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H.264 encoding - CPU vs GPU: Nvidia CUDA, AMD Stream, Intel MediaSDK and x264
by Guillaume Louel
Published on August 18, 2011

ArcSoft Media Converter, Inception 720p
Now let’s move on to the film Inception, still at 720p, and the short extract we have chosen of just 40 seconds, which does however include some particularly interesting explosions.

When you look at the SSIMs obtained across the whole length of the scene, the Arcsoft CPU encoder seems to stand out a bit more from the Radeon/HD 3000 versions, which are at a similar level. Given the limited length of the extract, we won’t give encoding times here.

We extracted a scene of 500 frames. During the first 400, shots with explosions alternate with shots of characters (51, 192, 352) before the scene finishes with a much calmer sequence.

Use an HTML5 compatible browser to view the graph!
Click here to view the PSNR graph of this scene.

The advantage of the Arcsoft CUDA encoder is that it allows you to detect scene changes easily: at each trough in quality, there’s a scene change! The Radeon encoder is a good way down on the Intel encoder in the explosion scenes, while the Intel is behind in the static scenes. Let’s check out what this translates to visually, firstly in an explosion frame:

Click here to display the frame comparison in a new tab.

We're going to put discussion of the quality of the CUDA version to one side and concentrate on the Radeon. While in the slow-moving Avatar scenes, the AMD encoder held its own better, here it's a long way behind. This isn’t really a surprise as the baseline profile is a lot less efficient than the others in terms of compression! Although the CPU version of Arcsoft preserves more detail, the result is far from great.

What about the end of the sequence where the scene is more static?

Click here to display the frame comparison in anew tab.

None of the encoders maintain the grain in the background. The Intel encoder is the least precise on the face and the cheek and the contour of the eye are particularly blurred. This is difficult to forgive when you think that your eye is drawn to this part of the image first.

Now for our last 720p test sequence.

K-On!! 720p
We encoded an entire episode (24 minutes 11 seconds) of this animation, with a relatively static image. At 4 Mbit/sec, this shouldn’t be too much of a challenge for our encoders.

The first surprise is that the Arcsoft Intel encoder didn't work here (the software crashed). The second surprise was that the score with the Arcsoft CUDA encoder, while still behind the Radeons, seemed to indicate that the result wouldn’t be the usual soup!

In terms of processing time, the CPU version was once again very fast, even if the Arcsoft CUDA encoders were faster.

Use an HTML5 compatible browser to view the graph!
Click here to view the PSNR graph of this scene.

The GeForce encoder still struggles with scene changes but the result still seems excellent. What does this give in practice?

Click here to display the frame comparison in a new tab.

This isn’t a mistake, the GeForce version is actually the one that conserves most detail! With the CPU and Radeon versions of the encoding, you lose all the grain. As this frame is static for several seconds, these two encoders have deliberately chosen to blur the scene, which is to be regretted! This is a tendency that we have already noticed and which also shows, once again, the pitfalls of relying on SSIM and PSNR type readings!

Click here to display the frame comparison in a new tab.

In a scene with fewer textures and more solid colours, the differences are minimal, with just a few slight artefacts. Nothing too concerning.

Let’s now see if moving up to 1080p and a higher bitrate changes things for the Arcsoft application.

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