Develop Reference

Understanding Drawables and Images sizes

Firstly I did a lot of search to understand how it work, but I don't find simples tutorials. An exemple, this view:
These are my drawables folders:
The selected device is: Pixel 5.0 1080 x 1920 (xxhdpi).
Immagine in this resolution (1080 x 1920) I set the image view on the top in blue color (Solutis) with 700px of width and 250 px of height, how I have to resize this images for each drawables folders ?
I found this informations:
LDPI - 0.75x
MDPI - Original size // means 1.0x here
HDPI - 1.5x
XHDPI - 2.0x
XXHDPI - 3x
XXXHDPI - 4.0x
And
LDPI: Portrait: 200 X 320px
MDPI: Portrait: 320 X 480px
HDPI: Portrait: 480 X 800px
XHDPI: Portrait: 720 X 1280px
XXHDPI: Portrait: 960 X 1600px
XXXHDPI: Portrait: 1440 x 2560px
Here I don't understand why when I select my vistuel device 1080 x 1920px on the Design Edit it say xxhdpi and xxhdpi is 960 X 1600px...
And what gonna be the different sizes of the image for the differents drawables ?
If someone can publish a project exemple, I wool look on, please.

Did you check the documentation? Here is the interesting part:
Density-independent pixel (dp)
A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way.
The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160). For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.
This means if your device as a higher density a bigger image will been chosen.

android: image in different devices

I want to insert images in my android application, first I will be create the images with photoshop then insert it in the projetct.
I have a problem in the size of the images:
how convert dpi to width and height in pixels
LDPI,MDPI,HDPI,XHDPI,XXHDPI= width px * height px
I want to know the range of values that can the variables width and height.
thanks.

LDPI = 120 pixels/inch
MDPI = 160 pixels/inch
HDPI = 240 pixels/inch
XHDPI = 320 pixels/inch
XXHDPI = 480 pixels/inch
XXXHDPI = 640 pixels/inch
These are approximations, actual device display resolutions vary somewhat
See this: https://developer.android.com/guide/practices/screens_support.html

I can not deal with non-standard resolution

I have design for my project. It for Nexus 6 with resolution 1440px x 2560px and 493 ppi. My design - 1440px x 2560px.
I know that:
ldpi 120 dpi
mdpi 160 dpi
tvdpi 213 dpi
hdpi 240 dpi
xhdpi 320 dpi
xxhdpi 480 dpi
xxxhdpi 640 dpi
I have - 493 ppi. what's this? I have a button size 100x50 pixels on the design.
how many dpi need button to look as well as in the design?

Hope this helps: Getting Your Apps Ready for Nexus 6 and Nexus 9
Nexus 6
Screen
The Nexus 6 boasts an impressive 5.96” Quad HD screen display at a
resolution of 2560 x 1440 (493 ppi). This translates to ~ 730 x 410 dp
(density independent pixels).
Check your assets
It has a quantized density of 560 dpi, which falls in between the
xxhdpi and xxxhdpi primary density buckets. For the Nexus 6, the
platform will scale down xxxhdpi assets, but if those aren’t
available, then it will scale up xxhdpi assets.
EDIT:
Consider this line
2560 x 1440 (493 ppi). This translates to ~ 730 x 410 dp (density
independent pixels)
It implies the scaling factor is 2560/730 = 1440/410 ~ 3.5
For a screen of width 410 and height 730 in dp, if you want to specify a button of half the screen width in dp, the width should be 410/2 = 205 dp which translates to 717px = 205*3.5 (width in dp * scaling factor)
A button of size 100px x 50px would translate to 28 x14 dp (100/3.5 * 50/3.5) on a 410 x 730 dp screen size.

If you are genuinely concerned about proper display resolution (which it sounds like you are) then you should never rely on the generic folders. The folders are there for generalized purposes - not for customized or high value display.
You should use displayMetrics to measure the screen size and select the appropriate asset, or even do customized image scaling. There are a lot of resources around this set of properties, but essentially it allows you to address the issue you are facing: set appropriate margins, padding, resolution, layout, etc. for a highly customized display.
Here are the docs:
http://developer.android.com/reference/android/util/DisplayMetrics.html
And then some "simple" implementations of getting the metrics:
How to get screen display metrics in application class
getting the screen density programmatically in android?

