Develop Reference

Understanding dpi and the drawable folders

Yesterday I already asked a question about this here. After that I saw this on the official android developer page:
So you can have a 1024x600px screen device with ldpi and a screen with the same resolution with mdpi.
Furthermore you can have a 600x1024px screen device with mdpi and a screen with the same resolution with hdpi.
I have an image, that takes 20 % of the screen's height and width on every device.
I want to make 5 versions of that image (with different resolution) for each drawable folder.
In my past thread I got this answer:
ldpi: 0.75
mdpi: 1.0
hdpi: 1.5
xhdpi: 2.0
xxhdpi: 3.0
xxxhdpi: 4.0
These are the scale factors.
So if my image has a resolution of 50 x 100 px in mdpi it is supposed to have a resolution of 75 x 150 px in hdpi.
I understand that.
The question now is, how am I supposed to know what resolution my image should have in the mdpi folder, if the resolutions for mdpi are totally different (as you can see in the image above -> from 320x480 to 1280x768).
Thanks !

The drawables should be created at a resolution that makes them the right physical size when displayed on the screen. If you have an icon that is supposed to be 1/2 inch by 1/2 inch on an MDPI screen, then the image should be 80px by 80px. This size should be irregardless of the physical dimensions of the screen; the same drawable on a 1280x768 MDPI tablet is supposed to be 1/2 inch by 1/2inch.
If the icon in question should be larger on the tablet for some reason, then you would need to introduce the drawable-swXYZdp-mdpi folder that Der Golem mentioned above. In this folder, you would be able to create a version of the icon that had a larger physical size (say 3/4 inch) that will be loaded on the tablet.

Designing for various resolutions under MDPI

I am designing an application wherein I specify the dimensions for different DPI sizes in the dimens.xml.
I am able to design for all the values except MDPI.
The problem with MDPI (landscape) is the different resolutions available like, 1280 X 800, 1024 X 600, 480 X 854, 480 X 800 and 320 X 480.
How do I design for all the above mentioned resolutions?

Use Android asset-studio or draw9patch for MDPI design.
For refer link Android asset-studio,Draw9patch

android: Galaxy Note 2 : for which pixel layout images/icon should be prepared?

I am developing an app which must support Samsung Galaxy Note - 2.
Now note-2's pixel resolution is 1,280 x 720. Reference Wikipedia.
While developing an app, I found that it was picking images from res/drawable-hdpi.
So for what resolution layout I should prepare the images ? Or do need to make separate resource folder ?

Resources are picked based on DPI a quick reference for what DPI each device uses can be found here:
http://blog.blundell-apps.com/list-of-android-devices-with-pixel-density-buckets/
For image scaling, the images should roughly be sized like so:
if an image is to use 100 dp then the actual images sizes will be:
ldpi: 75px
mdpi: 100px
hdpi: 150px
xhdpi: 200px
if you want to use 100 px on your NOTE 2 then the image sizes will be
ldpi: 50px
mdpi: 66.67px
hdpi: 100px
xhdpi: 133.33px
you can use the following site do do quick calculations:
http://labs.skinkers.com/content/android_dp_px_calculator/

Prepare images for hdpi resolution -- 480x800 hdpi.

Galaxy Note 2 is normal size, xhdpi density and long. You can prepare layout or drawable for xhdpi. For details, please check this post Galaxy Note 2 screen specifications. If you don't have xhdpi resource, android will try to find resource in hdpi.

The Galaxy Note uses hdpi which is a pixel density of 240dpi
Android Device Resolutions

Android: Background Image Size (in Pixel) which Support All Devices

I am creating an Application which will run on all Android Devices. I want to create xhdpi Graphics for My App. My App is full screen. I am confused in Creating graphics. can any one tell me the best sizes of my background image in pixels.
For Example:
xhdpi: 720x1280 px
hdpi: 480x800 px
mdpi: 320x480 px
ldpi: 240x320 px
Suggest me the best sizes which will appear on all devices good. Because my graphics are the core of my app.
I want that every device get the best image which it want. Android will be not involved in compressing or expanding of image.

