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High Dynamic Range Video with HDRx

May 09, 2013

High dynamic range (HDR) imaging is a powerful technique that combines multiple exposures into a single frame that encompasses the brightness range of the entire set. This has been an established technique with stills photography, but has only recently emerged as a possibility with motion capture—where the applications are even more expansive.

BACKGROUND

A camera’s dynamic range describes how much subject intensity can vary before being recorded as featureless black or white. Having a higher dynamic range therefore improves exposure and post-production flexibility, expands the possibilities for dramatic and unevenly-lit scenes, and enhances image quality and detail.

Dynamic range is typically specified in terms of stops, where each unit increase translates into a doubling of dynamic range. Current RED cameras capture over 13 stops of dynamic range in standard mode, which not only surpasses high-end DSLR cameras, but is also comparable to color film negatives. Although this is more than enough for most applications, sometimes even more is desired.

To effectively extend the dynamic range a camera can capture, many cinematographers employ graduated neutral density (GND) filters. This approach is typically used for simple intensity gradations, such as the linear transition from dark foliage to a brighter sky in landscape scenes. However, GND filters have to be physically positioned prior to capture, and cannot handle scenes with doorways, windows and other uneven lighting geometries.

To overcome these limitations, stills photographers created a technique called high dynamic range (HDR) imaging, which utilizes multiple exposures and a tripod-mounted camera. This has the power of handling virtually any lighting geometry, and is fully customizable in post-production. However, HDR has been elusive with video cameras, in part because this requires special-purpose electronics and software. Furthermore, mainstream stills HDR tools have become an artistic style as opposed to a dynamic range extension, so their look isn’t always desirable or realistic with cinema.

HOW IT WORKS

HDRx™ is a proprietary HDR video solution that was invented for RED’s EPIC and SCARLET cameras. It works by recording two exposures within the interval that a standard motion camera would record only one:

Standard Video
HDRx Enabled

The primary exposure is normal, and uses the standard aperture and shutter settings (the “A frame”). The secondary exposure is typically for highlight protection, and uses an adjustable shutter speed that is 2-6 stops faster (the “X frame”). For example, if the A frame is captured at 24 fps and 1/50 second shutter speed, then specifying 2 stops HDRx causes the X frame to be captured at a shutter speed of 1/200 second.

Both frames are then recorded as separate tracks within the same R3D file. The net result is two video streams that can be either used separately, or combined to encompass a higher dynamic range than any film stock.

MERGING FOR HDR

Merging for HDR in REDCINE-X works by effectively stacking the tonal levels from the A and X frames:

This approach produces tones that appear as though they were taken with a single higher dynamic range exposure, unlike the often artificial-looking HDR effect with stills that typically shows haloes and other artifacts. It also preserves the original tonal hierarchy, which means tones can be accurately mapped back to linear light for special effects or custom LUTs.

Merged A & X Frames

However, whenever you try to squeeze 13-18+ stops of dynamic range into a display with only 8-10 stops or less, the image will appear flat unless other measures are taken. Tools such as contrast curves, manual compositing and adaptive tone-mapping are all used to make high dynamic range images appear more natural on a standard display. Within REDCINE-X, the easiest approach is to apply a more aggressive contrast curve:

Optimal results are typically achieved by using only as much HDRx as necessary, and no more. A higher HDRx setting can make the blend more difficult since the A and X frames have less in common. The dynamic range of RED’s cameras is already quite high, so a little extension often goes a long way. When possible, it’s also always better to expose optimally using single exposures than to use HDRx to compensate for poor technique. Similarly, in post-production, it is easy to take the HDR effect too far. If an object was darker than the sky in the original scene, footage will lose its sense of realism if this tonal hierarchy is reversed in post-production.

Alternatively, sometimes having more dynamic range isn’t absolutely necessary, but instead saves time and reduces production costs. HDRx can decrease the chance of having to shoot at another time of day, use extensive ND filters or install additional lighting. Dialogue scenes within a moving car might require ND filters on the windows, for example, but with HDRx the car could remain unmodified.

CONTROLLING MOTION BLUR

An added benefit is that HDRx can also be used to control motion blur, which varies proportional to the shutter speed difference between the A and X frames. A 3-stop HDRx setting will mean that the blur distance in the X frame is an eighth of what it is in the A frame, for example.

Example “A Frame”
Corresponding “X Frame”
HDRx Setting:  + 2+ 3+ 4+ 5+ 6

Each frame pair can be merged using either a “simple blend,” which is a weighted average of the images, or “magic motion,” which is a motion conjoined version of the images. Similar to the effect created by using “first curtain” synchronization with a flash, each blending mode renders movement with both a sharp component and a more blurred component on the leading edge. This effect mimics how we perceive and recall real-world motion. At standard frame rates, the result is that motion typically appears smoother, but without necessarily compromising clarity as much as increasing the shutter angle.

