TIPS & TRICKS: Shooting HDR images with the Note 4 – recommended techniques

Started Jan 12, 2015 | Discussions thread
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Menneisyys Senior Member • Posts: 1,456
TIPS & TRICKS: Shooting HDR images with the Note 4 – recommended techniques
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One of the most popular features of the Note 4 flagship is its HDR mode. Unfortunately, the images it produces suffer from exactly the same problem as with the standard, non-HDR mode: very strong, in daylight, absolutely unnecessary noise reduction and oversharpening. In the article below, I explain how you can shoot significantly better-quality HDR images on your handset.

Important: as, as of now (ANK5 4.4.4 firmware), third-party apps cannot access the (almost) unprocessed sensor data on Note 4's with the Exynos CPU, this discussion only applies to devices with the Snapdragon 805 CPU. Sorry guys with the Exynos – currently, it's plain impossible to shoot decent pictures on your Note 4 and you can't do anything with it.

First and foremost, the most important thing to understand is that the way the stock Camera app of Note4 shoots HDR images is vastly different from both the previous Samsung flagships (for example, the S4C, which still used stitching using three images) and the way the iPhone takes images. With the Note4, physically only one image is shot, making sure the exposure is properly set to avoid burnt-in highlights (physically, this is achieved via using matrix / average, that is, full sensor metering, as opposed to the standard center-weighted one). The phone, then, just pulls the shadows to give the user a HDR-looking image.

This approach, while it has some major advantages (no stitching errors are guaranteed; no unnatural fusing artifacts; absolutely no need for de-ghosting), has the huge disadvantage of much worse image quality, particularly in the shadows, because of the heavy shadow pulling to make dark areas much brighter.

Let me introduce you to a scene demonstrating all these advantages and disadvantages, also providing you with a lot of tips on creating high dynamic range images in as good quality as possible.

Let's start with the following scene, in this case, shot with the stock Camera app in non-HDR mode:

(original at Flickr)

You can easily spot the biggest problem with this shot: the sky is burnt out in the upper left quarter – it's entirely white. HDR to the rescue!

Let's first see how just quickly enabling HDR shooting helped in the same stock Camera app:

(original at Flickr)

So far so good, you'd say. Nevertheless, as you already are aware of my not recommending the stock Camera app (unless you're on an Exynos-based Note), you may very well ask me about the problems of this shot.

First, not even this shot could render the sky without burning in in its entirety. Just check out the sky in the upper left corner. In the following crop, I've annotated the burnt-in area:

(original at Flickr)

Just compare the region pointed by the arrow to that of the following shot shot with a (strong) negative exposure compensation (-2):

(original at Flickr; full -2EV image shot, which will also be discussed in a later section, HERE. Note that, in the above two crops, the arrow points to different places relative to the trees. This is simply because I also took the relative position of the clouds between the two shots when I annotated the image. Because of the breeze, the clouds were also moving.)

Unfortunately, this problem in no way can be fixed. When you do enable HDR mode in the Camera app, it promptly deactivates exposure compensation. This can clearly be seen in the following screen, when not in HDR mode, can directly be accessed and set in Settings. In the following screenshot, I've used annotations to show you this (now-deactivated) menu item (see the rectange), along with the HDR menu item (see the two arrows) showing being active:

(original at Flickr)

This means you can't dial in an additional negative exposure compensation to make absolutely sure your shot won't contain burnt-in areas. Needless to say, as the Camera app switches to matrix / average metering as can also be seen in the now-deactivated “Metering modes” settings menu item in the above screenshot, you couldn't play with metering for a brighter area and locking the metering there either. All you can do composing your shot so that it contains a sufficiently large, high-brightness area to force the matrix metering to protect those highlights.

A Side Note: Problems with Matrix Metering and a Common Misconception

I did emphasize the Note 4 needs bright areas to be sufficiently large in order to protect them. If only a small percentage of the image is bright, it'll become burnt-in even in HDR mode – which is exactly the opposite of what HDR is for. That is, the stock Camera app on the Note4 is pretty immune to small-area highlights even in HDR mode – which is, again, a big problem.

Regretfully, a lot of people don't realize this. Exactly this is why for example GSMArena (erroreneously) stated in their roundup “The Galaxy Note 4 mode manages to bring out extra detail in the shadows (the Bistro sign is more legible) though the brightly lit areas become more washed out.” The image pair they show indeed shows the highlights to be a little bit more burnt out in the HDR shot on the right of the following crop:

(Flicker original)

This, however, is caused by the highlights being relatively small, not large enough to “kick in” the matrix metering to (drastically) reduce the number of photons entering the sensor by increasing the shutter speed and/or decreasing the ISO. This also means GSMArena's above statement in no way can be generalized – it's only in not-that-common cases that switching to the HDR mode burns in highlights even more than the non-HDR mode. If the highlighted areas are larger, matrix metering will properly work.

