Implementing Stickers on an Image in Phimpme

One feature we implemented in the Phimpme photo editing application is to enable users to add stickers on top of the image. In this blog, I will explain how stickers are implemented in Phimpme.

Features of Stickers

  • Users can resize the stickers.
  • Users can place the stickers wherever in the canvas.

Step.1-Storing the Stickers in assets folder

In Phimpme I stored the stickers in the assets folder. To distribute the stickers in different categories I made different folders according to the categories namely type1, type2, type3, type4 and so on.  

Displaying stickers

We used onBindViewHolder to Display the stickers in different categories like:

  • Facial
  • Express
  • Objects
  • Comments
  • Wishes
  • Emojis
  • Hashtags

String path will get the position of the particular type of stickers collection. This type is then loaded to the ImageLoader with the specific icon associating with the types.   

public void onBindViewHolder(mRecyclerAdapter.mViewHolder holder, final int position) {

   String path = pathList.get(position);
       ImageLoader.getInstance().displayImage("assets://" + path,holder.icon, imageOption);

   int size = (int) getActivity().getResources().getDimension(R.dimen.icon_item_image_size_filter_preview);
   LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(size,size);

   holder.itemView.setOnClickListener(new View.OnClickListener() {
       public void onClick(View v) {
           String data = (String) v.getTag();

Step.2- Applying a sticker on the image

When a particular sticker is selected selectedStickerItem() function is called.This function calls StickerView class to add the Bitmap on the image. It sends the path of the sticker as a parameter.  

public void selectedStickerItem(String path) {

In StickerView class the image of the sticker is then converted into a Bitmap. It creates an object(item) of StickerItem class. This object calls the init function, which handles the size of the sticker and the placement of the sticker on the image.

public void addBitImage(final Bitmap addBit) {
   StickerItem item = new StickerItem(this.getContext());
   item.init(addBit, this);
   if (currentItem != null) {
       currentItem.isDrawHelpTool = false;
   bank.put(++imageCount, item);

Step.3-Resizing the Sticker in the canvas

A bitmap or any image has two axes namely x and y. We can resize the image using matrix calculation.

float c_x = dstRect.centerX();
float c_y = dstRect.centerY();

float x = this.detectRotateRect.centerX();
float y = this.detectRotateRect.centerY();

We then calculate the source length and the current length:

float srcLen = (float) Math.sqrt(xa * xa + ya * ya);
float curLen = (float) Math.sqrt(xb * xb + yb * yb);

Then we calculate the scale. This is required to calculate the zoom ratio.

float scale = curLen / srcLen;

We need to rescale the bitmap. That is if the user rotates the sticker or zoom in or zoom out the sticker. A helpbox surrounds the stickers showing the actual size of the sticker. This helpbox which is rectangular shape helps in resizing the sticker.

RectUtil.scaleRect(this.dstRect, scale);// Zoom destination rectangle

// Recalculate the Toolbox coordinates
updateHelpBoxRect();// Recalculate
rotateRect.offsetTo(helpBox.right - BUTTON_WIDTH, helpBox.bottom
       - BUTTON_WIDTH);
deleteRect.offsetTo(helpBox.left - BUTTON_WIDTH,
       - BUTTON_WIDTH);

detectRotateRect.offsetTo(helpBox.right - BUTTON_WIDTH, helpBox.bottom
       - BUTTON_WIDTH);
detectDeleteRect.offsetTo(helpBox.left - BUTTON_WIDTH,
       - BUTTON_WIDTH);


In Phimpme a user can now place the sticker on top of the image. Resize the sticker, that is Zoom in the image or zoom out of the image. Move the image around the canvas. This will give users the flexibility to add multiple stickers on the image.