Understanding Density Independence Pixel - SP, DP, DIP And All In Android

Google defines pixel as "a minute area of illumination on a display screen, one of many from which an image is composed."
Screen Resolution
 Resolution is always referred to  in x*y form, which means x is the number of pixels in a horizontal direction and y is the number of pixels in a vertical direction. 

HD display 1920*1080 = 2,083,600 is the total pixels on screen

Pixels per inch
This is the measurement of the pixel density (resolution).

I have an image of 100*100 pixels which I need to print on a 1-inch square. Now, it will have a resolution of 100 pixels per inch.
<Button android:layout_width=”100dp” android:layout_height=”50dp” />
Let’s see the relation between pixels and density-independent pixels

A density-independent pixel (dp) is a unit of measure. When rendered to the screen, objects that are measured in density-independent pixels use the number of physical pixels calculated from the screen's pixel density.
  1. <TextView android:layout_width=”100dp” android:layout_height=”50dp” android:textSize="20sp" />   
What’s the conversion of pixel and dip?
px = dp * (dpi / 160)
You can set the child size by using specific units when you need the child to manage its own size. Android lets you specify sizes in pixels (px).
<Buttonandroid:layout_width="100px"... />
However, Android's best-practice guidelines recommend that you avoid using px.
Through these terminologies, we can say that now we can design the UI for Android based on the pixels of the displaying screen. But, this world consists of a variety of display screen sizes which is not possible for us to design with.
Here comes the Density Independent Pixel (dip).
In Android development, we have seen many developers using dp as a measurement unit for all the views. But, what’s dp? What's sp? And how does this dp/sp help us to achieve the same size in different screen sizes?
Android best-practice guidelines recommend that you use density-independent pixels to specify sizes.
Taking an example of three devices of the same physical size but different resolution.
If I define my button’s height and width in pixels, this is going to happen in different device resolutions. Button covers 2 pixels horizontally and 2 pixels vertically but the pixel density (resolution) is different which makes our button size small.
So, what’s the solution here? Now, I will use dp as a measurement unit.
Here, we can see the size of the button is the same in all the devices. What Android has done here is, map the number of pixels.
There is one more unit called SP. What is sp? And when should we use sp? And when should we use dp?
Suppose you defined the following button in your UI,
<Buttonandroid:layout_width="100dp"... />
The goal for the button is to occupy about the same area on the screen regardless of the device's screen density. On a high-resolution screen, this button would occupy more than 100 physical pixels.
A button with a width of 100 dp will appear about the same size on a 500-dpi screen and a 250-dpi screen. It will occupy a different number of physical pixels in the two cases to achieve the required size.
Scale Independent Pixels
This is same as dp, but by default, Android resizes it based on the font size of the user’s phone.
sp is only used for the text, never use it for the layout sizes.
In Android, we have a baseline density of 160 dots-per-inch(dpi). So, for a 160 dpi screen, we have 1 pixel = 1 dp and 320 dpi screen, we have 2 pixels = 1 dp which is 2x.
Let’s say we have a tablet of 1280*800 pixels, 160 dpi and phone of 800*1280 pixels, 320 dpi. Their pixel resolution is the same. But, what will be in the dp?
Tablet is of 160 dpi which is baseline so, 1280*800 dp but the phone is of 320 dpi which is half, 400*640dp. This helps us to design the layout for a tablet and phone very quickly. We can also think of a screen of 1280 dp versus 400 dp.
Let’s say we have two devices of the same size but different resolutions. One is 400*640 pixels, 160 dpi and another is 800*1280 pixels, 320 dpi.
Now, if we convert this in density-independent pixels, we have the same size of 400*640 dp. This makes us design a single layout for both the screens.
Now, we can understand the meaning of Density Independent Pixels, (Pixels not depending on the density).