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How to proper use DPI resolutions

I know I need to use different DPI for images and all graphics stuff, but I just don't understand if I have to create the different drawable folders by myself or if there's a proper way to do it, and plus, what to use them for.
For instance I need to add a wallpaper, where should I put it? And of what resolutions? What are the pixels needed for each?
Tried to read the Google page about this, but didn't get much, thank you!

I have to create the different drawable folders by myself or if
there's a proper way to do it?
Assuming you're using Android Studio:
what to use them for.
from http://developer.android.com/guide/practices/screens_support.html:
xlarge screens are at least 960dp x 720dp
large screens are at least 640dp x 480dp
normal screens are at least 470dp x 320dp
small screens are at least 426dp x 320dp
In that page you have documentation for screen sizes and densities.
Basically, when you put, let's say, an image in an activity, it will render according to the device's density corresponding folder.
a wallpaper, where should I put it?
The best is to have an image for each density, but if you don't have those provided, you can put it in the drawable folder, without any density qualifier, which means that your resource is the same for every screen density.
For a quick way of knowing your device screen stats, download ScreenInfo.

"...I just don't understand if I have to create the different drawable
folders by myself"
no, you don't have to, but you can.
"... if there's a proper way to do it"
If you right click on res then select new then select Image Asset, Android Studio has an Image Asset tool that guides you through the whole process. This is how you import various UI elements in their correct dpi and densities.
"... what to use them for"
This is upto you. But if you set an ImageButton to an drawable with various drawable resources for different screen dpi, the correct dpi drawable will be chosen according to the device's screen dpi when the app is running.
"wallpaper..?"
Put that in drawable-nodpi or just the regular drawable

Android Confusion on supporting multiple screen sizes

Ive read Android documentation on the subject of supporting multiple screen sizes but I just cant seem to wrap my head around it. I know qualifiers need to be set in layout names and android picks the one whos qualifier is closes to the width of the phone.
Im not to worried about tablets at the moment but when I develop for my physical device everything is fine and dandy all in default layout folder: layout/main.xml
But if I test on my friends phone Samsung Galaxy S4 everything is streched in a vertical way. I just dont know where to begin really...
Is there a standard set of folders that I could implement to lay my layouts in and then edit for optimized performance?

http://developer.android.com/training/basics/supporting-devices/screens.html. It's all explained in this tutorial link.

If one layout should support all screens then,
your app theame should be similer for tabs and mobiles
never use fixed values for layouts like (ex:300dp , 15dp), all are wrap , match , fill depends on requirement,
make all the icons, images in 2 or 4 different sizes
NOTE: if you have minor changes for smaller to larger device ui, then in on create get the device height and width , make your changes (images, layout widths etc..) runtime from java
If your app is not similer from small to big devices
Use layout(mobiles) ,layout-large(7 inch tabs) layout-xlarge(10 inch tabs)

Start with device independent pixels (dp) and scalable-independent pixels for fonts (sp)
If something is stretched vertically after that, it probably means it's a background image that's trying to fill up the entire height of a layout (by the way, next time you have a problem, please do post the relevant xml code and dimensions of the image, or at least a screenshot of the problem).
In that case, just use the different size qualifiers to solve your problem (not your density qualifiers). Density qualifiers don't help for widths or heights of bitmaps that are larger than the widths or the heights of device screens. This is just a rule of thumb that you should be aware of.
Use the toolbar select box in the layout designer of Android Studio/IntelliJ to quickly test multiple device screens all in one go. This is the fastest way to do it. Do not use Eclipse for that, its ADT layout designer doesn't have that capability (at least, not the last time I've checked). Even if you use Eclipse most of the time, it's worth using Android Studio/intelliJ just for that functionality, and then switching back to Eclipse (assuming it's still your favorite IDE after that) when you're done with testing that aspect of the layout.
Also, don't try to do a different layout for each size qualifier that you encounter. Only use different size qualifiers for the layouts that are actually giving you problems. And don't forget that common layout components that don't need to be duplicated can just be abstracted away in a parent layout. And if you can find a way to resolve your specific problem without the use of size qualifiers, that can work just as well. For instance, if instead of using a single image for your background, you could replace it with a larger image that you don't mind getting clipped at different aspect ratios, or replace the background image with something entirely different and abstract (like a solid color, a gradient, a tiling background, a large vector graphic, or a large patch-nine png) that looks ok at different aspect ratios, that could be even simpler still.

At the high level, you just need to take care of these things:
- Better have the images sliced for either XHDPI (720x1280) OR XXHDPI (1080x1920) resolution and keep them in respective folders. XHDPI images into drawable-xhdpi and XXHDPI images into drawable-xxhdpi folder. One set of image slices are enough.
- You don't need to write different XML layouts for the same screen if you are not supporting Tablets (except for very rare times)
- Try to avoid hard coding the android:layout_width & android:layout_height parameters. Use wrap_content, fill_parent OR match_parent.
- Try to avoid keeping any image slices inside simple drawable folder instead keep inside specific drawable folders like drawable-xhdpi , drawable-hdpi etc.
- Try to use, color codes & 9 patch images as much as possible instead of using images for everything which will reduce the build size and also helps in avoiding OutOfMemoryException.
- Inside simple drawable folder, keep all your XML drawables like background of a button with separate images for clicked & focused states etc.
If you follow these steps, you don't need to worry about supporting multiple screen resolutions for most of the cases.

