![]() ![]() The overlapping shapes show that the relationship cannot be modeled using a single curve. In the 2D histogram, brighter areas indicate where more pixels of a given input brightness are mapped to the same output. (d) HDR+, which preserves details everywhere. (c) Global curve optimizing for the subject. (a) The original, “un-tone-mapped” image. However, for an HDR scene with details in both shadows and highlights, no single curve is satisfactory.ĭifferent ways to tone-map a linear RGB image. The simplest method to reveal more detail is to apply a “global curve”, remapping all pixels with a particular brightness to some new value. While the linear RGB image contains detail in both the dark motorcycle and bright sky, the dynamic range is too high to see it. What gives HDR+ images their signature look is the "tone mapping" of this image, reducing the range to 8 bits and making it suitable for display.Ĭonsider the backlit photo of a motorcyclist, below. These images are aligned and merged to reduce noise in the shadows, producing a 14-bit intermediate “linear RGB image” with pixel values proportional to the scene brightness. When the user presses the shutter in the Pixel camera app, it captures 3-15 underexposed images. Live HDR+ on Pixel 4 and 4a helps the user compose their shot with a WYSIWYG viewfinder that closely resembles the final result. Together, Live HDR+ and dual exposure controls provide HDR imaging with real-time creative control. We also created dual exposure controls, which generalize the classic “exposure compensation” slider into two controls for separately adjusting the rendition of shadows and highlights. This provides a real-time preview of the final result, making HDR imaging more predictable. Starting with Pixel 4 and 4a, we have improved the viewfinder using a machine-learning-based approximation to HDR+, which we call Live HDR+. For example, bright white skies in the viewfinder might appear blue in the HDR+ result. Until recently, one challenge with HDR+ was that it could not be computed in real time (i.e., at 30 frames per second), which prevented the viewfinder from matching the final result. On Pixel phones, the engine behind HDR imaging is HDR+ burst photography, which involves capturing a rapid burst of deliberately underexposed images, combining them, and rendering them in a way that preserves detail across the range of tones. High dynamic range (HDR) imaging is a method for capturing scenes with a wide range of brightness, from deep shadows to bright highlights. Posted by Jiawen Chen and Sam Hasinoff, Software Engineers, Google Research ![]()
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