COLOR

  • Eye retina’s Bipolar Cells, Horizontal Cells, and Photoreceptors

    In the retina, photoreceptors, bipolar cells, and horizontal cells work together to process visual information before it reaches the brain. Here’s how each cell type contributes to vision:

     

    1. Photoreceptors

    • Types: There are two main types of photoreceptors: rods and cones.
      • Rods: Specialized for low-light and peripheral vision; they help us see in dim lighting and detect motion.
      • Cones: Specialized for color and detail; they function best in bright light and are concentrated in the central retina (the fovea), allowing for high-resolution vision.
    • Function: Photoreceptors convert light into electrical signals. When light hits the retina, photoreceptors undergo a chemical change, triggering an electrical response that initiates the visual process. Rods and cones detect different intensities and colors, providing the foundation for brightness and color perception.

     

    2. Bipolar Cells

    • Function: Bipolar cells act as intermediaries, connecting photoreceptors to ganglion cells, which send signals to the brain. They receive input from photoreceptors and relay it to the retinal ganglion cells.
    • On and Off Bipolar Cells: Some bipolar cells are ON cells, responding when light is detected (depolarizing in light), and others are OFF cells, responding in darkness (depolarizing in the absence of light). This division allows for more precise contrast detection and the ability to distinguish light from dark areas in the visual field.

     

    3. Horizontal Cells

    • Function: Horizontal cells connect photoreceptors to each other and create lateral interactions between them. They integrate signals from multiple photoreceptors, allowing them to adjust the sensitivity of neighboring photoreceptors in response to varying light conditions.
    • Lateral Inhibition: This process improves visual contrast and sharpness by making the borders between light and dark areas more distinct, enhancing our ability to perceive edges and fine detail.

     

    These three types of cells work together to help the retina preprocess visual information and perception, emphasizing contrast and adjusting for different lighting conditions before signals are sent to the brain for further processing and interpretation.

     

     

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    Read more: Eye retina’s Bipolar Cells, Horizontal Cells, and Photoreceptors
  • Black Body color aka the Planckian Locus curve for white point eye perception

    http://en.wikipedia.org/wiki/Black-body_radiation

     

    Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.

     

    A black-body at room temperature appears black, as most of the energy it radiates is infra-red and cannot be perceived by the human eye. At higher temperatures, black bodies glow with increasing intensity and colors that range from dull red to blindingly brilliant blue-white as the temperature increases.

    The Black Body Ultraviolet Catastrophe Experiment

     

    In photography, color temperature describes the spectrum of light which is radiated from a “blackbody” with that surface temperature. A blackbody is an object which absorbs all incident light — neither reflecting it nor allowing it to pass through.

     

    The Sun closely approximates a black-body radiator. Another rough analogue of blackbody radiation in our day to day experience might be in heating a metal or stone: these are said to become “red hot” when they attain one temperature, and then “white hot” for even higher temperatures. Similarly, black bodies at different temperatures also have varying color temperatures of “white light.”

     

    Despite its name, light which may appear white does not necessarily contain an even distribution of colors across the visible spectrum.

     

    Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, and it is believed that they emit black-body radiation (called Hawking radiation), with a temperature that depends on the mass of the hole.

     

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    Read more: Black Body color aka the Planckian Locus curve for white point eye perception
  • Scene Referred vs Display Referred color workflows

    Display Referred it is tied to the target hardware, as such it bakes color requirements into every type of media output request.

    Scene Referred uses a common unified wide gamut and targeting audience through CDL and DI libraries instead.
    So that color information stays untouched and only “transformed” as/when needed.

     

     

    Sources:
    – Victor Perez – Color Management Fundamentals & ACES Workflows in Nuke
    – https://z-fx.nl/ColorspACES.pdf
    – Wicus

     

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    Read more: Scene Referred vs Display Referred color workflows
  • Photography basics: Why Use a (MacBeth) Color Chart?

    Start here: https://www.pixelsham.com/2013/05/09/gretagmacbeth-color-checker-numeric-values/

     

    https://www.studiobinder.com/blog/what-is-a-color-checker-tool/

     

     

     

     

    In LightRoom

     

    in Final Cut

     

    in Nuke

    Note: In Foundry’s Nuke, the software will map 18% gray to whatever your center f/stop is set to in the viewer settings (f/8 by default… change that to EV by following the instructions below).
    You can experiment with this by attaching an Exposure node to a Constant set to 0.18, setting your viewer read-out to Spotmeter, and adjusting the stops in the node up and down. You will see that a full stop up or down will give you the respective next value on the aperture scale (f8, f11, f16 etc.).

    One stop doubles or halves the amount or light that hits the filmback/ccd, so everything works in powers of 2.
    So starting with 0.18 in your constant, you will see that raising it by a stop will give you .36 as a floating point number (in linear space), while your f/stop will be f/11 and so on.

     

    If you set your center stop to 0 (see below) you will get a relative readout in EVs, where EV 0 again equals 18% constant gray.

     

    In other words. Setting the center f-stop to 0 means that in a neutral plate, the middle gray in the macbeth chart will equal to exposure value 0. EV 0 corresponds to an exposure time of 1 sec and an aperture of f/1.0.

     

    This will set the sun usually around EV12-17 and the sky EV1-4 , depending on cloud coverage.

     

    To switch Foundry’s Nuke’s SpotMeter to return the EV of an image, click on the main viewport, and then press s, this opens the viewer’s properties. Now set the center f-stop to 0 in there. And the SpotMeter in the viewport will change from aperture and fstops to EV.

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    Read more: Photography basics: Why Use a (MacBeth) Color Chart?

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