Image Processing Reference
525 on 625 line raster
Widths scaled to preserve aspect ratio
Figure 34-5 525 lines on 625-line format.
The second quite common example is when 405-line UK transmissions are being dis-
played on 625-line TV (or being shown in the United States). The output quality will be
better if the picture size is reduced so that it occupies a 405-line window in the middle of
the 525-line raster rather than scaling it to fit. Over-scanning of TV sets will account for
some of the margin around the edges. The relative sizes of these margins are shown in
This leaves an unacceptably large blanked region around the picture when it is
broadcast in Europe, so 405-line TV is often scaled up to fit 625 lines. Otherwise the mar-
gin around the image is probably too wide. The 1:1 scaling is too small and 1:2 is too large.
So a non-integer scaling factor is used. This yields some very nasty line-based artifacts.
Oddly enough, 405 would probably look better on HDTV because a 1:2 scaling in the ver-
tical axis would be feasible.
The third case is when you are just enlarging an area of picture. Obviously, non-inte-
ger scaling factors will be problematic.
405 on 625 line raster
405 on 525 line raster
Figure 34-6 405 lines on 525 and 625 rasters.