Image resolution for mdpi, hdpi, xhdpi and xxhdpi

I have a background for my app in resolutions 720x1280 pixels, 1080x1920 pixels and 1440x2560 pixels.
In which folders (mdpi, hdpi, xhdpi and xxhdpi) should I put each background?

Please read the Android Documentation regarding screen sizes.
From a base image size, there is a 3:4:6:8:12:16 scaling ratio in drawable size by DPI.
LDPI - 0.75x
MDPI - Original size // means 1.0x here
HDPI - 1.5x
XHDPI - 2.0x
XXHDPI - 3x
XXXHDPI - 4.0x
For example, 100x100px image on a MDPI will be the same size of a 200x200px on a XHDPI screen.

Require Screen sizes for splash :
LDPI: Portrait: 200 X 320px
MDPI: Portrait: 320 X 480px
HDPI: Portrait: 480 X 800px
XHDPI: Portrait: 720 X 1280px
XXHDPI: Portrait: 960 X 1600px
XXXHDPI: Portrait: 1440 x 2560px
Require icon Sizes for App :
http://iconhandbook.co.uk/reference/chart/android/

DP size of any device is (actual resolution / density conversion factor).
Density conversion factor for density buckets are as follows:
ldpi: 0.75
mdpi: 1.0 (base density)
hdpi: 1.5
xhdpi: 2.0
xxhdpi: 3.0
xxxhdpi: 4.0
Examples of resolution/density conversion to DP:
ldpi device of 240 X 320 px will be of 320 X 426.66 DP. 240 / 0.75 = 320 dp 320 / 0.75 = 426.66 dp
xxhdpi device of 1080 x 1920 pixels (Samsung S4, S5) will be of 360 X 640 dp. 1080 / 3 = 360 dp 1920 / 3 = 640 dp
This image show more:
For more details about DIP read here.

Check the image above I hope it will help someone.
Link to the whole article itself

Your inputs lack one important information of device dimension.
Suppose now popular phone is 6 inch(the diagonal of the display), you will have following results
DPI: Dots per inch - number of dots(pixels) per segment(line) of 1 inch.
DPI=Diagonal/Device size
Scaling Ratio= Real DPI/160.
160 is basic density (MHDPI)
DP: (Density-independent Pixel)=1/160 inch, think of it as a measurement unit

in order to know the phone resolution simply create a image with label mdpi, hdpi, xhdpi and xxhdpi. put these images in respective folder like mdpi, hdpi, xhdpi and xxhdpi. create a image view in layout and load this image.
the phone will load the respective image from a specific folder. by this you will get the phone resolution or *dpi it is using.

android xdpi folders, what size for my image?

I've been reading the following: http://developer.android.com/guide/practices/screens_support.html
It's about the folders res-long-land-hdpi, res-notlong-land-dpi, res-notlong-port-ldpi etc. And what they actually mean and when they should be used.
The article also says the following:
xlarge screens are at least 960dp x 720dp
large screens are at least 640dp x 480dp
I'm developing for a device that is: 1280 x 720 and one that is 480 x 800
My app will only be available in portrait mode.
So that means i have to put images in the res-long-port-hdpi folder for the first device, and in res-notlong-port-ldpi for the second device right?
Then the next problem i have with this is. I have an image which is 332 x 226 pixels. This looks fine on the first (xlarge) device. But to what size do i have to rescale this image so that i can place it in the (lower resolution) res-xx-xx-ldpi folder?
I'm not sure how to calculate the new sizes for the different folders.

The size ratios should match the nominal pixel densities as Android defines them:
ldpi - 120 pixels/inch; dpi scale = .75 (4 dpi = 3 pixels)
mdpi - 160 pixels/inch; dpi scale = 1 (1 dpi = 1 pixel)
hdpi - 240 pixels/inch; dpi scale = 1.5 (2 dpi = 3 pixels)
xhdpi - 320 pixels/inch; dpi scale = 2 (1 dpi = 2 pixels)
So if you make your xhdpi images twice the size (in pixels) of the mdpi resources, they will be the same dpi, meaning they will be the same physical size on their respective screens.