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Thursday, August 25, 2011

••◊ GoPro - It Certainly Does Go.

Clarence bought a GoPro camera last weekend and let me borrow it for the week to figure out the work flow to integrate the footage into our normal HDSLR productions.  The first thing I had to do it pick this little action camera apart and figure out how it really works.


The first items I checked for are exposure lock and white balance.  No check on either.  Everything is auto, i.e. designed for common consumers as opposed to advanced consumers or professionals  What this means is that the camera is no-go for time lapse or high contrast night scenes.  Notice how all the GoPro example videos on their site are outdoors during the daytime?

Also the lens on the GoPro HD has a fixed focal length - EXTREME wide - and doesn't seem to include a mechanical aperture.  There's a reason that the footage is very jerky and artificial.  They are just adjusting shutter speed in order to control exposure and in daylight that shutter speed is just a small fraction of the frame rate - thus jerky motion.  What I haven't explored is frame rate conversion to 23.976fps (24p) for film making.  It might require some motion blur added in post to control the motion and not make it so jerky.  This is good and bad.  A fast shutter speed makes it easier to frame rate convert for artificial slow motion using Twixtor/Optical Flow/Pixel Motion .

The next area I wanted to look at is the color rendition.  There are two 1080p HD modes on the camera, PAL at 25fps (25p), and NTSC at 29.97fps (30p). Normally a prosumer camera would translate the internal camera capture color for the color differences between PAL (Europe) and NTSC (North America) displays, however from the two photos below you can see that there is no color difference between the PAL and NTSC mode.  Essentially the GoPro camera is ignoring color rendering difference between a PAL and NTSC (also rec.709 HD) display.  What is really surprising is that they don't offer a rec.709 (modern HDTV) or sRGB mode (PC) options.The top picture is from PAL mode and the lower picture is from NTSC mode.  What this means to the post work flow is basically *good luck* trying to get the footage to match to other cameras.  It's going to take tweaking and lots of it.

With just a glance at the gray tiles on the test chart you can see that this camera only has about 5-6 stops (my estimate) of exposure latitude.  Can't take this thing into a contrast-y scene without serious highlight blow out or shadow crushing.  Not necessarily a deal killer, just something to be aware of.



Now for a more scientific view of the color rendition I turn to my handy dandy Excel based vectorscope.  The first chart compared sRGB (PC display) with the GoPro output.  The circles are the desired color targets and the diamonds are the actual measured colors.  Skin tones look pretty good, however the primaries are way off.  Red, yellow, and green are all over saturated and miss their hue mark.  Magenta has the right hue, but is slightly under satured.  Blue is WAY off on hue and compressed toward cyan.  Cyan is WAY, WAY over saturated.  This tells me that the camera is going to have clipped color channels because of the over saturated color rendition they chose to implement.  Since the color seems to be so uncontrolled, it leaves me wondering what will happen when we try to use two of these cameras at the same time.  Will they render color in the same way?  Dunno, but will find out soon.  


Rec.709 (HDTV display) color rendering has about the same issues in the same places.


So what's really excellent about this glorified web-cam?  The accessories.  You can mount this camera to just about anywhere and it's indestructible.  A team used the camera on the 48 hour film project recently and the camera fell off the car mount while the car was moving along at speed.  The camera still worked after getting flung onto the asphalt.  Try that with your $4,000 HDSLR!  You can get the aforementioned car mount, surf board mount, bicycle mount, helmet mount, or just use a set of zip ties and duct tape.  You can even get an LCD display that mounts on the back to frame your shots and review footage in the field.  ...and for the really adventurous cross-eyed sort there's the 3-D configuration!  The accessory people at GoPro really know how to capture the market.

We're planning on strapping a couple of these cameras to fast Porsches and letting the cars rip along in a chase scene in the next few weeks.  Should be fun ...and if the cameras fall off, no big deal.

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