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--- isp:imagesignalprocessing [2018/05/29 22:41] – [Overall idea] Igor Yefmov
+++ isp:imagesignalprocessing [2018/06/04 12:50] (current) – removed Igor Yefmov
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-======= Image processing =======
-====== White Balance ======
-===== What is white color temperature =====
-A "white" color has different chromatic characteristics, depending on its "temperature". That temperature is traditionally expressed in Kelvin and corresponds to light, emitted by an ideal black body radiator, when heated to that temperature. Think of it as the color of a burning stick of wood at a camp site or a "red-hot iron" that looks more yellow when it actually melts in a furnace.
-{{:isp:color-temperature-1500x600.jpg?600|}}
-When a scene is illuminated by "white" color there's usually color correction needed to bring the "true" colors out. The "true" in this sense is an approximation of how our brains perceive colors during daylight, which is around ''6000''°K.
-For our purposes, we are using individual color channel **gains** to compensate for a given temperature. Lower temperature "white light" needs a lot of blue added to it and very little red and as the temperature climbs up, the amount of added red grows while the added blue goes down.
-===== Color correction for white temperature =====
-For the calibration purposes we have acquired a Philips "Hue White and Color Ambiance A19 LED Starter Kit" that allowed us to test various illumination scenarios for a range of color temperatures. For a given white color temperature setting we have dialed the red and blue gains to make the scene "white" (leaving the green gain at its constant value of ''1024'').
-Corrections to the red channel were way more noticeable than those to the blue one so we approximated the blue gains' graph with a single line, described by the formula ''B = 4205.4 - T*0.4087''.
-Red channel gains are approximated by two line segments, bordering the ''3500''°K mark:
-below ''3500''°K: ''R = 467.6 + T * 0.166'' \\
-above ''3500''°K: ''R = 82 + T * 0.277''
-{{:isp:wb_pig-2.jpg?600|}}
-==== Experimental data and verification ====
-Here is the table with the results of that testing and corresponding calculated values based on the above formulae:
-| ::: ^  Manually set  ^^  Calculated  ^^
-^  Color, K  ^  Red  ^  Blue  ^  Red  ^  Blue  ^
-| ''2000'' | ''800'' | ''3388'' | ''800'' | ''3388'' |
-| ''2296'' | ''840'' | ''3285'' | ''849'' | ''3267'' |
-| ''2592'' | ''913'' | ''3196'' | ''898'' | ''3146'' |
-| ''2888'' | ''959'' | ''3078'' | ''948'' | ''3025'' |
-| ''3184'' | ''1016'' | ''3005'' | ''997'' | ''2904'' |
-| ''3480'' | ''1046'' | ''2858'' | ''1046'' | ''2783'' |
-| ''3776'' | ''1119'' | ''2710'' | ''1128'' | ''2662'' |
-| ''4072'' | ''1195'' | ''2519'' | ''1210'' | ''2541'' |
-| ''4368'' | ''1296'' | ''2460'' | ''1292'' | ''2420'' |
-| ''4664'' | ''1370'' | ''2298'' | ''1374'' | ''2299'' |
-| ''4961'' | ''1458'' | ''2121'' | ''1456'' | ''2178'' |
-| ''5257'' | ''1561'' | ''2033'' | ''1538'' | ''2057'' |
-| ''5553'' | ''1620'' | ''1895'' | ''1620'' | ''1936'' |
-| ''5849'' | ''1750'' | ''1782'' | ''1702'' | ''1815'' |
-| ''6145'' | ''1738'' | ''1694'' | ''1784'' | ''1694'' |
-| ''6500'' | ''1738'' | ''1694'' | ''1882'' | ''1549'' |
-===== Practical example =====
-What does all that mean in practice? Suppose you have set up your scene with "white" lamps that are marked as ''4100''°K. Based on the above information you'd know that to get the best "natural" colors out of that scene means setting your red channel gain to ''1217'' and blue gain to ''2529'' while leaving the green gain at its default ''1024'' value.
-===== Automatic white balancing =====
-Quite often end users don't have the patience or the expertise (or both) to properly set up the white balance of the captured scene and prefer to rely on camera "just knowing what to do".
-Many algorithms are available today to deal with this, some very sophisticated (and computationally expensive) and others, way less computationally expensive but so simple that they are easily thrown off by thins like a huge green screen background or a bright super-yellow T-shirt.
-Below is one of those "simplistic" approaches with a little twist, that makes it an excellent option for real-time image processing on even the most basic of the devices.
-==== Overall idea ====
-The general idea is based around an assumption that if one takes a look around and measures the color of every pixel, then the grand total sum of all those values should be close enough to a pure grey color. This idea has a name and it "the grey world".
-==== Step-by-step algorithm ====

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