Right-to-left option for drawtext filter

[Andrey Utkin]

Summary of the bug:
Arabic text is displayed backwards comparing to supplied parameter.

How to reproduce:

This test requires some font supporting Arabic. Below i use a font supplied by third party, and i didn't get their agreement yet for publishing it here for bug investigation. I don't know which widespread free fonts support Arabic (comments are appreciated).

The issue is reproduced with following command line (i hope non-ASCII Arabic letters won't get garbled at transmission).

I'm not an expert on Arabic, but you can see that there's character like "i" on the right, and on rendered picture it goes on the left. I suppose that's some smartness of libfreetype. Does anybody know how this behaviour could get fixed? Maybe some flag to pass to libfreetype?

 $ ffplay -loglevel debug -f lavfi -i "color=color=white,drawtext=fontfile=./samples/editor/projects/stc/fonts/stc-regular-webfont.ttf:text=أحمد"
ffplay version N-64175-gb0f3b6d Copyright (c) 2003-2014 the FFmpeg developers
  built on Jul  3 2014 16:24:25 with gcc 4.7.3 (Gentoo 4.7.3-r1 p1.4, pie-0.5.5)
  configuration: --enable-libfreetype --enable-gpl --enable-libx264
  libavutil      52. 90.100 / 52. 90.100
  libavcodec     55. 68.100 / 55. 68.100
  libavformat    55. 44.100 / 55. 44.100
  libavdevice    55. 13.101 / 55. 13.101
  libavfilter     4.  9.100 /  4.  9.100
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
detected 4 logical cores  0 aq=    0KB vq=    0KB sq=    0B f=0/0   
[Parsed_color_0 @ 0x7fb5dc002060] Setting 'color' to value 'white'
[Parsed_color_0 @ 0x7fb5dc002060] size:320x240 rate:25/1 duration:-1.000000 sar:1/1
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Setting 'fontfile' to value './samples/editor/projects/stc/fonts/stc-regular-webfont.ttf'
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Setting 'text' to value 'أحمد'
[AVFilterGraph @ 0x7fb5dc001580] query_formats: 3 queried, 2 merged, 0 already done, 0 delayed
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:0 t:0.000000 text_w:30 text_h:18 x:0 y:0
[lavfi @ 0x7fb5dc000920] All info found
Input #0, lavfi, from 'color=color=white,drawtext=fontfile=./samples/editor/projects/stc/fonts/stc-regular-webfont.ttf:text=أحمد':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0, 1, 1/25: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 1/25, 25 tbr, 25 tbn, 25 tbc
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
Video frame changed from size:0x0 format:none serial:-1 to size:320x240 format:yuv420p serial:1
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:1 t:0.040000 text_w:30 text_h:18 x:0 y:0
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[ffplay_buffer @ 0x7fb5d4001620] Setting 'video_size' to value '320x240'
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:2 t:0.080000 text_w:30 text_h:18 x:0 y:0
[ffplay_buffer @ 0x7fb5d4001620] Setting 'pix_fmt' to value '0'
[ffplay_buffer @ 0x7fb5d4001620] Setting 'time_base' to value '1/25'
[ffplay_buffer @ 0x7fb5d4001620] Setting 'pixel_aspect' to value '1/1'
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[ffplay_buffer @ 0x7fb5d4001620] Setting 'frame_rate' to value '25/1'
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:3 t:0.120000 text_w:30 text_h:18 x:0 y:0
[ffplay_buffer @ 0x7fb5d4001620] w:320 h:240 pixfmt:yuv420p tb:1/25 fr:25/1 sar:1/1 sws_param:
[ffplay_crop @ 0x7fb5d4002840] Setting 'out_w' to value 'floor(in_w/2)*2'
[ffplay_crop @ 0x7fb5d4002840] Setting 'out_h' to value 'floor(in_h/2)*2'
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:4 t:0.160000 text_w:30 text_h:18 x:0 y:0
[AVFilterGraph @ 0x7fb5d4000d40] query_formats: 3 queried, 2 merged, 0 already done, 0 delayed
[ffplay_crop @ 0x7fb5d4002840] w:320 h:240 sar:1/1 -> w:320 h:240 sar:1/1
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:5 t:0.200000 text_w:30 text_h:18 x:0 y:0
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:6 t:0.240000 text_w:30 text_h:18 x:0 y:0
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:7 t:0.280000 text_w:30 text_h:18 x:0 y:0
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.
[Parsed_drawtext_1 @ 0x7fb5dc002b00] n:8 t:0.320000 text_w:30 text_h:18 x:0 y:0
[Parsed_drawtext_1 @ 0x7fb5dc002b00] Copying data in avfilter.

[Andrey Utkin]

Found a free Arabic-enabled font: DejaVuSans, in Gentoo it is available in media-fonts/dejavu package.

[gjdfgh]

libfreetype does not do text layout. For correct Arabic display, one would have to involve the libfribidi and harfbuzz libraries. Or use libass.

[Carl Eugen Hoyos]

A right-to-left option for the drawtext filter appears useful to me.

[Andrey Utkin]

I still don't understand why some explicit actions must be taken for correct displaying of any correct supplied UTF-8 string (i think Arabic text is nothing more than UTF-8 string).

[gjdfgh]

i think Arabic text is nothing more than UTF-8 string

It's not as simple, because Arabic text needs to be rendered in the opposite direction of western text. Freetype is quite low level and doesn't render text, it merely renders glyphs. Additionally to that, Arabic text has complex layout rules, which requires handling combining glyphs and such. libfribidi and Harfbuzz were created to handle these issues. A naive option to render text RTL might work in simple scenarios, but will surely fail in more complex cases, and thus would be a bad hack at best.

[Andrey Utkin]

There's a work in progress on a patch resolving this issue, stay tuned.

[Clément Bœsch]

Fixed by Marc Jeffreys in a0b71e9f3e95d22b098e4d81c26d3bfe1f53249f