Note that the matrix metering's inability to properly detect small highlights is one of the reasons you'll want to use third-party apps and do on-desktop HDR stitching instead of using the (semi-)HDR mode of the stock app. In most of them you can dial in exposure compensation. When paired with matrix metering, the (as of 4.4.4) minimal value of the exposure compensation, -2, will be sufficient to protect all but the harshest (natural) highlights. That is, in cases like this, in addition to switching to matrix metering, use an additional negative exposure compensation for additional highlight protection. This is what would have saved the above HDR shot too, should be there a way to apply an additional exposure compensation in HDR mode. As this is (at least currently) impossible, the just-explained matrix metering reliability problem is one of the reasons why you will NOT want to use the HDR mode of the Camera app.

What about the other problems?

The other problems plaguing the HDR shots of the Camera app are already known for anyone having read the first two parts of this article series (previous part HERE): oversharpening and, in most cases, unnecessarily excessive noise reduction. Let's take a look at the shadows in the above HDR shot: the white wall. A crop:

(Flicker original)

There is some very bad smearing in the crop, isn't there? Let's take a look at how exactly the same part is rendered by a decent desktop HDR stitcher, Photomatix Pro (as of version 5.0.5) using no more than two input images, one shot at -2 and the other at +2EV exposure compensation, using matrix metering, with some additional, very small (in this case, 6) color noise reduction in Lightroom:

(Flicker original; full image. Note that I've increased the brightness of the above crop so that there isn't any difference between the two images' brightness. This in no way had any effect on the quality (the effects of noise reduction / sharpening).)

As you can see, there is huge difference between the two crops. In the stock Camera one,

- there are major oversharpening halos around, among other things, the tree trunk

- in the window, the light brownish texture (in the reddish one) is almost completely smeared

- there is almost no texture in the tree trunk

- the colors are much less defined and much more smeared

- the (almost-)horizontal lines on the wall are, in many places, undefined.

than in the separately stitched HDR shot.

A Side Note: Consequences of Using Less Exposure

Readers well versed in photography, based on my earlier two (HDR vs. non-HDR) shots shot with the stock Camera app, may already know the HDR shot, as it uses matrix metering, is essentially noisier than the non-HDR one, which, as has also been proved by the much more burnt-out sky, used a higher exposure (essentially letting more photons on the sensor).

The results of this can clearly be seen in the images too. Just compare the above stock HDR crop (or full image) to the following one (a crop from the very first image (flickr link) in this article):

(flickr original or the crop. Note that I've raised the exposure of the crop itself, just as with the desktop-stitched, previous crop. This, just like in the previous case, had no effect on the noise reduction / oversharpening artifacts. HERE is the crop with the original brightness.)

The results are as anyone knowing the non-HDR shot used about twice the photons of that of the HDR one can expect: the colors are somewhat more defined. See for example the red + brown color in the window and the lower, horizontal part of the X-shaped boards at the top. In the HDR image (the one utilizing the fewest photons), the boards are almost completely devoid of color. In the non-HDR one (the just-shown one), at least the lower, horizontal ones are reminiscent of the original. That is, there's a bit less color smearing in the non-HDR shot. Again, this is to be expected as the HDR shot was shot at 1/3448s, while the non-HDR one at 1/1516s. That is, during shooting the non-HDR shot, the shutter let 3448/1516 = 2.27 times more photons on the sensor. No wonder the non-HDR shot is somewhat better. Nevertheless, it just can't come close to the stitched one, which was composed of a -2 and a +2EV exposure compensated shot, where the shot with the +2 EV was shot at 1/732s, meaning it let 1516/732=2.07 times more photons on the sensor. If you take a look at the following crop:

you can see that the desktop HDR stitcher (in this case, Photomatix Pro 5.0.5) has managed to keep the shadows of the resulting image as clean as the cleanest input image (in this case, the +2EV one).

(flickr crop link; full image link)

(NOTE: the article continues below, as a separate post titled Part II. (I don't want to lose hours of long work so I slice up my article to more manageable parts. Don't forget that, particularly because of the inline images, I need to do all the editing here in the DPReview Web interface.)

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