Tag Archives: Computational Photography

Experimental Nighttime Photography with Nexus and Pixel

On a full moon night last year I carried a professional DSLR camera, a heavy lens and a tripod up to a hilltop in the Marin Headlands just north of San Francisco to take a picture of the Golden Gate Bridge and the lights of the city behind it.
A view of the Golden Gate Bridge from the Marin Headlands, taken with a DSLR camera (Canon 1DX, Zeiss Otus 28mm f/1.4 ZE). Click here for the full resolution image.
I thought the photo of the moonlit landscape came out well so I showed it to my (then) teammates in Gcam, a Google Research team that focuses on computational photography - developing algorithms that assist in taking pictures, usually with smartphones and similar small cameras. Seeing my nighttime photo, one of the Gcam team members challenged me to re-take it, but with a phone camera instead of a DSLR. Even though cameras on cellphones have come a long way, I wasn’t sure whether it would be possible to come close to the DSLR shot.

Probably the most successful Gcam project to date is the image processing pipeline that enables the HDR+ mode in the camera app on Nexus and Pixel phones. HDR+ allows you to take photos at low-light levels by rapidly shooting a burst of up to ten short exposures and averaging them them into a single image, reducing blur due to camera shake while collecting enough total light to yield surprisingly good pictures. Of course there are limits to what HDR+ can do. Once it gets dark enough the camera just cannot gather enough light and challenging shots like nighttime landscapes are still beyond reach.

The Challenges
To learn what was possible with a cellphone camera in extremely low-light conditions, I looked to the experimental SeeInTheDark app, written by Marc Levoy and presented at the ICCV 2015 Extreme Imaging Workshop, which can produce pictures with even less light than HDR+. It does this by accumulating more exposures, and merging them under the assumption that the scene is static and any differences between successive exposures must be due to camera motion or sensor noise. The app reduces noise further by dropping image resolution to about 1 MPixel. With SeeInTheDark it is just possible to take pictures, albeit fairly grainy ones, by the light of the full moon.

However, in order to keep motion blur due to camera shake and moving objects in the scene at acceptable levels, both HDR+ and SeeInTheDark must keep the exposure times for individual frames below roughly one tenth of a second. Since the user can’t hold the camera perfectly still for extended periods, it doesn’t make sense to attempt to merge a large number of frames into a single picture. Therefore, HDR+ merges at most ten frames, while SeeInTheDark progressively discounts older frames as new ones are captured. This limits how much light the camera can gather and thus affects the quality of the final pictures at very low light levels.

Of course, if we want to take high-quality pictures of low-light scenes (such as a landscape illuminated only by the moon), increasing the exposure time to more than one second and mounting the phone on a tripod or placing it on some other solid support makes the task a lot easier. Google’s Nexus 6P and Pixel phones support exposure times of 4 and 2 seconds respectively. As long as the scene is static, we should be able to record and merge dozens of frames to produce a single final image, even if shooting those frames takes several minutes.

Even with the use of a tripod, a sharp picture requires the camera’s lens to be focused on the subject, and this can be tricky in scenes with very low light levels. The two autofocus mechanisms employed by cellphone cameras — contrast detection and phase detection — fail when it’s dark enough that the camera's image sensor returns mostly noise. Fortunately, the interesting parts of outdoor scenes tend to be far enough away that simply setting the focus distance to infinity produces sharp images.

Experiments & Results
Taking all this into account, I wrote a simple Android camera app with manual control over exposure time, ISO and focus distance. When the shutter button is pressed the app waits a few seconds and then records up to 64 frames with the selected settings. The app saves the raw frames captured from the sensor as DNG files, which can later be downloaded onto a PC for processing.