The following are the best dimensions for the app to run in all devices. For understanding multiple supporting screens you have to read
http://developer.android.com/guide/practices/screens_support.html
xxxhdpi: 1280x1920 px
xxhdpi: 960x1600 px
xhdpi: 640x960 px
hdpi: 480x800 px
mdpi: 320x480 px
ldpi: 240x320 px

Android Devices Matrices
ldpi mdpi hdpi xhdpi xxhdpi xxxhdpi
Launcher And Home 36*36 48*48 72*72 96*96 144*144 192*192
Toolbar And Tab 24*24 32*32 48*48 64*64 96*96 128*128
Notification 18*18 24*24 36*36 48*48 72*72 96*96
Background 240*320 320*480 480*800 768*1280 1080 *1920 1440*2560
(For good approach minus Toolbar Size From total height of Background Screen and then Design Graphics of Screens )
For More Help (This link includes tablets also):
https://design.google.com/devices/
Android Native Icons (Recommended) You can change color of these icons programmatically.
https://design.google.com/icons/

Check this. This image will show for all icon size for different screen sizes

I looked around the internet for correct dimensions for these densities for square images, but couldn't find anything reliable.
If it's any consolation, referring to Veerababu Medisetti's answer I used these dimensions for SQUARES :)
xxxhdpi: 1280x1280 px
xxhdpi: 960x960 px
xhdpi: 640x640 px
hdpi: 480x480 px
mdpi: 320x320 px
ldpi: 240x240 px

GIMP tool is exactly what you need to create the images for different pixel resolution devices.
Follow these steps:
Open the existing image in GIMP tool.
Go to "Image" menu, and select "Scale Image..."
Use below pixel dimension that you need:
xxxhdpi: 1280x1920 px
xxhdpi: 960x1600 px
xhdpi: 640x960 px
hdpi: 480x800 px
mdpi: 320x480 px
ldpi: 240x320 px
Then "Export" the image from "File" menu.

My understanding is that if you use a View object (as supposed to eg. android:windowBackground) Android will automatically scale your image to the correct size. The problem is that too much scaling can result in artifacts (both during up and down scaling) and blurring. Due to various resolutions and aspects ratios on the market, it's impossible to create "perfect" fits for every screen, but you can do your best to make sure only a little bit of scaling has to be done, and thus mitigate the unwanted side effects. So what I would do is:
Keep to the 3:4:6:8:12:16 scaling ratio between the six generalized densities (ldpi, mdpi, hdpi, etc).
You should not include xxxhdpi elements for your UI elements, this resolution is meant for upscaling launcher icons only (so mipmap folder only) ... You should not use the xxxhdpi qualifier for UI elements other than the launcher icon. ... although eg. on the Samsung edge 7 calling getDisplayMetrics().density returns 4 (xxxhdpi), so perhaps this info is outdated.
Then look at the new phone models on the market, and find the representative ones. Assumming the new google pixel is a good representation of an android phone: It has a 1080 x 1920 resolution at 441 dpi, and a screen size of 4.4 x 2.5 inches. Then from the the android developer docs:
ldpi (low) ~120dpi
mdpi (medium) ~160dpi
hdpi (high) ~240dpi
xhdpi (extra-high) ~320dpi
xxhdpi (extra-extra-high) ~480dpi
xxxhdpi (extra-extra-extra-high) ~640dpi
This corresponds to an xxhdpi screen. From here I could scale these 1080 x 1920 down by the (3:4:6:8:12) ratios above.
I could also acknowledge that downsampling is generally an easy way to scale and thus I might want slightly oversized bitmaps bundled in my apk (Note: higher memory consumption). Once more assuming that the width and height of the pixel screen is represetative, I would scale up the 1080x1920 by a factor of 480/441, leaving my maximum resolution background image at approx. 1200x2100, which should then be scaled by the 3:4:6:8:12.
Remember, you only need to provide density-specific drawables for bitmap files (.png, .jpg, or .gif) and Nine-Patch files (.9.png). If you use XML files to define drawable resources (eg. shapes), just put one copy in the default drawable directory.
If you ever have to accomodate really large or odd aspect ratios, create specific folders for these as well, using the flags for this, eg. sw, long, large, etc.
And no need to draw the background twice. Therefore set a style with <item name="android:windowBackground">#null</item>

How to resize images for medium screen size having images for large screen size in android

I've develop an app for android and now I support large screen devices. So I create an image for this device. Now I want to resize large images for medium screen sizes.
What is the proportion? For example if an image for large screen is 85x111(px), what is the dimension for medium?

There are different screen densities. ldpi, mdpi, hdpi, xhdpi , you have to create different images for all these densities, if you would like to support all screen.
see the links below.
http://developer.android.com/guide/practices/screens_support.html
http://developer.android.com/resources/dashboard/screens.html
hdpi ~ 240dpi - 240 pixels per inch.
mdpi ~ 160dpi - 160 pixels per inch.
so the scaling factor is 160/240 = 2/3. so if you have 85 x 111 in hdpi, for mdpi it should be 85*2/3 x 111*2/3.