Image Post-Processing

Tutorial 6 in the Basics of Twilight Render V2 Series

Post-Processing refers to image manipulation performed after a render is completed.  Thes are typically white-balancing or tone-mapping, de-noising, anti-aliasing, or additional effects like vignette, bloom, lens flares, etc..  Twilight Render has an great library of post-processing tools that allow you to edit your render while still inside SketchUp.  A complete rendered image can be finished without ever leaving SketchUp for an outside image editor!

education

 tonemap simple

Tone-mapping allows you to adjust the range of bright pixels and dark pixels in your image.  It lets you tone down washed out highlights, or improve the contrast of shadows.  Twilight Render provides two types of tone mapping: Simple, also known as gamma correction, and Linear.

Simple tone-mapping allows you to adjust two factors, Exposure and Gamma.  Anyone familiar with a camera knows that exposure controls how bright an image is.  Gamma, or gamma correction, is a non-linear adjustment of brightness to compensate for the difference in how our eyes view brightness and how computer monitors display it.

  • Increasing exposure directly increases the brightness of the image.
  • Because exposure increases bright pixels more than dark pixels, increasing the exposure also increases the contrast of the image.
  • Increasing exposure can bring out details in shadowy areas but usually reduces detail in bright areas.
  • Decreasing exposure, therfore, works oppositely by increasing detail in bright areas but reducing detail in dark areas.
  • Because Twilight Render's post-processing works on the high-dynamic range data of your render, you will retain bright highlights much better than image editors that work on low-dynamic range data (the standard image pixels).
  • Gamma works primarily on the mid-tones of your image, the pixels whose brightness falls near the middle.
  • Increasing gamma will brighten your image by pushing the mid-tones brighter.  The overall image brightness increase, but your dark shadows remain dark.
  • Decreasing gamma will darken your image by pusing the mid-tones darker.  The overall image brightness decreases, but your bright highlights remain bright.
 tonemap linear

Linear tone-mapping adjusts the brightness of your image by stretching or squishing the range of brightness.  By adjust the Light or Dark values, you are able to make highlights brighter and shadows darker.  Unlike Gamma, using Simple tone-mapping, which adjust the mid-tones of your image, Linear tone-mapping works primarily on the shadows and highlights of your image.

  • Decreasing the Dark value will reduce the brightness of the darker pixels in your image.  The lighter pixels will also be darkened, but only a little.
  • Increasing the Dark value will increase the brightness of the darker pixles in your image.  Again, lighter pixels will be affected, but not much.
  • Increasing and descreasing the Light value works exactly the same way, only primarily effect the lighter pixels.
  • By pushing the value away from each other (decreasing dark, increasing light), your image becomes polarized toward the extremes.
  • Pushing the values toward each (increasing dark, decreasing light), your image becomes more averaged.
  • Increasing or decreasing the values together (both values are equal) acts the same as changing the Exposure in Simple tone-mapping.

 

standard  dark b 
bright bright b

 

Bloom replicates the behavior of real-world lenses of blurring bright lights into the surrounding area.  This can produce dramatic effect by highlighting the source of light around windows, lights, and cameras, as well as the boundaries between light and shadow.

  • Bloom can be enabled and disabled by checking the box marked 'Bloom'.
  • The brightness of the bloom effect, how much the light 'drowns out' the surrounding area, is controlled by the Brightness value.  Values above 1.0 may cause odd, undesirable effects.
  • Sharpness sets how much the brightness of a pixel effects the Bloom.  Higher values of Sharpness will restrict the bloom effect to bright areas.  Lower values allow the bloom effect to occur in more ambient lit areas.
  • Size sets how wide the area is that is effected by the Bloom.  High values for Size spreads the bloom effect across a greater area (thus reducing the brightness of the effect).  Lower values concentrate the bloom effect into a smaller area and produce a brighter effect.
bloom a  bloom wide 
 bloom tight  bloom strong
 

Temperature Adjustment refers to adjusting the colors of an image to make it more blue or more orange, commonly called 'Colder' or 'Warmer'.  Adjusting the temperature of an image is particularly useful to remove unnatural or unwanted color casts, like the blue in outdoor images, or the yellow from flourescent lights.

  • Temperature Adjustment is enabled by clicking the checkbox next to Temperature Adjust.
  • 'Original' refers to the estimated actual temperature of the lighting in the image.
  • 'New' refers to what the temperature of the lighting in the image appears to be.

While this sounds a little confusing, in practice it's pretty easy.

  1. Decide what temperature the image should have been.  Warmer (orange) temperatures start at 1000 and go up to 5000; Cooler (blue) temperatures run from 10,000 to 7000. 
    • Should your image have been taken under a clear sky?  That's about 6500K
    • Was your image in a room under Flourescent lights?  That's about 4500K
    • Was it a cloudy, overcast day?  That's about 9000K
  2. Decide what temperature the image appears to be.  This can be a little tough, so some trials are probably going to be necessary.  Play around until you get the right tone.
    • Twilight Render's Physical Sky type tends to render with a Cooler (bluish) cast.  Determine what your intended color temperature was (6500 for a clear day) and use that as your 'Original' temperature, then set the 'New' temperature to 8000 (roughly what Physical Sky renders at).
  3. When 'New' is less than 'Original', your image will be Cooler (more blue).  When 'New' is greater than 'Original' your image will be warmer.
temp orig   temp fixed
temp overcast temp sunset

Vignette effect is when the area around the edges of the image is faded out, usually to black, but can be any color.  This effect is usually used to draw attention to the center of the image, or to give emphasis to shape of the image's primary subject.  Twilight Render allows you to add a vignette effect in any color, and to control the size and transparency of the fade.

  • Vignette is enabled by checking the checkbox next to 'Vignette'.
  • Hotspot sets the fraction of the image width and height outside of which the vignette fade begins.
  • Falloff sets the fraction of the image width and height at which the fade ends and vignette opacity is highest.  Note that this number can actually be outside the image boundaries (greater than 1).
  • Alpha sets the maximum opacity of the vignette effect.
 vignette
Monday, April 24, 2017

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