To test my app, I visited the Point Reyes lighthouse on the California coast some thirty miles northwest of San Francisco on a full moon night. I pointed a Nexus 6P phone at the building and shot a burst of 32 four-second frames at ISO 1600. After covering the camera lens with opaque adhesive tape I shot an additional 32 black frames. Back at the office I loaded the raw files into Photoshop. The individual frames were very grainy, as one would expect given the tiny sensor in a cellphone camera, but computing the mean of all 32 frames cleaned up most of the grain, and subtracting the mean of the 32 black frames removed faint grid-like patterns caused by local variations in the sensor's black level. The resulting image, shown below, looks surprisingly good.
Point Reyes lighthouse at night, photographed with Google Nexus 6P (full resolution image here).
The lantern in the lighthouse is overexposed, but the rest of the scene is sharp, not too grainy, and has pleasing, natural looking colors. For comparison, a hand-held HDR+ shot of the same scene looks like this:
Point Reyes Lighthouse at night, hand-held HDR+ shot (full resolution image here). The inset rectangle has been brightened in Photoshop to roughly match the previous picture.
Satisfied with these results, I wanted to see if I could capture a nighttime landscape as well as the stars in the clear sky above it, all in one picture. When I took the photo of the lighthouse a thin layer of clouds conspired with the bright moonlight to make the stars nearly invisible, but on a clear night a two or four second exposure can easily capture the brighter stars. The stars are not stationary, though; they appear to rotate around the celestial poles, completing a full turn every 24 hours. The motion is slow enough to be invisible in exposures of only a few seconds, but over the minutes it takes to record a few dozen frames the stars move enough to turn into streaks when the frames are merged. Here is an example:
The North Star above Mount Burdell, single 2-second exposure. (full resolution image here).
Mean of 32 2-second exposures (full resolution image here).
Seeing streaks instead of pinpoint stars in the sky can be avoided by shifting and rotating the original frames such that the stars align. Merging the aligned frames produces an image with a clean-looking sky, and many faint stars that were hidden by noise in the individual frames become visible. Of course, the ground is now motion-blurred as if the camera had followed the rotation of the sky.
Mean of 32 2-second exposures, stars aligned (full resolution image here).
We now have two images; one where the ground is sharp, and one where the sky is sharp, and we can combine them into a single picture that is sharp everywhere. In Photoshop the easiest way to do that is with a hand-painted layer mask. After adjusting brightness and colors to taste, slight cropping, and removing an ugly "No Trespassing" sign we get a presentable picture:
The North Star above Mount Burdell, shot with Google Pixel, final image (full resolution image here).
Using Even Less Light
The pictures I've shown so far were shot on nights with a full moon, when it was bright enough that one could easily walk outside without a lantern or a flashlight. I wanted to find out if it was possible to take cellphone photos in even less light. Using a Pixel phone, I tried a scene illuminated by a three-quarter moon low in the sky, and another one with no moon at all. Anticipating more noise in the individual exposures, I shot 64-frame bursts. The processed final images still look fine:
Wrecked fishing boat in Inverness and the Big Dipper, 64 2-second exposures, shot with Google Pixel (full resolution image here).
Stars above Pierce Point Ranch, 64 2-second exposures, shot with Google Pixel (full resolution image here).
In the second image the distant lights of the cities around the San Francisco Bay caused the sky near the horizon to glow, but without moonlight the night was still dark enough to make the Milky Way visible. The picture looks noticeably grainier than my earlier moonlight shots, but it's not too bad.

Pushing the Limits
How far can we go? Can we take a cellphone photo with only starlight - no moon, no artificial light sources nearby, and no background glow from a distant city?

To test this I drove to a point on the California coast a little north of the mouth of the Russian River, where nights can get really dark, and pointed my Pixel phone at the summer sky above the ocean. Combining 64 two-second exposures taken at ISO 12800, and 64 corresponding black black frames did produce a recognizable image of the Milky Way. The constellations Scorpius and Sagittarius are clearly visible, and squinting hard enough one can just barely make out the horizon and one or two rocks in the ocean, but overall, this is not a picture you'd want to print out and frame. Still, this may be the lowest-light cellphone photo ever taken.
Only starlight, shot with Google Pixel (full resolution image here).
Here we are approaching the limits of what the Pixel camera can do. The camera cannot handle exposure times longer than two seconds. If this restriction was removed we could expose individual frames for eight to ten seconds, and the stars still would not show noticeable motion blur. With longer exposures we could lower the ISO setting, which would significantly reduce noise in the individual frames, and we would get a correspondingly cleaner and more detailed final picture.

Getting back to the original challenge - using a cellphone to reproduce a night-time DSLR shot of the Golden Gate - I did that. Here is what I got:
Golden Gate Bridge at night, shot with Google Nexus 6P (full resolution image here).
The Moon above San Francisco, shot with Google Nexus 6P (full resolution image here).
At 9 to 10 MPixels the resolution of these pictures is not as high as what a DSLR camera might produce, but otherwise image quality is surprisingly good: the photos are sharp all the way into the corners, there is not much visible noise, the captured dynamic range is sufficient to avoid saturating all but the brightest highlights, and the colors are pleasing.

Trying to find out if phone cameras might be suitable for outdoor nighttime photography was a fun experiment, and clearly the result is yes, they are. However, arriving at the final images required a lot of careful post-processing on a desktop computer, and the procedure is too cumbersome for all but the most dedicated cellphone photographers. However, with the right software a phone should be able to process the images internally, and if steps such as painting layer masks by hand can be eliminated, it might be possible to do point-and-shoot photography in very low light conditions. Almost - the cellphone would still have to rest on the ground or be mounted on a tripod.

Here’s a Google Photos album with more examples of photos that were created with the technique described above.

