GPU notes for Lightroom CC (2015)

Hi everyone,

I wanted to share some additional information regarding GPU support in Lr CC.

Lr can now use graphics processors (GPUs) to accelerate interactive image editing in Develop. A big reason that we started here is the recent development and increased availability of high-res displays, such as 4K and 5K monitors. To give you some numbers: a standard HD screen is 2 megapixels (MP), a MacBook Retina Pro 15" is 5 MP, a 4K display is 8 MP, and a 5K display is a whopping 15 MP. This means on a 4K display we need to render and display 4 times as many pixels as on a standard HD display. Using the GPU can provide a significant speedup (10x or more) on high-res displays. The bigger the screen, the bigger the win.

For example, on my test system with a 4K display, adjusting the White Balance and Exposure sliders in Lightroom 5.7 (without GPU support) is about 5 frames/second -- manageable, but choppy and hard to control. The same sliders in Lightroom 6.0 now run smoothly at 60 FPS.

So why doesn't everything feel faster?

Well, GPUs can be enormously helpful in speeding up many tasks. But they're complex and involve some tradeoffs, which I'd like to take a moment to explain.

First, rewriting software to take full advantage of GPUs is a lot of work and takes time. Especially for software like Lightroom, which offers a rich feature set developed over many years and release versions. So the first tradeoff is that, for this particular version of Lightroom, we weren't able to take advantage of the GPU to speed up everything. Given our limited time, we needed to pick and choose specific areas of Lightroom to optimize. The area that I started with was interactive image editing in Develop, and even then, I didn't manage to speed up everything yet (more on this later).

Second, GPUs are marvelous at high-speed computation, but there's some overhead. For example, it takes time to get data from the main processor (CPU) over to the GPU. In the case of high-res images and big screens, that can take a LOT of time. This means that some operations may actually take longer when using the GPU, such as the time to load the full-resolution image, and the time to switch from one image to another.

Third, GPUs aren't best for everything. For example, decompressing sequential bits of data from a file -- like most raw files, for instance -- sees little to no benefit from a GPU implementation.

Fourth, Lightroom has a sophisticated raw processing pipeline (such as tone mapping HDR images with Highlights and Shadows), and running this efficiently on a GPU requires a fairly powerful GPU. Cards that may work with in the Photoshop app itself may not necessarily work with Lightroom. While cards that are 4 to 5 years old may technically work, they may provide little to no benefit over the regular CPU when processing images in Lr, and in some cases may be slower. Higher-end GPUs from the last 2 to 3 years should work better.

So let's clear up what's currently GPU accelerated in Lr CC and what's not:

First of all, Develop is the only module that currently has GPU acceleration whatsoever. This means that other functions and modules, such as Library, Export, and Quick Develop, do not use the GPU (performance should be the same for those functions regardless of whether you have GPU enabled or disabled in the prefs).

Within Develop, most image editing controls have full GPU acceleration, including the basic and tone panel, panning and zooming, crop and straighten, lens corrections, gradients, and radial filter. Some controls, such as local brush adjustments and spot clone/heal, do not -- at least, not yet.

While the above description may be disappointing to some of you, let's be clear: This is the beginning of the GPU story for Lightroom, not the end. The vision here is to expand our use of the GPU and other technologies over time to improve performance. I know that many photographers have been asking us for improved performance for a long time, and we're trying to respond to that. Please understand this is a big step in that direction, but it's just the first step. The rest of it will take some time.

Summary:

1. GPU support is currently available in Develop only.

2. Most (but not all) Develop controls benefit from GPU acceleration.

3. Using the GPU involves some overhead (there's no free lunch). This may make some operations take longer, such as image-to-image switching or zooming to 1:1. Newer GPUs and computer systems minimize this overhead.

4. The GPU performance improvement in Develop is more noticeable on higher-resolution displays such as 4K. The bigger the display, the bigger the win.

5. Prefer newer GPUs (faster models within the last 3 years). Lightroom may technically work on older GPUs (4 to 5 years old) but likely will not benefit much. At least 1 GB of GPU memory. 2 GB is better.

6. We're currently investigating using GPUs and other technologies to improve performance in Develop and other areas of the app going forward.

The above notes also apply to Camera Raw 9.0 for Photoshop/Bridge CC.

Eric Chan

Camera Raw Engineer