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The quality analysis
comparison performed between CineForm's 10-bit
Visually Perfect codec and the DVCPRO HD codec
was performed at CineForm's facilities by
CineForm employees. The analysis uses
visual inspection, image differencing, and PSNR (peak signal-to-noise ratio)
calculations as
measures of image quality. PSNR analysis uses a
standard mathematical model to measure an
objective difference between two images.
It is commonly used in the development and
analysis of compression algorithms, and for
comparing visual quality between different
compression systems. PSNR is calculated by the
following formula:
PSNR = 20*log10
(255 / RMSE) // For 8bit inputs.
Where RMSE is
the square root of the mean squared error
for the entire image.
Pseudo code (for
those who wish to perform their own PSNR
analysis):
For (each pixel)
{
Difference = Pixel from Image A
– Pixel from Image B
SummedError = SummedError +
Difference * Difference
}
MeanSquaredError = SummedError / Number of
Pixels
RMSE = sqrtt (MeanSquaredError)
PSNR = 20*log10 (255 / RMSE)
For the Quality results shown
in the table, PSNR was calculated on the
uncompressed luma (Y) channel of the YUV signal. Luma
is the best channel for showing errors the human
eye is most likely to perceive. Also, by
analyzing the luma channel directly we avoid
any PSNR error that may occur in YUV to RGB
conversion; therefore we get a more accurate
measure of the codec’s quality performance.
NOTE: The Luma
channel for both sources (CFHD 10-bit and DVCPRO
HD 8-bit) was scaled up to a 16-bit value for
analysis (i.e. the data was normalized –
requiring 255 in the formulas to be 65535.) If
you perform your own analysis, be weary that
most off-the-shelf PSNR tools will only
consider 8-bit sources.
PSNR was calculated
using the first-captured image as the reference
image, not the original 1920x1080 source image. The reason for this is the DVCPRO HD codec
always resamples 1920x1080 frames to 1280x1080,
substantially reducing (by 1/3) its spatial
resolution. Consequently, using the
original image as the reference for PSNR
analysis would have further reduced the measured
PSNR of the DVCPRO HD codec. Consequently
the PSNR analysis uses the first-captured image
as the reference for both codecs.
To perform each step
in the multi-generation testing, a 1920x1080
uncompressed RGB frame was converted to 10-bit
YUV 4:2:2 and presented to each codec. For
testing the DVCPRO HD codec the data was sent across HD-SDI for
compression in the AJ-HD130DCP DVCPRO HD deck.
For testing CineForm's CFHD codec the
uncompressed YUV source was presented to the
production version of CineForm's software codec
that ships with Prospect HD. This effectively simulates a DVCPRO HD camera
acquiring its first image, or the CineForm codec
acquiring its first image from an uncompressed
camera head. By performing the PSNR analysis in
this way we are testing the performance of each codec for multi-generational characteristics – i.e. we
are testing how well it preserves the capture
quality through a series of rendering stages.
No additional compression was used in the signal
processing paths of either codec.
The PSNR graph on
the Quality Analysis page shows the boundary
generally considered the dividing point between
"visually lossless" and "visible". This boundary is
subjective, as some people will see
errors that others will not. In practice,
it becomes very difficult to
see differences for PSNR greater than 50dB between the source and the
compressed image.
The difference
images were calculated by comparing the source
RGB image to the resulting compressed images.
(Remember, the PSNR analysis used the
first-captured image, not the original image). This is why both codecs show some loss in their
first generation. Some small differences
are due to the RGB-to-YUV conversion (common to most
compressed video formats), however most of
the error shown with the DVCPRO HD codec indicates the
lower resolution nature of this compressed
format. The difference images where created by extracting
uncompressed frames from the timeline (in RGB.) These
frames were loading into an image-processing
tool (GIMP) where the source and
resulting compressed images were "differenced"
(not subtracted which can hide errors.) These
difference images were lifted to a middle grey
by setting the brightness to 127, a luminance
region where
the eye is more sensitive. In order to
enhance the visibility of codec-induced
errors, the
contrast for all difference images was lifted
about 400%. |
