• sRGB vs REC709 – An introduction and FFmpeg implementations

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    1. Basic Comparison

    • What they are
      • sRGB: A standard “web”/computer-display RGB color space defined by IEC 61966-2-1. It’s used for most monitors, cameras, printers, and the vast majority of images on the Internet.
      • Rec. 709: An HD-video color space defined by ITU-R BT.709. It’s the go-to standard for HDTV broadcasts, Blu-ray discs, and professional video pipelines.
    • Why they exist
      • sRGB: Ensures consistent colors across different consumer devices (PCs, phones, webcams).
      • Rec. 709: Ensures consistent colors across video production and playback chains (cameras → editing → broadcast → TV).
    • What you’ll see
      • On your desktop or phone, images tagged sRGB will look “right” without extra tweaking.
      • On an HDTV or video-editing timeline, footage tagged Rec. 709 will display accurate contrast and hue on broadcast-grade monitors.

    2. Digging Deeper

    FeaturesRGBRec. 709
    White pointD65 (6504 K), same for bothD65 (6504 K)
    Primaries (x,y)R: (0.640, 0.330) G: (0.300, 0.600) B: (0.150, 0.060)R: (0.640, 0.330) G: (0.300, 0.600) B: (0.150, 0.060)
    Gamut sizeIdentical triangle on CIE 1931 chartIdentical to sRGB
    Gamma / transferPiecewise curve: approximate 2.2 with linear toePure power-law γ≈2.4 (often approximated as 2.2 in practice)
    Matrix coefficientsN/A (pure RGB usage)Y = 0.2126 R + 0.7152 G + 0.0722 B (Rec. 709 matrix)
    Typical bit-depth8-bit/channel (with 16-bit variants)8-bit/channel (10-bit for professional video)
    Usage metadataTagged as “sRGB” in image files (PNG, JPEG, etc.)Tagged as “bt709” in video containers (MP4, MOV)
    Color rangeFull-range RGB (0–255)Studio-range Y′CbCr (Y′ [16–235], Cb/Cr [16–240])


    Why the Small Differences Matter

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  • What light is best to illuminate gems for resale

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    www.palagems.com/gem-lighting2

     

    Artificial light sources, not unlike the diverse phases of natural light, vary considerably in their properties. As a result, some lamps render an object’s color better than others do.

     

    The most important criterion for assessing the color-rendering ability of any lamp is its spectral power distribution curve.

     

    Natural daylight varies too much in strength and spectral composition to be taken seriously as a lighting standard for grading and dealing colored stones. For anything to be a standard, it must be constant in its properties, which natural light is not.

     

    For dealers in particular to make the transition from natural light to an artificial light source, that source must offer:
    1- A degree of illuminance at least as strong as the common phases of natural daylight.
    2- Spectral properties identical or comparable to a phase of natural daylight.

     

    A source combining these two things makes gems appear much the same as when viewed under a given phase of natural light. From the viewpoint of many dealers, this corresponds to a naturalappearance.

     

    The 6000° Kelvin xenon short-arc lamp appears closest to meeting the criteria for a standard light source. Besides the strong illuminance this lamp affords, its spectrum is very similar to CIE standard illuminants of similar color temperature.