This gives cinematographers the ability to adjust motion blur in post-production—a capability previously only possible with expensive and time-consuming visual effects. Just as motion blur in stills could be controlled using the flash ratio and shutter speed, with HDRx this can be controlled using a slider in post-production software. Note that the slider also affects the degree of dynamic range extension, and thus image contrast as well (as illustrated above).

OTHER APPLICATIONS

Many other HDRx possibilities exist, and not all have to do with dynamic range. Applications include:

Highlight Insurance. Many only use the X track as a backup to safeguard against costly re-shoots due to overexposure. This is particularly helpful when shooting scenes containing dramatic or rapidly changing lighting, or with time lapse work when exposure settings have to remain constant. It can also reduce image noise since exposures do not need to pad the highlights.

Pulling Stills. One of the most popular applications for HDRx is to use the X track for sharper stills. Although these images will initially appear darker, the ISO setting can be increased in post-production to compensate for any decrease in light from the faster shutter speed. Most find that an HDRx setting of 2-3 stops works best for this purpose.

Motion Tracking. The X frame is sometimes also used to achieve more accurate motion tracking. Since there is less motion blur, tracking algorithms can better estimate each object’s position, and thus also their speed and direction of motion between frames. This can be particularly helpful with motion capture applications.

Transitions. Another option could be to utilize HDRx temporally as opposed to spatially. For example, one could transition from the A frame to the X frame when moving from indoors to outdoors. This would be helpful if the lens aperture and depth of field needed to remain constant.

Keying. The sharper edges in the X frame can improve mask or keying quality, especially for situations with a moving subject against a backdrop.

IN PRACTICE

Most use the A frame as a normal exposure and the X frame as the under-exposed highlight protection frame. This is generally recommended since it makes HDRx easily reversible if the X frames are later deemed unnecessary. However, one could instead shoot “over-under,” where the A frame is overexposed and the X frame is underexposed (but less so than otherwise). One might do this if they wanted to apply some of the dynamic range extension to the shadows in addition to just the highlights.

Once captured, HDRx files will be immediately recognized by REDCINE-X, and an “HDR” label will get appended to their thumbnail title. The HDRx extension setting can also be viewed within the file’s metadata by right-clicking on the thumbnail or filename:

A section within the “look” panel will become available which gives control over how to display and process the file. The HDR mode can be selected to show either the A or X frame alone, or to use a merged version of these frames with either a simple blend or with magic motion. The slider within the last two modes effectively determines how bright the merged HDR will appear. Within the simple blend tool, the slider controls the relative weight of the A and X frames, where a setting of 0 gives them equal weight and -1 or +1 means the output is entirely the X or A frame, respectively. With the magic motion tool, the slider controls the lightness threshold where tones from the A frame will take precedence over those in the X frame.

Any color grading applied to either frame will affect both. To affect each frame separately, or for more complex blending, many perform additional grading using third party packages such as After Effects, Resolve and Scratch. In that case, HDRx is often thought of as two plates that need blending as opposed to an automated process – much as one would think about shooting for a green screen. Motion blur may therefore also require more attention than normal, since this can create edge artifacts if it extends into very bright or dark regions.

Regardless, since HDRx preserves each exposure pair separately, the post-production possibilities are virtually endless. For example, if one prefers the look of a traditional graduated ND filter, this effect can be applied digitally by blending frame pairs using a simple linear gradient. Alternatively, one could create a custom mask to match the lighting geometry of their scene. If the stills HDR effect is desired, this can also be implemented just as it would with two stills frames.

COMPLICATIONS & CONCLUSIONS

Whether HDRx is the right tool for your shoot may depend on other considerations. Since twice as many frames are captured, storage and data rate requirements will correspondingly double. This also means that the effective maximum frame rate will become half of your camera’s normal capabilities. In addition, specifying a large shutter angle for the primary exposure may limit the range of shutter speeds available with the secondary exposure.

As with any tool, there are situations where HDRx is well-suited to the task, but other times when another tool would be a better option. Very rapid subject motion or camera movements are not recommended, for example, just as with any normal film or digital cinema camera capturing at standard frame rates. When in doubt, always perform your own camera tests to assess whether HDRx works with the subject matter and workflow you’ll have in production.

Ultimately though, a new tool expands the creative possibilities. Many of the stylistic decisions that previously had to be made prior to capture can now be made in post-production. In that respect, having an HDRx capability is a lot like always having an additional camera rig available, and is just as much a problem-solving tool as it is a creative tool. Until recently, cinematographers also had to think in terms of what was possible using the traditional 10-13 stops (or less) that had been available. Now, for the first time, these constraints are being lifted and we are venturing into unexplored territory.

FURTHER READING


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