Nick Whittome MVP
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This article was derived from a post made by Adam Szofran, ACES Developer, on AVSIM.COM forums. All credit should be given to Adam, I have simply translated here as requested.
The symptom of this problem is increasingly blurry terrain textures the further and faster you fly. When you slow down or stop, high-resolution textures eventually appear after some delay. This is probably the more traditional and most frequent cause of the blurries people have seen in Combat Flight Simulator 2 (CFS2), Flight Simulator 2002 (FS8), and Flight Simulator 2004 (FS9).
This problem was caused by the scheduler for background tasks in the FS engine putting too much emphasis on maintaining high and even frame rates and not enough emphasis on preventing a backlog of background work, such as loading terrain textures.
To reduce this cause of the blurries, the ACES team reworked how Flight Simulator's scheduler prioritizes background tasks in FSX. Now much more CPU time is devoted to loading scenery data, including terrain textures, at the expense of somewhat lower frame rates. This change has mostly solved this cause of the blurries, although the scenery loader can still get behind at extremely low frame rates (less than 10 fps) or at very large airspeeds (faster than 600 knots).
There are several ways to adjust how much CPU time FS devotes to loading scenery and textures. The easiest way is to set the target frame rate slider to a value that your machine can consistently achieve. The lower you set the slider, the more CPU time is diverted from rendering to loading data. Another thing you can do is to modify the following variable in FSX.CFG:
Add this entry, just below the [MAIN] section in FSX.CFG
This variable determines the amount of CPU time given to loading scenery data as a fraction of the time spent rendering. For example, the default value of 0.33 means that for every 3 milliseconds spent rendering, FS will give 1 millisecond to the scenery loader. If necessary, you can use a larger value to devote more time to loading. Or, if you don't have a problem with the blurries and you want slightly higher frame rates, then you can use a smaller value. Again, this variable is only available in the final release version of FSX and is not available in the beta or demo.
The FSX.CFG File is located in the users application data path:
C:\Documents and Settings\username\Application Data\Microsoft\FSX
Other reasons for "The Blurries" may include:
Exhaustion of video memory.
If a particular scene requires more video memory than your video card has available, the Flight Simulator graphics engine has no choice but to remove some of the highest resolution mipmaps from video memory to save space, leaving only lower-resolution mipmaps for rendering. This usually appears as a sudden change. One moment, all the textures in the scene are sharp and full-resolution and suddenly everything becomes more blurry. It can also get progressively worse over several frames as FS works to fit more textures into video memory.
There are several ways to combat this problem.
a) If you are running at a very high screen resolution, lower it slightly to free up additional video memory.
b) Some video cards use lots of video memory for anti-aliasing, so try turning it off.
c) Turn down the setting of the "Global Max Texture Size" slider.
d) Lower the setting of the AutoGen and scenery complexity sliders.
e) Reduce the setting of the terrain texture resolution slider.
f) Turn down the amount of AI traffic.
Poor filtering settings.
Unfortunately, not all video cards and drivers respond the same way when Flight Simulator interacts with Direct3D to set up bilinear, trilinear, or anisotropic filtering. This means that in spite of our best efforts, the texture filter on your video card may not be set up to perform optimally. This can result in textures looking excessively blurry when viewed at an oblique angle. To fix this, try each of the different filter modes (bilinear, trilinear, anisotropic) to see if any of them perform better. If not, then try overriding Flight Simulator's filter settings with the settings in your display driver. You can do this with ATI and NVIDIA drivers by clicking on the ATI or NVIDIA icon in the tray next to the clock on your desktop and making the appropriate selections.