Jim, I must confess that I share your apparent frustration with Dalsa.
Most notably, I am frustrated by their claims of having created a 4k camera.
The fact is that their "breakthrough" 4k camera truly has no more than a
MAXIMUM of 4 megapixels of resolving power. This is due to the inherent
drawbacks of using a Bayer pattern CFA based sensor. Many people reading
this may think they already know about these problems, but they most likely
attribute them simply to sharpening, chroma subsampling, or compression.
This is very often not the case. If anyone is interested in knowing more
about what I am talking about, please read on. Some of the problems induced
by Bayer pattern sampling are explained below. Those of you with a passion
for using three-chip cameras, read on; you'll get a kick out of this.
To understand this, let's first consider the true meaning of the word "pixel".
The word "pixel" comes from the two words "picture element". A pixel is the
building "block" of a digital picture. As we all know by now, each pixel is
made up of three color components: R, G, and B.
The big confusion in megapixels is created when manufacturers call a color-
blind photodiode a pixel. A photodiode is NOT an entire pixel, it's true
effect is that of a single color component of a pixel. A photodiode cannot
detect color, only brightness. Therefore, a color filtering dye must be placed
over the photodiode so that it will only see ONE of the primary colors. Three
of these photodiodes are necessary to produce a full color pixel. So, in
reality, the so-called 8 megapixel Dalsa camera is more correctly called an
8 megaphotodiode camera.
Each photodiode is, however, treated as though it were a full color pixel by
filling in the two missing color component values. This is done by examining
the nearest photodiodes that are of the same color as the missing components.
For example, if finding the missing red and blue values for a green photodiode,
look at the nearest red and blue photodiodes and use their values. This is
known as interpolation and it is basically used in trying to determine the
missing values that were most likely to have been in a certain location.
Virtually every single-chip digital camera (including Dalsa's) uses a Bayer
pattern CFA. In every Bayer pattern CFA, there are just as many green "pixels"
as there are red and blue "pixels" combined! That means that an 8 megapixel
camera, for example, truly has only 4 million green "pixels", 2 million red
"pixels" and 2 million blue "pixels". Bayer pattern CFAs were designed to
contain more green "pixels" to take advantage of the fact that the human
eye (and brain) perceives most of an image's detail from the green wavelengths
of the spectrum. So, in simple terms, the Bayer pattern CFA provides more
green "pixels" on the sensor to give the image it's detail and the red and
blue "pixels" are there to provide color to the image.
Now, because the green component of an 8 mega"pixel" image was captured with
only 4 million photodiodes, and because green provides the detail for the
image, virtually all single-chip digital cameras have only half the advertised
pixel count; and that is true only under ideal conditions.
Now you may be asking, "If the red and blue components only have 1/2 the
pixel count of the green component, then why do they appear to be the same
resolution as the green component?". The simple answer to this question is
"edge detection". Because the green component has the most detail, it's edges
are carefully copied to the red and blue components. This process not only
has the effect of increased resolution; it also helps to hide the artifacts
caused by sampling the three color components for a single pixel from differing
locations on the xy axis of the sensor.
If you don't believe this to be true, you can prove to yourself that it is
true. Simply use you Bayer CFA based digital camera to take a picture of a
completely red colored object on a black background. Because the green channel
will contain no edge detail, no edges can be copied from it. That means that
the red channel must be left untouched and therefore shows it's original edge
detail. Hence, you will notice an increased softness and pixelation. This is
NOT due to JPEG subsampling of the red component. This can be proven by looking
at the red component in an area where the green component contained an edge.
It will have a sharper edge than the area where green had no edge. There are
many irreparable artifacts introduced by copying edges from one component to
another; but they are too complicated to go into at this time.
This ultimately means that with ANY Bayer CFA based digital camera, if you
take a picture of a completely red or blue object on a black background, the
actual pixel count will be 1/4 of the advertised pixel count. This also means
that if you were to take a picture of a completely gray scene, the green
component would be able to provide almost completely accurate edge data to
the red and blue components with very few adverse effects; thus providing a
nearly completely accurate 1/2 advertised pixel count. A gray scene is an
example of the most ideal condition for a Bayer CFA. A completely red or
blue object on a black background is an example of the worst possible
condition for a Bayer CFA. So, when using a Bayer CFA based digital camera,
the best possible pixel count that can be expected is 1/2 of the advertised
pixel count and the worst that can be expected is 1/4 the advertised pixel
Now you are probably asking "So, if an 8 mega"pixel" image really only has a
maximum pixel count of 4 megapixels, then why would an 8 megapixel image be
needed to maintain ALL of the detail?". This is because the green "pixels"
on a Bayer pattern CFA based sensor are basically arranged in a diagonal
fashion. Every column and every row contains a green photodiode. This is not
the case with red and blue photodiodes. If you remove any column or row to
reduce the size of the image, you will be removing a green "pixel". Green
"pixels" provide the image with it's detail, so removing them should be
I hope this can be understood easily enough. It's quite a complicated subject,
so I apologize if I haven't made everything clear.
So, Dalsa, I suppose you just forgot to mention anywhere the fact that your
camera has unavoidable artifacts and lower resolution than advertised due to
the use of a Bayer based CFA?
Oh, I must also say that I am annoyed by Dalsa's claims of
"12 stops of Latitude". Has anyone else noticed the many
discrepancies on Dalsa's web site? I will cite some examples
from here http://www.dalsa.com...n/dc_sensor.asp
and here http://www.dalsa.com...igin/origin.asp
First, they say "This gives us more dynamic range, which
means much wider exposure latitude than any other cinema-
tography sensor, CCD or CMOS.". Doesn't that sound as if
they are implying that their sensor now has a larger
dynamic range than negative film stocks? I mean, after
all, isn't film considered a "cinematography sensor"?
Later, on the same page, they say "Origin's exposure
latitude is comparable to the best film stocks". So now
it's only comparable and not better?
Second, they say that their camera "...offers at least 12
stops of linear response...". On another page, they say
their sensor offers "...more than 12 stops of exposure
latitude...". You'll notice great ignorance at work here.
In one place, they use the phrase "linear response" which
is basically the same thing as "dynamic range". In another
place, they use the phrase "exposure latitude". Now somebody
please correct me if my many years of experience have
provided me with incorrect knowledge: dynamic range and
exposure latitude are two different things!
Dynamic range refers to the total number of stops between
the brightest white and darkest black a sensor can sense
in a single exposure. Exposure latitude refers to the
number of stops left over after subtracting the displayed
dynamic range from the total dynamic range. A good example
of this is: if your displayed dynamic range is 12 stops
and you have a total dynamic range of 12 stops, then you
have no latitude. But, if your displayed dynamic range is
only 5 stops, then you can move that range around inside
of the total dynamic range a total of 7 stops.