PhotoScan: Taking Glare-Free Pictures of Pictures

Yesterday, we released an update to PhotoScan, an app for iOS and Android that allows you to digitize photo prints with just a smartphone. One of the key features of PhotoScan is the ability to remove glare from prints, which are often glossy and reflective, as are the plastic album pages or glass-covered picture frames that host them. To create this feature, we developed a unique blend of computer vision and image processing techniques that can carefully align and combine several slightly different pictures of a print to separate the glare from the image underneath.
Left: A regular digital picture of a physical print. Right: Glare-free digital output from PhotoScan
When taking a single picture of a photo, determining which regions of the picture are the actual photo and which regions are glare is challenging to do automatically. Moreover, the glare may often saturate regions in the picture, rendering it impossible to see or recover the parts of the photo underneath it. But if we take several pictures of the photo while moving the camera, the position of the glare tends to change, covering different regions of the photo. In most cases we found that every pixel of the photo is likely not to be covered by glare in at least one of the pictures. While no single view may be glare-free, we can combine multiple pictures of the printed photo taken at different angles to remove the glare. The challenge is that the images need to be aligned very accurately in order to combine them properly, and this processing needs to run very quickly on the phone to provide a near instant experience.
Left: The captured, input images (5 in total). Right: If we stabilize the images on the photo, we can see just the glare moving, covering different parts of the photo. Notice no single image is glare-free.
Our technique is inspired by our earlier work published at SIGGRAPH 2015, which we dubbed “obstruction-free photography”. It uses similar principles to remove various types of obstructions from the field of view. However, the algorithm we originally proposed was based on a generative model where the motion and appearance of both the main scene and the obstruction layer are estimated. While that model is quite powerful and can remove a variety of obstructions, it is too computationally expensive to be run on smartphones. We therefore developed a simpler model that treats glare as an outlier, and only attempts to register the underlying, glare-free photo. While this model is simpler, the task is still quite challenging as the registration needs to be highly accurate and robust.

How it Works
We start from a series of pictures of the print taken by the user while moving the camera. The first picture - the “reference frame” - defines the desired output viewpoint. The user is then instructed to take four additional frames. In each additional frame, we detect sparse feature points (we compute ORB features on Harris corners) and use them to establish homographies mapping each frame to the reference frame.
Left: Detected feature matches between the reference frame and each other frame (left), and the warped frames according to the estimated homographies (right).
While the technique may sound straightforward, there is a catch - homographies are only able to align flat images. But printed photos are often not entirely flat (as is the case with the example shown above). Therefore, we use optical flow — a fundamental, computer vision representation for motion, which establishes pixel-wise mapping between two images — to correct the non-planarities. We start from the homography-aligned frames, and compute “flow fields” to warp the images and further refine the registration. In the example below, notice how the corners of the photo on the left slightly “move” after registering the frames using only homographies. The right hand side shows how the photo is better aligned after refining the registration using optical flow.
Comparison between the warped frames using homographies (left) and after the additional warp refinement using optical flow (right).
The difference in the registration is subtle, but has a big impact on the end result. Notice how small misalignments manifest themselves as duplicated image structures in the result, and how these artifacts are alleviated with the additional flow refinement.
Comparison between the glare removal result with (right) and without (left) optical flow refinement. In the result using homographies only (left), notice artifacts around the eye, nose and teeth of the person, and duplicated stems and flower petals on the fabric.
Here too, the challenge was to make optical flow, a naturally slow algorithm, work very quickly on the phone. Instead of computing optical flow at each pixel as done traditionally (the number of flow vectors computed is equal to the number of input pixels), we represent a flow field by a smaller number of control points, and express the motion at each pixel in the image as a function of the motion at the control points. Specifically, we divide each image into tiled, non-overlapping cells to form a coarse grid, and represent the flow of a pixel in a cell as the bilinear combination of the flow at the four corners of the cell that contains it.

The grid setup for grid optical flow. A point p is represented as the bilinear interpolation of the four corner points of the cell that encapsulates it.
Left: Illustration of the computed flow field on one of the frames. Right: The flow color coding: orientation and magnitude represented by hue and saturation, respectively.
This results in a much smaller problem to solve, since the number of flow vectors to compute now equals the number of grid points, which is typically much smaller than the number of pixels. This process is similar in nature to the spline-based image registration described in Szeliski and Coughlan (1997). With this algorithm, we were able to reduce the optical flow computation time by a factor of ~40 on a Pixel phone!
Flipping between the homography-registered frame and the flow-refined warped frame (using the above flow field), superimposed on the (clean) reference frame, shows how the computed flow field “snaps” image parts to their corresponding parts in the reference frame, improving the registration.
Finally, in order to compose the glare-free output, for any given location in the registered frames, we examine the pixel values, and use a soft minimum algorithm to obtain the darkest observed value. More specifically, we compute the expectation of the minimum brightness over the registered frames, assigning less weight to pixels close to the (warped) image boundaries. We use this method rather than computing the minimum directly across the frames due to the fact that corresponding pixels at each frame may have slightly different brightness. Therefore, per-pixel minimum can produce visible seams due to sudden intensity changes at boundaries between overlaid images.
Regular minimum (left) versus soft minimum (right) over the registered frames.
The algorithm can support a variety of scanning conditions — matte and gloss prints, photos inside or outside albums, magazine covers.

Input     Registered     Glare-free
To get the final result, the Photos team has developed a method that automatically detects and crops the photo area, and rectifies it to a frontal view. Because of perspective distortion, the scanned rectangular photo usually appears to be a quadrangle on the image. The method analyzes image signals, like color and edges, to figure out the exact boundary of the original photo on the scanned image, then applies a geometric transformation to rectify the quadrangle area back to its original rectangular shape yielding high-quality, glare-free digital version of the photo.
So overall, quite a lot going on under the hood, and all done almost instantaneously on your phone! To give PhotoScan a try, download the app on Android or iOS.