Wednesday, July 25, 2012

Thoughts on Blackmagic Cinema Camera

Blackmagic Cinema Camera is to be released in a few days, on 27th of July to be exact. I have been following the discussions about this camera in forum and here are my two cents on this topic.

Since 2008 when Vincent Laforet shot the famous Reverie short that started the whole DSLR film mania, little has happened camera-wize on the low-budget end of nice looking filmmaking. Hacks for Canon cameras and Panasonic GH2 have opened some additional possibilities with higher bit-rates and other rather insignificant features but nothing really exiting has happened. It seems that Canon has dropped the ball in DSLR development because new camera models have brought absolutely nothing new feature or quality wise. Probably it is so because of things like C300 and C500 but they are 5x more expensive than say a 5dmkII.

In may 2012, Blackmagic Design, company best known for video capture cards, converters etc, announced their plans to make a 2.5K CMOS sensor camera that shoots CinemaDNG raw for only 2995$. And not only announced, they showed a working prototype also! RED once also promised 3K for 3K$ but this utopia never realized and Scarlet became the somewhat lighter twin brother for Epic with a much more heftier price tag than 3K$.

So, only three months ago Blackmagic came out of nowhere with it's new camera and surprised everyones socks off. What's so interesting about this camera besides the relatively cheap price?

2.5K CMOS sensor

BMCC has an approximately micro-4/3 sized sensor that measures 15.81 x 8.88 mm with 2432x1366 elements. I say elements because it is not absolutely correct to call these pixels as they don't give you RGB data for each one. As a CMOS sensor, it is basically a single sensor overlaid with Bayer filter. This means that each element "sees" either green, red or blue depending on the position. Most common Bayer filter has twice as many green elements as red or blue because green is most helpful for luminance recording and human eye is also most sensitive to green color.

One could argue that Bayer sensor is similar to 4:2:2 chroma subsampling but this is wrong because they are different schemes for completely different purposes. Chroma subsampling means throwing chroma samples in YCrCb color space which takes place after all the other image reconstruction processes have been done. Bayer pattern with it's distribution of green, red and blue cells contains information about real RGB values and captures as much information as possible with given sensor.

Constructing full RGB image from Bayer pattern is called debayering. Raw image from Bayer sensor is basically monochromatic, it does not have colors as such because each element registered only the light level it received through Bayer filter. Debayering algorithms use clever schemes to interpolate the missing values for all RGB colors and thus reconstruct full RGB raster image. Very basic algorithms do simple interpolation between elements of single color, advanced algorithms use all surrounding elements regardless of "color" and use local gradients to choose, which direction should be chosen for interpolation. It might be called inverse gradient-strength weighted interpolation or something in that direction...

So, as this camera has a Bayer pattern sensor, it does not really have the 2.5K image color-resolution wise. But depending on the debayer algorithm, it comes close. It will be a wonderful fullHD camera for sure and possibly even beyond that. Filming for 2K digital cinema will probably be no problem because even Avatar was up-scaled from 1920 pixels to fill the 1998 pixel wide DCP container. Avatar was filmed mostly with Sony F35 cameras that output true 1920x1080 RGB image (debayered and downscaled from 3.5K sensor) but debayered and downscaled BMCC image probably comes pretty damn close.

Next post will be about dynamic range and encoding.

No comments:

Post a Comment