[FFmpeg-trac] #3302(avfilter:open): Cannot draw opaque text on transparent frame

FFmpeg trac at avcodec.org
Fri Jun 20 19:08:16 CEST 2014

#3302: Cannot draw opaque text on transparent frame
             Reporter:  Krieger     |                    Owner:
                 Type:  defect      |                   Status:  open
             Priority:  normal      |                Component:  avfilter
              Version:  git-master  |               Resolution:
             Keywords:  drawtext    |               Blocked By:
             Blocking:              |  Reproduced by developer:  1
Analyzed by developer:  0           |

Comment (by Krieger):

 This issue is still actual. Drawing a text over transparent background
 doesn't work, although should.

 This bug may be related to http://trac.ffmpeg.org/ticket/3317

 Seems the problem is that ff_blend_mask() from drawutils.c treats alpha,
 the 4th component on the 1st plane of dst, just like all other components
 (R, G, B). And, for example, in case of

 {{{ffmpeg -f lavfi -i
 "color=color=white at 0.0:size=2x2,format=pix_fmts=rgba,drawtext=text=TEST:fontsize=50:fontfile=/usr/share/fonts/corefonts/verdana.ttf:fontcolor=red at 1.0,format=pix_fmts=rgba"
 -vframes 1 -vcodec png /tmp/tmp.png}}}

 in the end of this routine i have such contents:

 Breakpoint 3, ff_blend_mask (draw=0x19e91d0, color=0x19e9200,
 dst=0x19f6580, dst_linesize=0x19f65c0, dst_w=2, dst_h=2, mask=0x19f6c90
 '\377' <repeats 124 times>, mask_linesize=31,
     mask_w=2, mask_h=1, l2depth=3, endianness=0, x0=0, y0=1) at
 504     }
 (gdb) x/8xb dst[0]
 0x19f6840:      0xff    0xff    0xff    0x00    0xff    0xff    0xff
 (gdb) x/8xb (dst[0] + dst_linesize[0])
 0x19f6860:      0xff    0x00    0x00    0x00    0xff    0x00    0x00

 It shows that topmost line is 2 white pixels (background color), and the
 bottom line is 2 red pixels (color of text). However, red-colored pixels
 have alpha = 0 and the whole picture is represented as fully transparent.

 It is possible to parametrize the algorithm for resulting alpha. So that
 old behaviour can be preserved, but also we get a new mode to preserve
 alpha value of mask (i.e. ignore background alpha), which can be desired
 in some cases; more strategies can be added.

Ticket URL: <https://trac.ffmpeg.org/ticket/3302#comment:2>
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