💡🎬HDR

💡🎬HDR

Since you should now understand the concept of gamma from previous sections, let’s dive into some alternate EOTFs.

HDR can mean several different things:

  • A method of capturing a high contrast scene with multiple photographs then “tone mapping” them all into a visible range, sometimes with extreme stylistic consequences.
  • A method of capturing a high contrast scene’s lighting for use in photo-based illumination for lighting 3D visual effects elements with the lighting of the actual scene.
  • A technology and process used to show higher dynamic range than has previously been possible with limited dynamic range displays. This is what we’re talking about here.

Historically we’ve only had 6 stops or so to work with. The standard SDR (Standard Dynamic Range) is only about 100 nits when calibrated to spec. With HDR displays, we’re often talking ten times that amount.

But HDR isn’t about blinding an audience with a sunset or neon signage. In my view, it’s more exciting in how it can create an increase in perceived textural detail. Areas of specularity that were previously white blobs can reveal surface texture in, sometimes subtle, but improved ways.

Not everyone is a fan of HDR or believes it to make all that marked a difference.

Linear isn’t really 1:1 if it’s increasing according to stops (a log scale)

With linear recording, your brightest stop contains as much information as the entire rest of the picture

The problem arises with the realization that the exposure values with which we measure light are not linear. An exposure “stop” represents a doubling or halving of the actual light level, not an increase by some arbitrary linear value on a scale. You could, in fact, say exposure values are themselves logarithmic!

So, working backward, using the camera example: in a linear recording, the voltages coming from each pixel off the camera’s sensor are assigned one of those 256 values, based on voltage level, which is determined by the actual exposure level of the image focused by the lens. Now, since exposure values are logarithmic, the voltage levels being output by the sensor to the A/D converter reflect that. And when those voltages arrive at the A/D converter, it records them using a linear function. The downside is that this method top-loads the 256 values, and the dynamic range won’t be equally represented across the 8 bits.