App widget inconsistent sizing between tablet and phone

I am attempting to design an app which reproduces the shortcut styles of the standard app shortcut home screen icons but as a widget. Note, this is currently just looking at the standard Android homescreen.
I have made the following observations using the "Dump View Hierarchy for UI Automator" tool in the DDMS -> Devices view of Eclipse:
The space given in the 1 x 1 square changes depending on screen density, orientation and whether the phone home screen is in "phone mode" or "tablet mode" (start a 4.x emulator and you can tell by the background amongst other things). Screen density effects the size as expected (0.75, 1, 1.5, 2.0 density factor scaling), but the orientation and 'home screen mode' are quite unpredictable. Specifically:
DIMENSIONS FOR A 1x1 'SQUARE' ON THE HOMESCREEN
Phone mode:
Portrait: 80dp x 100dp = 4:5 aspect ratio
Landscape (if available - not thoroughly tested): 106dp x 74dp = 53:37 aspect ratio
Tablet mode:
Portrait: 96dp x 96dp = 1:1 aspect ratio
Landscape: 96dp x 96dp = 1:1 aspect ratio
As you can see, there is barely any consistency whatsoever, but these are the dimensions given to a 'square' on the home screen. It gets even worse when looking at the space actually given to widgets (yes, it does differ from the square space available and used by the system shortcut widgets):
DIMENSIONS GIVE TO A WIDGET IN A 1x1 'SQUARE'
Phone mode:
Portrait: 80dp x 100dp = 4:5 aspect ratio
Landscape (if available - not thoroughly tested): 90dp x 58dp = 45:29 aspect ratio
Tablet mode:
Portrait: 72dp x 72dp = 1:1 aspect ratio
Landscape: 72dp x 72dp = 1:1 aspect ratio
So I'm stuck with:
Inconsistent aspect ratios between phone and tablet modes
Smaller areas given for an app widget then that of an app shortcut in some cases. Consequently, the relative and perceived size of widgets will look very different on a tablet vs phone?
The smaller areas aren't even vertically centred, they are mostly top aligned with a 2dp top margin which means they don't even look aligned with a neighbouring application shortcut to start with, let alone when trying to produce a similar looking shortcut through a widget
To visualise, the following images show a default system shortcut (with a red box surrounding showing the 'square dimensions - which both system shortcuts and widgets always share). Next to it is my widget with the blue highlight showing the bounds of a MATCH_PARENT setup, hence where the widget can actually draw within its 'square':
Phone - Port:
Tablet - Land:
Tablet - Port:
Most important to take from here, is the blue area of the widgets on a tablet does not even include the space where the system shortcut draws its text?
Does anyone have insights into:
Why the home screen is so inconsistent?
Why app widgets are not given the same drawing space as that of a system shortcut / or should it be and I am doing something wrong?
Should I be doing something else, or is it just accepted that for a 1x1 widget which takes up all its given drawing space, it will look 'smaller' (compared to its surroundings) on a tablet than it would on a phone?

And even better, many launchers allow the user to customize the size of their grid. Per the design guidelines for App Widgets, you cannot rely on an exact size on all devices. Instead, determine a minimum width and height and let the home screen determine how many squares the layout takes up.
They do provide some guidance on what you can expect for the minimum size of a cell (and all your numbers are much larger than that it seems), but again custom launchers may reduce this minimum even lower.
I find it best to focus on what components need to be in your layout (buttons, etc) and then follow the Metrics and Grids design guidelines to determine your minimal size (noting the margins are automatically added on Android 4.0+ devices but not on <4.0 devices). Then make sure your layouts are dynamic enough to fill in the space provided to them (centering text, etc as appropriate). Remember widgets are assumed to be a framed, single entry, rather than floating text as noted on the App Widget Design guidelines.

Hey you can manage the scree sizes in layouts you want to create different layouts for different screen sizes and also for landscape and portrait .
create the following layouts in your resource folder like this :
layout-large
layout-small
layout-xlarge
layout-landscape
And then copy your XML files in to it and check in preview screens for different devices you can have a idea based on that you can modify the size of the widgets in your homescreen.

java android - fit items on a image background with different screen size

This is my first post and i have (i think) a uncommon request...
I'm programming on the last Eclipse Mac, for every android SDK, and i'm using more XML for layout than runtime... So i search a solution in XML if possible.
Here is the situation: i work with an image background wich simulate item like buttons, image, text... etc. On this image, i put buttons, image, and text (buttonview textview etc...) and place it precisely on their places on the image. This solution is very powerful to have good design BUT, if i change the resolution of the screen, and/or its size, each item won't be at its place, and will be translated (horiz and/or vertic) for some "dp"... (and yes i use dp, not mm or px or whatever)
I'm really embarrased because i think thanks to "dp" it keep proportionnality but.. not !
My question is how can i fix my items at their places on the image background, for different screen size/resolution !!
Thanks in advance everyone,
My Best From Lyon,France

First realise that if you want to make it pixel perfect for all screen sizes your out of luck.
Second accept that you can't make it perfect for all screen sizes.
Third you can get far with creating different layout for different screen sizes. You can read a lot about supporting multiple screen sizes here. One important thing to take from here is that you can make layout for the different screen sizes or different density sizes.

