rgb24 -> yuv420p conversion is not precise?

[ami_stuff]

I don't know if this is a bug (or it should be like it is), but when you convert the attached rgb24 sample to yuv420p then for example the red line in the picture will be not smooth anymore

C:\>ffmpeg -i tetest.tif -pix_fmt yuv420p out.tif
ffmpeg version N-35966-g1d0ae92, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec 19 2011 13:01:15 with gcc 4.6.2
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libope
ncore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --en
able-libmp3lame --enable-libopenjpeg --enable-librtmp --enable-libschroedinger -
-enable-libspeex --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwben
c --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-
libxvid --enable-zlib
  libavutil    51. 32. 0 / 51. 32. 0
  libavcodec   53. 46. 1 / 53. 46. 1
  libavformat  53. 27. 0 / 53. 27. 0
  libavdevice  53.  4. 0 / 53.  4. 0
  libavfilter   2. 53. 0 /  2. 53. 0
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
Input #0, image2, from 'tetest.tif':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: tiff, rgb24, 1024x768, 25 tbr, 25 tbn, 25 tbc
[buffer @ 020E1200] w:1024 h:768 pixfmt:rgb24 tb:1/1000000 sar:0/1 sws_param:
[buffersink @ 020E1480] auto-inserting filter 'auto-inserted scale 0' between th
e filter 'src' and the filter 'out'
[scale @ 020E1880] w:1024 h:768 fmt:rgb24 -> w:1024 h:768 fmt:yuv420p flags:0x4
Output #0, image2, to 'out.tif':
  Metadata:
    encoder         : Lavf53.27.0
    Stream #0:0: Video: tiff, yuv420p, 1024x768, q=2-31, 200 kb/s, 90k tbn, 25 t
bc
Stream mapping:
  Stream #0:0 -> #0:0 (tiff -> tiff)
Press [q] to stop, [?] for help
frame=    1 fps=  0 q=0.0 Lsize=       0kB time=00:00:00.04 bitrate=   0.0kbits/
s
video:1158kB audio:0kB global headers:0kB muxing overhead -100.000000%

[Elon Musk]

I'm pretty much sure this can not be fixed.

[Elon Musk]

I really do not see how this could be fixed.

[Michael Niedermayer]

Try this:
ffmpeg -i tetest.tif -sws_flags lanczos+accurate_rnd+full_chroma_int+full_chroma_inp -pix_fmt yuv420p out.y4m
ffmpeg -i out.y4m -sws_flags lanczos+accurate_rnd+full_chroma_int+full_chroma_inp out.bmp

and view the bmp with something not based on SDL (that is something not converting to YUV420 again)

[gjdfgh]

It would be nice if these magical flags would be documented anywhere. Especially how to get highest quality output. (Maybe even add a #define for this.)