Frame Rating Dissected: Full Details on Capture-based Graphics Performance Testing
Data Analysis via FCAT
Finally, we get into the art of the Frame Rating process, analyzing the data that is presented to us via the overlay, capture and extractor. NVIDIA wrote a set of Perl scripts they are calling FCAT (Frame Capture and Analysis Tool) that takes as input the XLS file mentioned above and generates a set of graphs that attempt to tell a performance story. There are Perl scripts that generate batch files that call other Perl scripts – it’s not pretty but it works.
For those of you interested, I have uploaded the most recent copy of FCAT right here for you to download and look over. I am curious to get some feedback from the community on these and look forward to any improvements the engaged users can come up with.
While there are literally dozens of file created for each “run” of benchmarks, there are several resulting graphs that FCAT produces, as well as several more that we are generating with additional code of our own.
The PLOT File
The primary file that is generated from the extracted data is a plot of calculated frame times including runts. The numbers here represent the amount of time that frames appear on the screen for the user, a “thinner” line across the time span represents frame times that are consistent and thus should produce the smoothest animation to the gamer. A “wider” line or one with a lot of peaks and valleys indicates a lot more variance and is likely caused by a lot of runts being displayed.
The PERcentile File
Scott introduced the idea of frame time percentiles months ago but now that we have some different data using direct capture as opposed to FRAPS, the results might be even more telling. In this case, FCAT is showing percentiles not by frame time but instead by instantaneous FPS. This will tell you the minimum frame rate that will appear on the screen at any given percent of time during our benchmark run. The 50th percentile should be very close to the average total frame rate of the benchmark but as we creep closer to the 100% we see how the frame rate will be affected.
The closer this line is to being perfectly parallel the better as that would mean we are running at a constant frame rate the entire time. A steep decline on the right hand side tells us that frame times are varying more and more frequently and might indicate potential stutter in the animation.
The RUN File
While the two graphs above show combined results for a set of cards being compared, the RUN file will show you the results from a single card on that particular result. It is in this graph that you can see interesting data about runts, drops, average frame rate and the actual frame rate of your gaming experience.
For tests that show no runts or drops, the data is pretty clean. This is the standard frame rate per second over a span of time graph that has become the standard for performance evaluation on graphics cards.
A test that does have runts and drops will look much different. The black bar labeled FRAPS indicates the average frame rate over time that traditional testing would show if you counted the drops and runts in the equation – as FRAPS FPS measurement does. Any area in red is a dropped frame – the wider the amount of red you see, the more colored bars from our overlay were missing in the captured video file, indicating the gamer never saw those frames in any form.
The wide yellow area is the representation of runts, the thin bands of color in our captured video, that we have determined do not add to the animation of the image on the screen. The larger the area of yellow the more often those runts are appearing.
Finally, the blue line is the measured FPS over each second after removing the runts and drops. We are going to be calling this metric the “observed frame rate” as it measures the actual speed of the animation that the gamer experiences.
The PCPER FRAPS File
While the graphs above are produced by the default version of the scripts from NVIDIA, I have modified and added to them in a few ways to produce additional data for our readers. The first file shows a sub-set of the data from the RUN file above, the average frame rate over time as defined by FRAPS, though we are combining all of the GPUs we are comparing into a single graph. This will basically emulate the data we have been showing you for the past several years.
The PCPER Observed FPS File
This graph takes a different subset of data points and plots them similarly to the FRAPS file above, but this time we are look at the “observed” average frame rates, shown previously as the blue bars in the RUN file above. This takes out the dropped and runts frames, giving you the performance metrics that actually matter – how many frames are being shown to the gamer to improve the animation sequences.
As you’ll see in our full results on the coming pages, seeing a big difference between the FRAPS FPS graphic and the Observed FPS will indicate cases where it is likely the gamer is not getting the full benefit of the hardware investment in their PC.
The PCPER Frame Time Variance File
Of all the data we are presenting, this is probably the one that needs the most discussion. In an attempt to create a new metric for gaming and graphics performance, I wanted to try to find a way to define stutter based on the data sets we had collected. As I mentioned earlier, we can define a single stutter as a variance level between t_game and t_display. This variance can be introduced in t_game, t_display, or on both levels. Since we can currently only reliably test the t_display rate, how can we create a definition of stutter that makes sense and that can be applied across multiple games and platforms?
We define a single frame variance as the difference between the current frame time and the previous frame time – how consistent the two frames presented to the gamer. However, as I found in my testing plotting the value of this frame variance is nearly a perfect match to the data presented by the minimum FPS (PER) file created by FCAT. To be more specific, stutter is only perceived when there is a break from the previous animation frame rates.
Our current running theory for a stutter evaluation is this: find the current frame time variance by comparing the current frame time to the running average of the frame times of the previous 20 frames. Then, by sorting these frame times and plotting them in a percentile form we can get an interesting look at potential stutter. Comparing the frame times to a running average rather than just to the previous frame should prevent potential problems from legitimate performance peaks or valleys found when moving from a highly compute intensive scene to a lower one.
While we are still trying to figure out if this is the best way to visualize stutter in a game, we have seen enough evidence in our game play testing and by comparing the above graphic to other data generated through our Frame rating system to be reasonably confident in our assertions. So much in fact that I am going to going this data the PCPER ISU, which beers fans will appreciate the acronym of International Stutter Units.
To compare these results you want to see a line that is as close the 0ms mark as possible indicating very little frame rate variance when compared to a running average of previous frames. There will be some inevitable incline as we reach the 90+ percentile but that is expected with any game play sequence that varies from scene to scene. What we do not want to see is a sharper line up that would indicate higher frame variance (ISU) and could be an indication that the game sees microstuttering and hitching problems.
I think the best way to really understand all of this data is just to start looking at it and discuss it as we go along. We are going to start with battle between similarly priced graphics cards from both AMD and NVIDIA with the GeForce GTX 680 and the Radeon HD 7970 GHz Edition and use six different PC games at different resolutions including 1920x1080, 2560x1440 and even 5760x1080 for Surround/Eyefinity comparisons. The games we are going to test with include Battlefield 3, Crysis 3, DiRT 3, Far Cry 3, Skyrim and Sleeping Dogs. No, we did not purposefully try to find as many games with ‘3’ in them as possible.
|Test System Setup|
|CPU||Intel Core i7-3960X Sandy Bridge-E|
|Motherboard||ASUS P9X79 Deluxe|
|Memory||Corsair Dominator DDR3-1600 16GB|
|Hard Drive||OCZ Agility 4 256GB SSD|
AMD Radeon HD 7970 GHz Edition 3GB
NVIDIA GeForce GTX 680 2GB
AMD: 13.2 beta 7
NVIDIA: 314.07 beta
|Power Supply||Corsair AX1200i|
|Operating System||Windows 8 Pro x64|
Testing both 1920x1080 and 2560x1440 resolutions was very direct as we simply used our capture configuration as described on the previous pages. However, testing AMD Eyefinity and NVIDIA Surround required a bit more imagination since we need to use three different monitor connections and the higher data rates than the Datapath VisionDVI-DL is capable of. By capturing the left hand monitor though we are able to limit our actual resolution to 1920x1080 for the capture card while still presenting a full 5760x1080 to the gamer. This allows us to capture the overlay and thus measure frame rates and frame times as expected.