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Gamma color machine vision12/17/2023 This represents the net effect of all gamma values that have been applied to an image, and is also referred to as the "viewing gamma." For faithful reproduction of a scene, this should ideally be close to a straight line (gamma = 1.0). A higher display gamma results in a darker image with greater contrast.ģ. The main purpose of the display gamma is to compensate for a file's gamma - thereby ensuring that the image isn't unrealistically brightened when displayed on your screen. This refers to the net influence of your video card and display device, so it may in fact be comprised of several gammas. It redistributes native camera tonal levels into ones which are more perceptually uniform, thereby making the most efficient use of a given bit depth.Ģ. This is applied either by your camera or RAW development software whenever a captured image is converted into a standard JPEG or TIFF file. Depicts a display gamma equal to the standard of 2.2ġ. Depicts an image in the sRGB color space (which encodes using a gamma of approx. The following diagram illustrates how all of this fits together:ġ. Fortunately this second step (the "display gamma") is automatically performed by your monitor and video card. In other words, the purpose of gamma encoding is for recording the image - not for displaying the image. A gamma encoded image has to have "gamma correction" applied when it is viewed - which effectively converts it back into light from the original scene. The next step is where most people get confused, so take this part slowly. GAMMA WORKFLOW: ENCODING & CORRECTIONĭespite all of these benefits, gamma encoding adds a layer of complexity to the whole process of recording and displaying images. If linear encoding were used instead, 8X as many levels (11 bits) would've been required to avoid image posterization. However, real-world images typically have at least 256 levels (8 bits), which is enough to make tones appear smooth and continuous in a print. This also ensures that subsequent image editing, color and histograms are all based on natural, perceptually uniform tones. On the other hand, the gamma encoded gradient distributes the tones roughly evenly across the entire range ("perceptually uniform"). Notice how the linear encoding uses insufficient levels to describe the dark tones - even though this leads to an excess of levels to describe the bright tones. Note: Above gamma encoded gradient shown using a standard value of 1/2.2 See the tutorial on bit depth for a background on the relationship between levels and bits. Otherwise, an excess of bits would be devoted to describe the brighter tones (where the camera is relatively more sensitive), and a shortage of bits would be left to describe the darker tones (where the camera is relatively less sensitive): Since gamma encoding redistributes tonal levels closer to how our eyes perceive them, fewer bits are needed to describe a given tonal range. Gamma encoded images store tones more efficiently. This formula causes the blue line above to curve. Technical Note: Gamma is defined by V out = V in gamma, where V out is the output luminance value and V in is the input/actual luminance value. When a digital image is saved, it's therefore "gamma encoded" - so that twice the value in a file more closely corresponds to what we would perceive as being twice as bright. Otherwise the typical range in brightness we encounter outdoors would be too overwhelming.īut how does all of this relate to gamma? In this case, gamma is what translates between our eye's light sensitivity and that of the camera. There's a biological reason for this peculiarity: it enables our vision to operate over a broader range of luminance. For extremely dim scenes, such as under starlight, our eyes begin to see linearly like cameras do.Ĭompared to a camera, we are much more sensitive to changes in dark tones than we are to similar changes in bright tones. Actual perception will depend on viewing conditions, and may be affected by other nearby tones. Accuracy of comparison depends on having a well-calibrated monitor set to a display gamma of 2.2. Refer to the tutorial on the photoshop curves tool if you're having trouble interpreting the graph. Reference Tone Select: Perceived as 50% as Bright by Our Eyes
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