Let's start with a simple question: how accurate is your gaming mouse? If your answer is in terms of DPI--maybe the number you've read off the side of the mouse' package--you're omitting a lot of attributes and variables that affect the accuracy and performance of a typical mouse. That's part of the reason it's difficult to objectively evaluate a gaming mouse. So much of user experience lies in subjective factors: the physical sculpting and weight of a mouse, your preference for button surface textures, etc. These are the things that you notice immediately and affect your day-to-day use, while sensor quality more often than not just has to pass a threshold of acceptable responsiveness and accuracy for non-professional gamers.
When I visited Logitech's Borel Innovation Center facility last week, I spoke with the company's engineers about the process of designing a gaming mouse and learned about the tracking variables that they care about when testing mouse accuracy.
Full disclosure: Logitech paid for my trip to their laboratories in Lausanne, Switzerland, but we were under no obligation to produce video or write about anything I saw there. The information I learned from Logitech's engineers is genuinely interesting to me from both a consumer and product reviewer's perspective, and the insights about mouse tracking variables are applicable any gaming mouse, whether it's made by Logitech or a competitor.
Logitech Senior Engineer François Morier explained that when developing tracking sensors for new generations of gaming mice, engineers look to four areas for improvement. They are, as indicated in the photo below: Tracking Quality, Surface Coverage, Power Consumption, and Size Reduction. These goals are relatively self-explanatory; it makes sense for Logitech to want to improve the accuracy of their sensors while also reducing its size and power consumption. But advancements in each of these areas doesn't always happen concurrently, nor are they always complimentary. Improving surface coverage, like being able to use a mouse on transparent glass, may come at the cost of accuracy or power consumption because a different imager technology is used. The advancement of each goal may require a tradeoff with another, and it's the job of mouse engineers to figure out the right balance of features to meet the needs of different users.
If you use a wired gaming mouse on a mousepad, the attribute you likely care about most is Tracking Quality. That's where specs like DPI comes in. DPI (dots per inch) is a metric taken from printer technology, and corresponds with the resolution of the mouse sensor. Mouse engineers actually refer to this as CPI, or counts per inch, because that more accurately describes what the sensor imager is doing when reading the data of light reflecting off the mouse surface. Think of it the megapixels of a camera.
Something interesting I learned was that an increase in DPI doesn't necessary imply a larger sensor imager. Just like with the megapixel myth in photography, a higher DPI isn't always better. It depends on how that higher number is achieved. In the case of mice, where the sensor size is often fixed for cost or physical space constraints, the higher DPI resolution is achieved through magnification in the plastic lensing. The lens (or series of lenses) that optical light has to pass through to get from tracking surface to the sensor can give the imager more data, which the digital signal processor can split up to into more "counts" to analyze and track. But because the sensor hasn't changed in size, this data may be "noisier" and more prone to errors.
Sensor resolution is one of many tracking variables that Logitech engineers consider then testing the accuracy of their new sensors. These are the variables that you should care about too when buying a new mouse, and most can be evaluated with simple testing using a drawing application on your desktop. With sensor resolution already covered, let's examine the other variables:
Angle Error. Accurately tracking horizontal and vertical movements is much easier for a mouse than tracking a perfect diagonal one. That's because diagonal tracking is derived from the combined data of horizontal and vertical movement. Mice sensors and processors try to compensate for that with some degree of prediction. So when the mouse thinks you're moving in a diagonal line, it will figure out the angle of movement and align the tracking point to that line. This avoids the "staircase" look of a diagonal line. But the snapping prediction can't be too strong, or it'll prevent you from being able to draw accurate curves. One way to test for this is to draw a circle very slowly.
Resolution vs. Speed Error. As I mentioned above, one consequence of increasing the resolution of the mouse sensor is that the data becomes a little fuzzier. As a result, the tracking of high-speed movement may become inconsistent. For example, if you move your mouse quickly from one end of the mousepad to the other, you would expect that moving the same distance back at the same speed would put your cursor back at its starting point. If a mouse fails to accomplish that, then it may be compromising speed accuracy for resolution.
Ripple. The effect of ripple and jitter is another one of those things that you might not notice in-game (or with mouse smoothing turned on), but it can be detected in design applications. If when drawing a straight horizontal or vertical line, you see slight deviations (or "jumps") along both sides of the line, the sensor may be inaccurately tracking noisy data. Again, this is something that can happen at high DPI settings.
Pixel Walk. This phenomenon occurs when moving your mouse at very low speeds, and is a good test to gauge for low-speed accuracy. To conserve power in wireless mice, tracking is sometimes disabled when the mouse isn't moving, and activated when the sensor detects a certain threshold of movement. That threshold is very tricky for mouse engineers, because they want to keep it as high as possible to conserve battery consumption while also as low as possible to track very slow and nuanced movements. When you're moving a mouse at the speed that hovers right at that threshold, tracking may start and stop, causing the cursor to jump.
Timing and Circle Drift. Drawing perfect circles is a really good way to test the accuracy of a mouse sensor. In this test, the timing of the tracker can be gauged by drawing a perfect circle several times. In the ideal scenario, the cursor will end each circle where it started, drawing several circles over the sample spot. But if sensor's timing is off, the the circles will be more like ovals and spiral away from the start position.
That should give you a good place to start for testing your own mouse. Logitech--and I assume other mice makers--have their own computerized tools for testing these tracking variables. But even without a machine that can draw a perfect circle, you should be able tune your settings to detect (and in some cases avoid) anomalies like angle error and line jittering.