All Things Color for Film and Digital Cinema
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Technical Data

Waveform Vs Histogram interpretation in Digital Cinema Cameras

Understanding how to read Histograms present on many new digital cinema cameras can be tricky and are easily misenterpreted.

For analysis, I will use the below still from the film Daybreakers as reference.

Ethan Hawke in "Daybreakers" (2010)

The following snapshots have been taken using DpxRead available on the Panavision website.

Immediately we can see that there is a massive difference in the way these two images are represented in each of the graphical/statistical graphs.

Histograms represent the volume/percentage of light levels exposed within a particular image. The resulting graph shows the distribution/intensity plot of those levels.

Histogram Exposure

The above image reference image  is quite ‘moody’. The histogram shows us this quite literally but surprisingly shows nothing of where the midtones sit. This is because Histograms work with percentages and Ratios of light. If for the most part an image is dark, say 60% of the overall area , then the rest of the histogram has to be interpreted with the remaining 40 percent of image area. For that reason the intensity represented by the histogram for the remaining light values is visually a lot lower than the Dark spike shown in our reference.

To better show the way a histogram graphs an images lightness values I have put the below gradient into the scopes.

Linear Grayscale Ramp

We can see that because there is an even amount of each light level within the gradient image the Histogram shows an even intensity/distribution of each ‘level’.

Video Waveforms on the other hand give us a lot more visual information with which to evaluate your exposure and contrast ratio. As well as showing us the distribution of light values the graph is also plotted across the horizontal plane of the image. With this additional ‘axis’ one can easily determine where within the frame a particular item sits in its digital exposure value. This makes it easy to find for example the exposure of someones skin tone in relation to the background subject matter.

Histograms are Cheap and Easy to display from a programmers POV but in my opinion are quite useless in representing photographic content and should not be used for indepth exposure analysis.

Adrian


18 Percent Grey …. “Middle Grey” and Magic Numbers

 

    For a while I have been pondering the function of the 18 percent grey card. Why 18%, what is its historical reference…ect.

    After a lot of reference reading I came up with the following ….

    Traditionally 18 % refers to the statistical average reflectance of a photographed scene. ‘Normally’ exposed skin tones also generate an average incident light reading of about 18%.

    Photographically, if you were to make a set of 11 patches starting with 100% reflectivity and each subsequent patch was halved in reflectance, you would end up with a logarithmic scale where the light intensity is being halved each patch. The seventh step, middle grey, would yield a photographic Status M Neg density value of 0.7

    Using our Density math described in previous posts we know Transmission = 1/10^Density   

    Therefor    T=1/10^0.7= 0.18      AND       Transmission is directly proportionate to Luminance L*. Not boring you with the math the result is 49.496.

   So    0.7 Density = 0.18 Transmission = 50% Luminance(L*).

 

 

Fig.A - Status M against Status A Density

Fig.A - Status M against Status A Density

 

 

  Interestingly If we map our 21 step sensitometry readings of a 21 strip grey scale test wedge over Neg and Print densities we see that 0.7D is the cross over point !   See Fig.A

 

 

Fig.C

Fig.B

 

 

 

 

Looking at the image below, Fig.C, you will notice that this point of around 18 % grey is the mid point at which the cineon Log file is expanded when overlayed with a Print emulation 3DLookUp table. Both Mid Grey LAD Patch’s are almost identical as seen on the corresponding waveform Representations. 

 

 

18percent

Fig.C

 

Looking at the graph in Fig.C one can also see the reference point of 18 % on each of the mapped targets averaging around 50% luminance. Interestingly with this chart I have mapped Cineon Log levels against the 2.2 and 2.5 video gamma transfer functions and CIE Luminace L* values. They are all relatively close to each other in their Logarithmic encoding. I can see im going to have to rewrite this as its going to get messy from about here on in, although quite fascinating. 2.5 gamma looks like its the best match for CIE L* but somehow we got stuck with 2.2. Near enough is good enough I guess. It was decided back in the 8Bit video days that to help save on visual data that video/TV could also have a perceptual gamma encoding, once again mimicking the eyes response to nature. A gamma of 2.2 was decided apon.  

 

 

Adrian


Optical Density Explained. Density Pt 2.

As mentioned in Part 1, optical density or ‘log density’ is the scale used for measuring photographic optical ‘lightness’. 

In describing densitometry, three main terms are used. Transmission(T), Opacity(O) and Density(D).

Transmission (T) is a measure of the light passing ability of a material expressed as a percentage. It is calculated by taking the measured transmitted light and dividing it by the incidental light(the light source).                T= transmitted light/incident light

Opacity (O) is the ability of a medium to absorb light. Opacity is measured as     O= 1/transmitted light 

Optical Density (D) is the l                                                                                                  D=Log(O)

                                                               or                  D= (LOG10(1/T) 

Using the basic principles above we can best show the relationship between Transmission, Opacity and Density using a graph mapping out a black to white scale over a number of steps.

10-step-opacity_density

 

To the Left you can see how as the ‘linear’ transmittance of light increases the measured “optical density” of the image decreases. Obviously the more light being allowed through a medium the less ‘dense’ it becomes. 

 

 

 

Adrian


Technical Data

dlad4801This Category will over time will be a collection of technical data collated from the plethora of information out there

regarding formats, imaging acquisition systems, workflows, Color responsiveness and tools and their use in the world

of color management. 

 

Adrian