Android Graphics: Drawing same size circles

Question

I have a major issue I am working on for days now. It is much understandable by looking at the requirement first. I will list down my requirement as simple as possible in point form below.

1. I have 5 android phones. 5 different brands, different screen sizes.
2. Imagine the screen sizes are 4inch, 4.5inch, 5inch, 5.1inch and 5.2inch
3. I have an android app and it has a drawing canvas.
4. Now using canvas.drawCircle(x / 2, y / 2, radius, paint) I am drawing a circle. Imagine the radius is 100 (100 pixels?)
5. I install this app in my smallest screen phone, 4inch. Then I use a ruler and measure the the circle diameter. Imagine the circle diameter is "exact" 3cm.
6. Now I install this in my other phones.
7. Unfortunately, in my 4.5 inch phone the circle diameter is 3.2cm. In 5 inch phone it is 3.3 cm. In 5.1 inch phone it is 2.8cm and so on.
8. However I want my circle diameter to be 3cm (exact 3cm) in every phone.

Above requirement is something I am trying for days now. I tried the following to make sure I get a circle with no size change across all screens.

1. Using ImageView - I tried using an ImageView and added a circle image. I have given the width, height fixed. I have tryied setting the values in px, dp, inches etc. I also tried scalling options available for ImageView. Also tried placing the same image in drawable-nodpi folder. Tried creating drawable folders for all sizes (ex: drawable-mdpi, drawable-hdpi). Non of this stopped the image from being scaled.
2. Using Canvas - As explained in the above, I tried using canvas and drawing the image. The drawing scales.
3. Finding pixels per inch.- Tried finding pixels per inch of each phone programatically thinking I can find a way to develop a logic from it to dynamically draw the images in same size. I used several links including - Calculate PPI of Android Device. Most of them talk about getting screen resolution only.

How can I fulfill my requirement of drawing same size circles? If it can be done by knowing PPI, how should I do it?

Answer 1

The Answer before has worked for me but as long as it didn't for you I will try explain something I think it will help.

I have a device that have a density of 1 (160 px per inch), and the Display metrics told me that i have 320 * 480 pixels, with simple calculation I should know my mobile width and height in inch like this : 320 /160 = 2 inch for width , 480/160 = 3 . for now everything was just fine until i got the ruler and measured it and the surprise was this : I have a 1.65 * 2.48 inch !!.

Then I noticed that there is something called physical pixels per inch of the screen this was the relief for me and you can get it like this :

getResources().getDisplayMetrics().xdpi;  //for width physical dpi
getResources().getDisplayMetrics().ydpi;  //for height physical dpi

And now I can calculate my Physical width and height of my device like this :

my physical dpi for both width and height is 193.5238 so...

320 / 193.5238 = 1.65 inch width 480 / 193.5238 = 2.48 inch height

And that was correct !!.

Now back to your problem lets get the 3cm in pixels for any Phone :

getting the width in cm :

width in cm = width in inch * 2.54 (because 1 inch equals 2.54 cm)

getting the amount of pixels in each cm :

width in pixels / width in cm = Pixels per cm for width (px/cm)

now you want a 3cm then the pixels according to it is:

3 * Number = 3cm Pixels

all of the above can be shortened like this :

float devicePixelsWidth = getResources().getDisplayMetrics().widthPixels;
float deviceActualDpi = getResources().getDisplayMetrics().xdpi ;
float deviceActualInchWidth = devicePixelsWidth / deviceActualDpi ;
float deviceActualCMWidth = deviceActualInchWidth * 2.54f ;

float PixelsForActual3CM = devicePixelsWidth / deviceActualCMWidth * 3;

In the end, all of the above has been tested and approved by me and has the same accuracy of the previous Answer method :)

Answer 2

maybe this is not answer, but might be helpful

float _100MmAsPx = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_MM,
    100/*mm unit set*/, getResources().getDisplayMetrics());

above gives me 1661.5354 on Nexus 5x and 1889.7638 Nexus 5 (emulator). both have fullHD display, but 5 has 4.95 inch versus 5.2 inch in 5x.

it means 100 millimeters is 1662 pixels on 6 and 1890 on 5x