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A clever idea, however his data source is irrevocably corrupted by the auto exposure and auto white balance of his webcam. The result is, sadly, more influenced
by sjwright 15y ago
A clever idea, however his data source is irrevocably corrupted by the auto exposure and auto white balance of his webcam. The result is, sadly, more influenced by the algorithms in his camera than the sky itself.
Notice how there's some outrageous and short-lived tones during sunrise and sunset -- that's the white balance jumping around (Possibly an automatic saturation gain too?)
Then notice how the brightness doesn't peak and fall during the day, instead jumping quickly from darkness to "full" brightness with frequent dramatic shifts far greater than reality would allow. That's because the moving cloud cover is triggering the auto exposure to jump around incessantly.
The ideal fix would be to replace the webcam with a digital camera capable of tethered shooting and fully manual operation. You'd fix the shutter, aperture and ISO values to a level that minimizes clipping, set the white balance to daylight, and point the camera in a direction least likely to be in the sun's path (in the southern hemisphere, that's south).
In order to capture the dynamic range fully, you could allow the camera to automatically choose the shutter speed, then compensate in the algorithm by reading the shutter speed from the EXIF data.
That said, full points for the idea, and for a great web app to visualize the data!
- seabee 15y agoSome webcams have manual settings, I know mine does. Just enabling those should get you 90% of the way of a tethered digital camera, "all that remains" is getting sufficient dynamic range and fixing the curves/compensating for human perception.
- sjwright 15y agoMany cheap/consumer devices let you disable some automatic adjustment, but rarely do they let you disable everything. For example, I've seen webcams that can fix the white balance but have automatic saturation adjustment that can't be turned off. The cheap option that comes to mind would be an old and/or second hand Canon PowerShot capable of running CHDK firmware. Ask friends and family; you'd probably be able to score one free from a dusty cupboard draw.
- sneak 15y agoGood insight... but New York City is in the northern hemispshere.
- sjwright 15y agoYeah, which is why I qualified it. I assume it's the opposite and the sun drops to the south in a northern winter, but I simply don't know if my assumption is right.
- sp332 15y agoYup, I live in northeast USA and the sun has a distinct southern slant even in summer. In winter it hardly gets over the trees.
- joshwa 15y agoNotice how there's some outrageous and short-lived tones during sunrise and sunset -- that's the white balance jumping around (Possibly an automatic saturation gain too?) The ones at sunrise are weird (blue throws off the white balance) but the sunset ones are actually accurate-- that's the color of a NY sunset in the summer. Surreal, eh? I miss New York.
- yid 15y agoHow I wish more grad students dissected empirical data like this!
- hammock 15y agoI think the word you are looking for is irreparably, not irrevocably. And it looks like, more than the auto-white balance thing, he's getting washed-out skies from overexposure. This is a very common problem in photography- ever take a photo somewhere with a blue sky dotted with clouds but in the photo the sky comes out solid white or gray? Go back and look at some of your old photos, you will notice it. There is no easy solution without sacrificing exposure of the lower half (what is usually the subject) however in this case the subject IS the sky, so select a camera that has manual exposure, set it a little lower and you'll be all right.