In a previous post introducing AccuRaw EXR, the new version of my AccuRaw raw processing software, I mentioned the importance of a "non-clipping workflow" for the promise of the EXR format to be a reality. One of the biggest advantages of the ILM's OpenEXR format is that image highlights aren't clipped, no matter how badly overexposed. This makes manipulating images in multiple tools a lot easier. However, it's not enough to have a image format that doesn't clip highlights - the whole workflow need to avoid clipping highlights.
The need not to clip highlights is why adding EXR support to AccuRaw was more than just a matter of adding an new export format. The core processing engine, and some of the controls, had to change to avoid clipping.
The easiest way to show the difference is with a practical example. The image below, figure 1, is a crop of a shot that I took in La Serena, Chile a number of years ago (while sitting out a weather related flight delay).
 |
| Fig. 1 : The original image with no adjustments. |
The second image, figure 2, is the result of deliberately overexposing the image in AccuRaw, in this case by adjusting the "ETTR" slider. The ETTR slider, as AccuRaw users will know, makes it's adjustments right at the beginning of AccuRaw's image processing pipeline. This allows exposure offsets in the original raw image to be compensated for without introducing any non-linearities - using the exposure slider for this risks a non-linear end result, as the exposure adjustment occurs later in the processing pipeline. But in this case, I'm using the slider to show that:
- AccuRaw EXR's processing engine is non-clipping, and
- AccuRaw EXR's processing engine is highly linear.
 |
| Fig. 2 : The image, 3 stops overexposed. |
The next images are screenshots from Blackmagic Design's Fusion 8 software package. Fusion 8 is a compositing tool that's extensively used in the movie industry. For example, it can be used to composite a still image, taken e.g., by a DSLR, into video footage. In this case, I'm using it because it has full (non-clipping!) support for EXR. Here I should point out that while a fair number of apps have some level of EXR support, not many of them are non-clipping. Apple's Preview, found on all Mac's, for example, will read EXR's, but clips any highlight above 1 when images are opened. (It also doesn't properly support EXR chromaticity tags.)
 |
| Fig. 3 : The original and adjusted image in Fusion 8. |
The first Fusion screenshot, figure 3, simply shows the original image and the overexposed image, with any adjustments in Fusion. For those who know Fusion, the Fusion workflow is configured for linear light operation, with the two image viewers in the top half of the screen configured with a LUT to show an "as will be seen" image.
 |
| Fig. 4 : Image recovery in Fusion 8, using EXR format images, and a non-clipping work flow. The images are effectively indistinguishable. |
The second Fusion screenshot, figure 4, shows the results of using Fusion "BC" tool to recover the overexposed image. As can be seen, the recovered and original image are identical. This shows both AccuRaw's linearity, and that AccuRaw doesn't clip highlights.
 |
| Fig. 5 : Attempted image recovery in Fusion 8, from a JPEG image. The inability to recover highlights in a clipping workflow is clear. |
By way of contrast, the final screenshot, figure 5, shows what happens when exactly the same recovery operation is attempted on a clipped image, in this case a JPEG image, which is inherently a clipping format. The difference is stark.
Add a comment