Easy Way to Cut and Paste Bitmaps

• Download CutBitmapTutorial.zip - 584.9 KB

Introduction

I have explained how to combine many bitmaps into one here. In this tip, I will guide the way to create a bitmap by cutting from a part of an original bitmap.

Using the code

At first, create an image original.png has size (400px, 400px) as below

Create bitmap from this image

Bitmap origialBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.original);        

Create empty bitmap

Bitmap cutBitmap = Bitmap.createBitmap(origialBitmap.getWidth() /          2,         origialBitmap.getHeight() /          2, Config.ARGB_8888);        

Normally, to cut an specified area of a bitmap, they use.

          public          void          drawBitmap (Bitmap          originalBitmap, Rect          src, Rect          dst, Paint          paint)

• src: The rectangle is used to specified the area will be cut in originalBitmap
• dst: After basing on src rectangle to cut the from an area of originalBitmap, the result bitmap will be translated/scaled by dst rectangle. With my experience, the easiest way is: size of src = size of cutbitmap = size of dst. It will help to keep the ratio.

How to cut left|top?

Based on the explanation above, if we want to cut left|top to get image ①, to create src rectangle, please see this picture:

To cut bitmap in red rectangle, we are easy to create src rectangle as below:

Rect srcRect =          new          Rect(0,          0, origialBitmap.getWidth() /          2, origialBitmap.getHeight() /          2);        

Using above src to cut bitmap, we will have:

About the dest rectangle, the easiest way is creating it with the same size with cutBitmap size

Rect srcRect =          new          Rect(0,          0, origialBitmap.getWidth() /          2, origialBitmap.getHeight() /          2);        

Draw bitmap!

canvas.drawBitmap(origialBitmap, srcRect, desRect, null);        

Please view the result in device

After cutting the left|top, it is easy to cut

right|top

          private          Bitmap cutRightTop() {     Bitmap origialBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.original);     Bitmap cutBitmap = Bitmap.createBitmap(origialBitmap.getWidth() /          2,             origialBitmap.getHeight() /          2, Config.ARGB_8888);     Canvas canvas =          new          Canvas(cutBitmap);     Rect desRect =          new          Rect(0,          0, origialBitmap.getWidth() /          2, origialBitmap.getHeight() /          2);     Rect srcRect =          new          Rect(origialBitmap.getWidth() /          2,          0, origialBitmap.getWidth(),             origialBitmap.getHeight() /          2);     canvas.drawBitmap(origialBitmap, srcRect, desRect, null);          return          cutBitmap;  }

left|bottom

          private          Bitmap cutLeftBottom() {     Bitmap origialBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.original);     Bitmap cutBitmap = Bitmap.createBitmap(origialBitmap.getWidth() /          2,             origialBitmap.getHeight() /          2, Config.ARGB_8888);     Canvas canvas =          new          Canvas(cutBitmap);     Rect srcRect =          new          Rect(0, origialBitmap.getHeight() /          2, origialBitmap.getWidth() /          2,             origialBitmap.getHeight());     Rect desRect =          new          Rect(0,          0, origialBitmap.getWidth() /          2, origialBitmap.getHeight() /          2);     canvas.drawBitmap(origialBitmap, srcRect, desRect, null);          return          cutBitmap; }        

right|bottom

          private          Bitmap cutRightBottom() {     Bitmap origialBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.original);     Bitmap cutBitmap = Bitmap.createBitmap(origialBitmap.getWidth() /          2,             origialBitmap.getHeight() /          2, Config.ARGB_8888);     Canvas canvas =          new          Canvas(cutBitmap);     Rect desRect =          new          Rect(0,          0, origialBitmap.getWidth() /          2, origialBitmap.getHeight() /          2);     Rect srcRect =          new          Rect(origialBitmap.getWidth() /          2, origialBitmap.getHeight() /          2,             origialBitmap.getWidth(),             origialBitmap.getHeight());     canvas.drawBitmap(origialBitmap, srcRect, desRect, null);          return          cutBitmap; }        

Points of Interest

Scale ratio

If you want to create bitmap with both number 1 and 2 in result, at first, src rectangle will be:

Rect srcRect =          new          Rect(0,          0, origialBitmap.getWidth(), origialBitmap.getHeight() /          2);        

But if we keep cut bitmap size and dest as old source code, what will happen? It is the result:

We can see the bitmap result has not good scaling. How to fix? Please make the size of cut bitmap and dest rectangle is the same with srcRect.

Bitmap cutBitmap = Bitmap.createBitmap(origialBitmap.getWidth(),         origialBitmap.getHeight() /          2, Config.ARGB_8888); Rect desRect =          new          Rect(0,          0, origialBitmap.getWidth(),         origialBitmap.getHeight() /          2);

The result will be:

But in fact, sometime, user wants to create cutBitmap smaller (or bigger than the size of srcRect). Suppose that scale to 0.5. Please note that:

dest = 0.5 * src

Rect desRect =          new          Rect(0,          0, (int) (origialBitmap.getWidth() *          0.5), (int) (origialBitmap.getHeight() /          2          *          0.5));

After that,  the cutBitmap size will base on desRect is the best way.

Bitmap cutBitmap = Bitmap.createBitmap(desRect.width(), desRect.height(), Config.ARGB_8888);

The result of scaling 0.5 will be:

Summary

To cut bitmap, please:

• Specified the cutting area, then making the src rectangle firstly.
• What scale ratio user wants? Base on it to create dest = ratio * src.
• Specified the cuttingBitmap size base on dest rectangleis the best way.

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Source: https://www.codeproject.com/tips/640058/how-to-cut-a-bitmap

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