draw square 3x3 cm in Android

问题

I need to draw an exact square 3x3 cm on the screen, you need the exact dimensions in any type of screen as design work.

The result in a Samsung Ace is 2.8 cm, the result is not exact.

Java...

DisplayMetrics metrics = new DisplayMetrics();

getWindowManager().getDefaultDisplay().getMetrics(metrics);

float ld =  30 * metrics.densityDpi * (metrics.density/25.4f);

bitmap = Bitmap.createBitmap((int) ld, (int) ld, Bitmap.Config.ARGB_8888);

canvas = new Canvas(bitmap);
paint = new Paint();

paint.setColor(Color.WHITE);

imageView.setImageBitmap(bitmap);

canvas.drawRect(0, 0, (float) ld, (float) ld, paint);

XML...

<ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/image_view"
        android:scaleType="center"/>

thank you very much

回答1:

I think you need to use the dpi in the specific directions x and y eg:

// We want 30/25.4 inches but in dots
float inches = 30/25.4f;

float xdpi = getResources().getDisplayMetrics().xdpi;
float xDots = inches * xdpi;
float ydpi = getResources().getDisplayMetrics().ydpi;
float yDots = inches * ydpi;

Paint paint = new Paint();
paint.setColor(Color.WHITE);
canvas.drawRect(0, 0, xDots, yDots, paint);

Then you will have more chance, but they are not accurate for every phone. I got the following results after placing this code in the onDraw() method of and Android app:

Nexus 7:        2.8 cm
Galaxy Nexus    3.0 cm
HTC Sensation   3.0 cm
HTC Desire HD   2.7 cm

So it is not exact in all cases.

来源：https://stackoverflow.com/questions/15035792/draw-square-3x3-cm-in-android