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  • If you put a colorful image into photoshop or instagram and blur it, youll see a weird,

  • dark boundary between adjacent bright colors.

  • Yuk!

  • In the real world, out of focus colors blend smoothly, going from red to yellow to green

  • not red to brown to green!

  • This color blending problem isn’t limited to digital photo blurring, either – pretty

  • much any time a computer blurs an image or tries to use transparent edges, youll see

  • the same hideous sludge.

  • There’s a very simple explanation for this ugliness – and a simple way to fix it.

  • It all starts with how we perceive brightness.

  • Human vision, like our hearing, works on a relative, roughly logarithmic scale: this

  • means that flipping from one light to two changes the percieved brightness a TON more

  • than going from a hundred and one to a hundred and two, despite adding the same physical

  • amount of light.

  • Our eyes and brains are simply better at detecting small differences in the absolute brightness

  • of dark scenes, and bad at detecting the same differences in bright scenes.

  • Computers and digital image sensors, on the other hand, detect brightness purely based

  • on the number of photons hitting a photodetectorso additional photons register the same

  • increase in brightness regardless of the surrounding scene.

  • When a digital image is stored, the computer records a brightness value for each colors

  • red, green and blueat each point of the image.

  • Typically, zero represents zero brightness and one represents 100 percent brightness.

  • So 0.5 is half as bright as 1, right?

  • NOPE.

  • This color might LOOK like it’s halfway between black and white, but that’s because

  • of our logarithmic visionin terms of absolute physical brightness, it’s only

  • one fifth as many photons as white.

  • Even more crazy, an image value of 0.25 has just one twentieth the photons of white!

  • Digital imaging has a good reason for being designed in this darker-than-the-numbers-suggest

  • way: remember, human vision is better at detecting small differences in the brightness of dark

  • scenes, which software engineers took advantage of as a way of saving disk space in the early

  • days of digital imaging.

  • The trick is simple: when a digital camera captures an image, instead of storing the

  • brightness values it gives, store their square rootsthis samples the gradations of dark

  • colors with more data points and bright colors with fewer data points, roughly imitating

  • the characteristics of human vision.

  • When you need to display the image on a monitor, just square the brightness back to present

  • the colors properly.

  • This is all well and gooduntil you decide to modify the image file.

  • Blurring, for example, is achieved by replacing each pixel with an average of the colors of

  • nearby pixels.

  • Simple enough.

  • But depending on whether you take the average before or after the square-rooting gives different

  • results!!

  • And unfortunately, the vast majority of computer software does this incorrectly.

  • Like, if you want to blur a red and green boundary, you’d expect the middle to be

  • half red and half green.

  • And most computers attempt that by lazily averaging the brightness values of the image

  • FILE, forgetting that the actual brightness values were square-rooted by the camera for

  • better data storage!

  • So the average ends up being too dark, precisely because an average of two square roots is

  • always less than the square root of an average.

  • To correctly blend the red and green and avoid the ugly dark sludge, the computer SHOULD

  • have first squared each of the brightnesses to undo the camera’s square rooting, then

  • averaged them, and then squared-rooted it backlook how much nicer it is!!

  • Unfortunately, the vast majority of software, ranging from iOS to instagram to the standard

  • settings in Adobe Photoshop, takes the lazy, ugly, and wrong approach to image brightness.

  • And while there are advanced settings in photoshop and other professional graphics software that

  • let you use the mathematically and physically correct blending, shouldn’t beauty just

  • be

  • the default?

If you put a colorful image into photoshop or instagram and blur it, youll see a weird,

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B1 brightness image digital square dark average

Computer Color is Broken

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    林宜悉 posted on 2020/03/28
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