Image Processing Reference
In-Depth Information
Some compression is taking place as the movie is laser scanned onto film. This can be
disappointing to hear if you have labored over an 8K square image. Nevertheless, an image
that is created and worked on at 8K is going to look a lot better in the final low-resolution
output than if you had worked at the lower resolution, so the effort is not wasted.
Knowing about this compromise in resolving power significantly alters the cost
dynamics of the disk storage you buy when equipping the distribution and play-out systems
in a cinema complex. It also helps facilitate the delivery and transmission of a movie via a
network connection from the studio to the theater. Video compression helps some more, too,
but it does not have to compress the image quite as harshly to accomplish useful bit rates.
Delivering an appropriately sized image directly to a D-cinema projector eliminates
the intermediate process of creating a 35mm print. This improves the quality of the view-
ing experience for the customer and saves money for the distributor. Typical resolutions
for this are 2200
1080 is also popular and is a size which
coincidentally is also used for high-definition TV in some deployments.
1125 pixels, although 1920
Down-Sampling to SDTV Format
Let's say you are creating some program material in HDTV format, for example, rendered
at 1920
1080 and interlaced at 25 fps. If this is intended for delivery at standard definition
for broadcasting purposes, you must render a special low-resolution version unless you
trust the broadcasters to convert it themselves. The visible area on a 625-line raster is only
579 lines, which does not have an integer relationship to 1080. Even more problematic is
the 480 visible lines on the 525-line systems, which have added complexity of scanning at
30 fps. The change in time base causes more problems with keeping the motion smooth.
This is especially tricky if the motion was rendered at field rate rather than frame rate. The
interpolation artifacts are avoided by rendering two additional copies of the movie with the
correct scale factor and frame timing. Most production companies will not have the luxury
of time or computing capacity to do this so some shortcuts will be necessary.
Up-Sampling to HDTV Format
Time pressure being what it is, redesigning your rendering process could cut some corners
off the time required to complete your project. For example, a raster only 720 lines high is
probably detailed enough to be displayed at high definition. Up-sampling this 720-line
image to 1080 lines is feasible for an HDTV output. A picture size that is 720 lines high is
one of the HDTV standards currently deployed anyway. If you create content for multiple
territories, then rendering at 25 fps will be less work than 30 fps. Pulldown will yield the
30 fps version as it does for movie film. Products such as Adobe After Effects will produce
your up-sampled and down-sampled versions. The savings allow you to render 1080
60i, which reduces your render workload to about 30% of
what it would be at full resolution.
Compression is not just about bit rates coming out of a tool like Discreet Cleaner; it
is a mindset. You have to think about your entire workflow process and not waste time
creating data that is going to disappear when the video is compressed.
25p instead of 1920
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