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Still image extracted
from sequence of "Magic Hour" StEM footage used in tests |
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CineForm 444 Quality
Analysis Versus HDCam SR |
| HDCAM-SR has
achieved an important position in the market, being used
routinely for on-set capture and final mastering of
feature film projects. The quality of HDCam SR is quite
high and has repeatedly proven itself worthy of both. So
as CineForm introduces its 12-bit CineForm 444 format it
is important to test visual fidelity comparatively
against HDCam SR. |
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Test Methodology |
In designing the
comparative quality tests, two sets of uncompressed
dual-link HD-SDI sources were used:
- "Magic Hour" sequence of StEM
(Standard Evaluation Material) commissioned by DCI
(Digital Cinema Initiative) for purposes of testing the
quality of compression codecs.
- Viper camera shot in dual-link
Filmstream mode (courtesy of Thomson).
In both cases the
uncompressed source served as the reference for
performing PSNR (Peak Signal-to-Noise Ratio) tests which
measure, in decibels (dB) how close the compressed
images (after reconstruction) match the uncompressed
source images. The higher the number the better
the result.
In this write-up, we
present the visual quality performance results obtained
from analyzing the StEM footage for both HDCam SR and
CineForm 444. Results from the Viper Filmstream
tests are very similar, and are publicly available as a
series of posts on the blog of David Newman at
http://cineform.blogspot.com/.
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 |
The figure at
left defines the hardware configuration for the quality
tests using the StEM material.
Our source material was
580 frames of the "Magic Hour" sequence from the
Standard test Evaluation Material (StEM) which is used
for DCI compliance testing. The footage is quite
demanding (intentionally) on compression, with a lot of
film grain, complex motion and significant detail within
each frame. Before we performed the tests, using After
Effects 7, we resized the StEM sequence to 10-bit 1920 x
1080 RGB (from its original 4096 x 1714 16-bit TIFF
files) for compatibility with dual-link HD-SDI.
The resized StEM footage was stored on a workstation
that included a Blackmagic Decklink dual-link HD-SDI
card for content delivery.
Content was then played
out from the uncompressed workstation into both the SRW1
(SQ-440 Mbps mode) and the Wafian HR-2. The Wafian
HR-2 recorded the StEM footage using two different
quality settings defined for CineForm 444:
Filmscan
1 and Keying 1
In software, from the
original uncompressed StEM footage, we later created
CineForm content using two additional CineForm 444
modes:
Filmscan 2 and Keying
2
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| After recording
onto the SRW1, its material was then played back out and
re-captured on the uncompressed workstation so we now
had a "before" and "after" sequence for PSNR analysis.
As a matter of
background, the CineForm designation of "Filmscan" is
used because these modes are designed to accurately
reproduce the grain characteristics of 35mm and 16mm
film scans. The "Keying" designation is used to
define internal quantization settings for the codec that
are more optimum for recorded material that is intended
for keying. The differences between the "Keying"
and non-"Keying" images will show up later in the
analysis results. Finishing the analogy, the "Filmscan
1 / Keying 1" designations are the modes CineForm
normally recommends for our customers doing film or
digital cinema projects. The "Filmscan 2 / Keying
2" names do offer additional measurable visual fidelity,
but the files are about 40% larger, and any differences
are not visually apparent. |
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Test Results Summary |
|
(Click to
see larger version) |
At left is
the summary PSNR results (click on the chart to
enlarge). Overall PSNR results are calculated
using the ITU-709 luminance conversion algorithm for
signals containing individual channels of of R, G, and
B. The higher the numbers, the more accurately the
compressed signal approaches the uncompressed source
signal. The red
line is the HDCam SR PSNR performance resulting from the
SQ-440 Mbps mode. The variation in its PSNR is
characteristic of fixed bitrate algorithms which adjust
quality up and down based on varying scene complexity.
The other lines on the
chart show the PSNR for the four different CineForm 444
recording modes. Notice the PSNR for each remains
relatively constant as is characteristic of variable
bitrate (constant quality) algorithms.
CineForm 444 in Filmscan
1 mode exceeds the quality of SQ-440 Mbps by about 3dB.
CineForm 444 in Filmscan 2 mode exceeds the quality of
SQ-440 Mbps by about 5 dB. |
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Individual Charts of
Test Results |
HDCam SRW1 - PSNR of individual RGB
channels |
 |
|
Above is the
PSNR graph for the individual R, G, and B channels from
the SRW1, and below are the same graphs for the
individual channels of the four separate recordings of
CineForm 444. To the right of each graph are the
statistics for (calculated) luminance plus the
individual R, G, and B channels.
Some
interesting observations about the PSNR of the
individual CineForm graphs are worth noting. In
each of the sets of two charts, you'll notice that the
green channel in the leftmost chart (of the two) remains
the same as the "keying" chart to its immediate right.
The difference between the two is that the red and blue
channels rise about 4dB in the keying mode as part of
the algorithm differences optimized for keying. In
the non-keying modes the algorithm assigns different
weights to red and blue based on the characteristics of
the human visual system. |
|
CineForm 444 - -
Individual channels of RGB for
Filmscan 1 (L) and Keying 1 (R). |
 |
CineForm 444 - - Individual channels
of RGB for
Filmscan 2 (L) and Keying 2 (R). |
 |
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Final Observations |
| The implications
of the test results are straightforward. Recording
direct-to-disk using CineForm 444 as an alternative to
HDCam SR will not compromise the fidelity of the
recorded material. Specifically, these results
show that recorded fidelity of CineForm 444 is higher
than the same images recorded to tape. Although
tape remains the dominate recording solution in the
market, as the production world moves towards IT
solutions, it is important to realize that disk-based
compression can deliver necessary visual fidelity.
Some other workflow
benefits of are worth noting:
- An entire two hour
feature can be mastered to a single 350GB hard drive
for long term archive which offers longer shelf life
than tape ,
- Lossless duplication
for storage redundancy and data migration is simple
,
- Delivery for
film-out or mastering to other delivery formats is
straightforward because the source material is
always online, and
- drives now cost less
than the equivalent storage on tape(s).
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