Background images must be at a maximum of 1024x768 pixels for 4:3, 1366x768 pixels for 16:9. Although 1366x768 (16:9) is recommended, most background ratios that are 4:3 or 16:9 should also work reasonably well (for example, 800x600 or 852x480). Using an image with a different ratio is fine, but will result in letterboxing (black bars at the top/bottom of the screen) or upscaling (resized image to fill the whole screen).the first half of the last sentence is just wrong. osu does not letterbox backgrounds (unless it's part of an SB that does not have widescreen support checked, in which case that is not within the scope of this rule). The easiest way to amend this would be as follows:
Background images must be at a maximum of 1024x768 pixels for 4:3, 1366x768 pixels for 16:9. Although 1366x768 (16:9) is recommended, most background ratios that are 4:3 or 16:9 should also work reasonably well (for example, 800x600 or 852x480). Using an image with a different ratio is fine, but will result in letterboxing (black bars at the top/bottom of the screen) or upscaling (resized image to fill the whole screen).
However, if I were to write this rule from scratch, this is how I would put it:Background images may have a maximum height of 768 pixels. An aspect ratio of 16:9 (e.g. 1366x768) is recommended, but other aspect ratios (i.e. 4:3, 16:10, etc.) are allowed. Images with lower vertical or horizontal resolutions than that of the player's screen will be upscaled to fill the player's screen.
I personally think that this is better than the simple amendment to the original. see the following box for why:reasoning, in a box since this is getting lengthy
i like backgroundsBackground images may have a maximum height of 768 pixels.
height is literally the only thing enforced by osu itself; huge images wider than 16:9 are only downscaled by osu to a height of 768, with widths still wider than 1366 (first image is the bg I used on a map (21:9 aspect ratio, 2100x900 resolution), the second is what osu resized it to automatically (21:9 aspect ratio, 1792x768 resolution))
An aspect ratio of 16:9 (e.g. 1366x768) is recommended, but other aspect ratios (i.e. 4:3, 16:10, etc.) are allowed.
16:9 is the widest and most common aspect ratio, so that is why i would mention it this way. wider aspect ratios inherently have higher viable resolutions in osu than thinner aspect ratios, since height is the only limit.Images with lower vertical or horizontal resolutions than that of the player's screen will be upscaled to fill the player's screen.
basically just a rewording of the original last sentence, omitting the flawed letterboxing part.