What usage of the "keep screen ratio"?

[ASD]

How I do to use the "Keep screen ratio" option?

Is this option perhaps keep 4:3 ratio in wide screen?

Please tell me the example.

[Nobuyuki]

From the help file:

Keep screen ratio
In full screen mode, the resolution mode of the computer will change to match as best as possible both the size of the window and the width/height ratio of the current screen. This option avoids your application window to get distorted on wide screens.

Screen ratio tolerance
This option allows you to define a tolerance for the "Keep screen ratio" option. This tolerance is a percentage of the current screen ratio. It allows you to switch to a screen resolution close to the window size of the application even if its width/height ratio is not strictly identical to the current screen ratio.
Quick example : Suppose that the current resolution of the user's machine is 1440x900 (ratio = 1440/900 = 1.6), the window size of your application is 640x400 (ratio = 1.6) and the full screen resolutions available on the user's machines are :
640x480 (r=1.33)
720x480 (r=1.5)
720x576 (r=1.25)
800x600 (r=1.33)
1024x768 (r=1.33)
1152x864 (r=1.33)
1280x720 (r=1.77)
1440x900 (r=1.6)
As you can see, the only resolution with the same ratio is 1440x900. So if you want to execute your application in a screen resolution with the exact same ratio, you have no choice, your application will be played in a small 640x480 window in a big 1440x900 screen. If you set a tolerance of 8%, then any ratio between 1.47 and 1.72 will be considered as acceptable (between 8% below and above 1.6). So in this case, the 720x480 resolution will be chosen as it fits in the tolerance interval and is the smallest resolution greater than the window size of your application. The window of your application will be slightly distorted but this distortion is much less than if it was executed in 640x480 full screen mode.

When "Keep screen ratio" is enabled, it forces selection of a full screen resolution which preserves the same ratio as the game's window, with a 0% tolerance. This can create black bars on the sides of the application. By increasing tolerance, slight image distortion will occur on screen, but will stretch to cover the black bars up to n% of the original screen ratio.