None of the "subject" type of photos (such as Cats or Birds as opposed to "patterns") are mine but collected from the Internet and modified to work well with the format of most "smart phones." I'm new to smart phones but not new to computer graphics, which I've been doing for over twenty years. The main issue of phone wallpapers is the "force crop" that is applied to an image when you install one. For now, I'm familiar only with the Android -- not the other operating systems such as Blackberry or Windows with regard to how wallpapers are handled.
Below are some diagrams to show how source images are cropped on Android. Let's use the following test image for best illustration.
The rainbow colored framework of the picture will be completely cropped at the sides of the image when your phone is vertical. When your phone is horizontal the top and bottom bands will be cropped. Aside from that, the status bar at the top of the phone's display (usually opaque black but shown here as opaque blue) covers part of what's left from the cropping. In addition, the "dock" at the bottom of the phone has a background itself (usually a translucent band letting some degree of the wallpaper image show through it and through any icons placed on it).
So the tan-colored areas are what would be visible depending on orientation -- the area within the "X" cross hairs is what would show on the display regardless of orientation.
A curiosity is that, when an Android wallpaper is oriented horizontally, the apparent image size is reduced to 95 per cent. I have no idea of what the purpose or the necessity of this is.
On the left is the original picture I found of a very cute kitten. One problem with it is that the subject is not centered. Another problem is that there is not enough of a margin around three sides (top, bottom and right) for making a nicely cropped image. Adapting the image for a wallpaper required me to create parts of it that didn't exist.
When installed on your phone the image looks like this:
-- not my best wallpaper because the ears and paws are slightly clipped in the horizontal. Let's just say that this one is better optimized for the vertical.
Likewise, if you apply a horizontally oblong picture it fits nicely only when the phone is horizontal. Broad, empty space will form at top and bottom when the phone is vertical.
So, that is why my wallpapers (especially subject wallpapers) are made square and optimized for either orientation -- to allow the phones operating system to crop an image in a way that doesn't affect the overall composition very much, and without installing extra apps. My guess is that the Force Crop is just a practical solution the Android developers came up with to make the wallpaper display more adaptable to a "landscape" view as well as the vertical (portrait view).
Also, most phones do not auto-rotate wallpapers though "slide" phones automatically do this when you slide the keyboard out. There is an app that will make non-slide phones auto-rotate wallpapers but I don't know the name of it so you would have to Google it if you're interested.
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