situations, I need to rotate the text label (rectangle
>     and text) 90 degrees clockwise. I do not expect the text rotation
>     to be vertical as shown here:
>
>     |̄ ̄ ̄ |
>     | A |
>     |   |
>     | T |
>     | E |
>     | X |
>     | T |
>     |   |
>     | L |
>     | A |
>     | B |
>     | E |
>     | L |
>     |___|
>
>     Instead, I am expecting the text to be rotated (in position) with
>     the label.
>
>     Sometimes (about 25% of the time), Cairo seems to be rotating
>     random text characters and drawing them over the top of other
>     correctly rotated characters.
>
>     /* Draw the labels background */
>     cairo_rectangle( cr, x, y, width, height );
>     cairo_set_source_rgba( cr, red, green, blue, alpha );
>     cairo_fill( vg->cairo_context );
>
>     /* Rotate 90 degrees clockwise and show the text
>     */cairo_translate( cr, x, y );
>     cairo_rotate( cr, 1.5708 );
>     cairo_show_glyphs( cr, glyphs, num_glyphs );
>
>
>     Notes:
>     1. The glyphs are created by freetype libraries.
>     2. The system is using Wayland to draw the text labels.
>
>
>     I've tried quite a few things, the most successful was to add a
>     call to show_glyphs before translating, which reduces the issue to
>     show approximatley 5% of the time:
>
>     /* Rotate 90 degrees clockwise and show the text */
>     cairo_show_glyphs( cr, glyphs, num_glyphs );
>     cairo_translate( cr, x, y );
>     cairo_rotate( cr, 1.5708 );
>     cairo_show_glyphs( cr, glyphs, num_glyphs );
>
>
>     I've taken a photo of the screen to show what the issue looks
>     like, it can be viewed here:
>     http://postimg.org/image/x4g4fne5h/
>
>
>     I'm using:
>     - latest cairo 1.14.6
>     - freetype to create the font glyphs
>     - Wayland
>
>
>     Thanks,
>     Matt.
>     -- 
>     cairo mailing list
>     cairo at cairographics.org <mailto:cairo at cairographics.org>
>     http://lists.cairographics.org/mailman/listinfo/cairo
>
>

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