Android How to overlay a bitmap and draw over a bitmap

Question

I have three questions actually    Is it better to draw an image on a bitmap or create a bitmap as resource and then draw it over a bitmap  Performance wise  which one is better  If I want to draw something transparent over a bitmap  how would I go about doing it  If I want to overlay one transparent bitmap over another  how would I do it    Sorry for the long list  but in the interest of learning  I would like to explore both the approaches

User · Answer

If the purpose is to obtain a bitmap  this is very simple   Canvas canvas   new Canvas    canvas setBitmap image   canvas drawBitmap image2  new Matrix    null     In the end  image will contain the overlap of image and image2

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public static Bitmap createSingleImageFromMultipleImages Bitmap firstImage  Bitmap secondImage  ImageView secondImageView        Bitmap result   Bitmap createBitmap firstImage getWidth    firstImage getHeight    firstImage getConfig         Canvas canvas   new Canvas result       canvas drawBitmap firstImage  0f  0f  null       canvas drawBitmap secondImage  secondImageView getX    secondImageView getY    null        return result

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I can t believe no one has answered this yet  A rare occurrence on SO   1  The question doesn t quite make sense to me  But I ll give it a stab  If you re asking about direct drawing to a canvas  polygons  shading  text etc     vs  loading a bitmap and blitting it onto the canvas that would depend on the complexity of your drawing   As the drawing gets more complex the CPU time required will increase accordingly  However  blitting a bitmap onto a canvas will always be a constant time which is proportional to the size of the bitmap   2  Without knowing what  something  is how can I show you how to do it  You should be able to figure out  2 from the answer for  3   3  Assumptions    bmp1 is larger than bmp2  You want them both overlaid from the top left corner       private Bitmap overlay Bitmap bmp1  Bitmap bmp2            Bitmap bmOverlay   Bitmap createBitmap bmp1 getWidth    bmp1 getHeight    bmp1 getConfig             Canvas canvas   new Canvas bmOverlay           canvas drawBitmap bmp1  new Matrix    null           canvas drawBitmap bmp2  new Matrix    null           return bmOverlay

User · Answer

I think this example will definitely help you overlay a transparent image on top of another image  This is made possible by drawing both the images on canvas and returning a bitmap image    Read more or download demo here  private Bitmap createSingleImageFromMultipleImages Bitmap firstImage  Bitmap secondImage            Bitmap result   Bitmap createBitmap firstImage getWidth    firstImage getHeight    firstImage getConfig             Canvas canvas   new Canvas result           canvas drawBitmap firstImage  0f  0f  null           canvas drawBitmap secondImage  10  10  null           return result          and call the above function on button click and pass the two images to our function as shown below  public void buttonMerge View view             Bitmap bigImage   BitmapFactory decodeResource getResources    R drawable img1           Bitmap smallImage   BitmapFactory decodeResource getResources    R drawable img2           Bitmap mergedImages   createSingleImageFromMultipleImages bigImage  smallImage            img setImageBitmap mergedImages           For more than two images  you can follow this link  how to merge multiple images programmatically on android

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For Kotlin fans    U can create  a more generic extension      private fun Bitmap addOverlay  DimenRes marginTop  Int   DimenRes marginLeft  Int  overlay  Bitmap   Bitmap            val bitmapWidth   this width         val bitmapHeight   this height         val marginLeft   shareBitmapWidth - overlay width - resources getDimension marginLeft          val finalBitmap   Bitmap createBitmap bitmapWidth  bitmapHeight  this              config           val canvas   Canvas finalBitmap          canvas drawBitmap this  Matrix    null          canvas drawBitmap overlay  marginLeft  resources getDimension marginTop   null          return finalBitmap           Then use it as follow     bitmap addOverlay  R dimen top margin  R dimen left margin  overlayBitmap

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public static Bitmap overlayBitmapToCenter Bitmap bitmap1  Bitmap bitmap2        int bitmap1Width   bitmap1 getWidth        int bitmap1Height   bitmap1 getHeight        int bitmap2Width   bitmap2 getWidth        int bitmap2Height   bitmap2 getHeight         float marginLeft    float   bitmap1Width   0 5 - bitmap2Width   0 5       float marginTop    float   bitmap1Height   0 5 - bitmap2Height   0 5        Bitmap overlayBitmap   Bitmap createBitmap bitmap1Width  bitmap1Height  bitmap1 getConfig         Canvas canvas   new Canvas overlayBitmap       canvas drawBitmap bitmap1  new Matrix    null       canvas drawBitmap bitmap2  marginLeft  marginTop  null       return overlayBitmap

User · Answer

You can do something like this   public void putOverlay Bitmap bitmap  Bitmap overlay        Canvas canvas   new Canvas bitmap       Paint paint   new Paint Paint FILTER BITMAP FLAG       canvas drawBitmap overlay  0  0  paint        The idea is very simple  Once you associate a bitmap with a canvas  you can call any of the canvas  methods to draw over the bitmap   This will work for bitmaps that have transparency  A bitmap will have transparency  if it has an alpha channel  Look at Bitmap Config  You d probably want to use ARGB 8888   Important  Look at this Android sample for the different ways you can perform drawing  It will help you a lot   Performance wise  memory-wise  to be exact   Bitmaps are the best objects to use  since they simply wrap a native bitmap  An ImageView is a subclass of View  and a BitmapDrawable holds a Bitmap inside  but it holds many other things as well  But this is an over-simplification  You can suggest a performance-specific scenario for a precise answer

[android] Android: How to overlay a bitmap and draw over a bitmap?

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