Best practices to use when targeting multiple screen resolutions on Android

When designing an UI, we need to target many android mutations and various screen resolutions.
How to differentiate layout for 480 * 800 and 480 * 854 screen resolutions in android?
how to layout support for various screen in android?
Both questions above recommend to use different Layout code for different screens. Is it really necessary?
http://developer.android.com/guide/practices/screens_support.html
recommends to
Provide different layouts for different screen sizes
Provide different bitmap drawables for different screen densities
But I still think, we can support multiple screens without creating standalone layout for each screen resolution. Or am I wrong?
I know, and I do use, 9-patch for scalable background images (or form inputs, and so on)
and I do use dp (density pixel) to declare sizes relatively to screen density
But what are next best practices ?

I think the answer to this has changed recently, but I'll go over historical solutions.
Pre-Honeycomb, the solution was to create a dynamic layout that could adjust itself to the size of the screen (using 9-patches, dips, layouts that expand/shrink, etc.). This meant that none of your Views could be "pixel perfect", but you could adjust to different aspect ratios that you encounter. Sometimes you would come up with different layouts for landscape and portrait, but beyond that customization wasn't necessary.
With the release of Honeycomb, the problem got a lot more complex. Now you've got dramatically different screen sizes, where your app stretching doesn't look good at all. You've got tablets that stretch from 7" to 10" - which one is "large" vs. "xlarge"? 3.0 and 3.1 were an awkward period, where you had to detect the API version/screen size and configure your app accordingly (supposing your app supports both phones and tablets).
However, everything's changed in 3.2 and beyond. Now, the best practice can be described thus: think like a web designer, not an Android designer. Instead of thinking about phone vs. tablet vs. landscape vs. portrait layouts, think instead of layouts that work on particular screen sizes.
This thought process is detailed by this blog post, and these slides, but I think it's best demonstrated by going to some sample web pages and seeing it in action. Try visiting this page (or this page, or this page) and changing the size of your browser. Notice how they dynamically change layout based on the width - this is what you want in Android as well.
So now you've got a layout that works between screen width 150dp and 400dp; another one that works between 401dp and 800dp; a third that handles 800dp and 1000dp, etc. This way of thinking does not end up with you, as a developer, doing too much more work than before - instead of defining a phone layout, a tablet layout (each with a portrait/landscape layout), you just define a few layouts that work with different widths.
Right now we're in an awkward transitional stage as most people don't have devices that support this practice. So "best practice" is essentially all of the above. In a few years, when everyone's got ICS and beyond, then we can all breath a sigh of relief.
(I apologize if you were looking for specific solutions; this answer ended up being relatively theoretical rather than having concrete answers, mostly because I felt the question was pretty open-ended. If you've got a specific problem you want to solve, I can try to address that elsewhere.)

If you build your views using "dp" it would, basically, be the same size for eack screen size.
In most cases you will prefer that your view will resize itself proportional to the screen size.
Of course, in most cases you will need to build separate layouts for tablets.
But, besides I can recommend you to do the next steps:
1. Add this library to your project.
2. Now in your layout you can write views like that:
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="I'm scalable!"
android:textSize="#dimen/_12sdp"/>
In this example your TextView will scale on each screen size.

These are some suggestions, in random order:
Avoid AbsoluteLayout, I know that it is deprecated, but you can still use it; forget it and use LinearLayout and RelativeLayout instead
Use 9-patch to create buttons and backgrounds, so that they will appear the same for each resolution
Use DisplayMetrics() to get informations about the screen, to show different things in different screens (i.e. to show AdMob "Banner" in smartphones and AdMob "Leaderboard" in tablets)
Create WVGA layout first (this is only a personal opinion, but I've found it a nice way of working): as far as I know the most smartphones are WVGA or at least HVGA (QVGA is not so frequent), so creating a perfect WVGA layout will make you able to satisfy lots of phones, than you can create a different layout for lpdi-screen and tablets (that are a market-share minority too, at the moment)
In conclusion I recommend you to create a layout for phones and on for tablets, that can adjust itself to screen resolution: this will fit the most devices; for borderline devices (i.e. 7" tablets or 5.3" phone) I'd work with different drawables/different XML layouts.

I would suggest as much as possible use Relative calculations rather than Absolute. Avoid Hard coding of numbers for padding, margins etc. Simply give them a value in relation to desired property like Width, height of screen or any component.