Friday, August 21, 2009

Drawable Image View in Android

I recently needed an image view that scaled a bitmap to its size and could be drawn on using a touchscreen.

Its not very polished or extensible (the line size and color is hardcoded), but it works well for a prototype: Update: I went ahead and made the color and line width configurable:

import android.content.Context;
import android.view.MotionEvent;
import android.view.View;

public class DrawableImageView extends View {
private Bitmap mBitmap;
private Bitmap pic;
private Canvas mCanvas;
private final Paint mPaint;
private int a = 255;
private int r = 255;
private int g = 255;
private int b = 255;
private float width = 4;

public DrawableImageView(Context c, Bitmap img) {
pic = img;
mPaint = new Paint();

Bitmap newBitmap = Bitmap.createBitmap(img.getWidth(), img.getHeight(), Bitmap.Config.RGB_565);
Canvas newCanvas = new Canvas();
if (img != null) {
newCanvas.drawBitmap(img, 0, 0, null);
mBitmap = newBitmap;
mCanvas = newCanvas;


public DrawableImageView(Context c, Bitmap img, int alpha, int red, int green, int blue) {
this(c, img);
setColor(alpha, red, green, blue);
public DrawableImageView(Context c, Bitmap img, int alpha, int red, int green, int blue, float w) {
this(c, img, alpha, red, green, blue);
width = w;

public Bitmap getBitmap() {return mBitmap;}
public void setWidth(float w) {width = w;}
public void setColor(int alpha, int red, int green, int blue) {
a = alpha;
r = red;
g = green;
b = blue;
public void Undo() {
mCanvas.drawBitmap(pic, 0, 0, null);

float scaleX;
float scaleY;
float scale;
@Override protected void onSizeChanged(int w, int h, int oldw, int oldh) {
scaleX = (float) w/mBitmap.getWidth();
scaleY = (float) h/mBitmap.getHeight();
scale = scaleX > scaleY ? scaleY : scaleX;

@Override protected void onDraw(Canvas canvas) {
if (mBitmap != null) {
Matrix matrix = new Matrix();
matrix.postScale(scale, scale);
canvas.drawBitmap(mBitmap, matrix, null);
//canvas.drawBitmap(mBitmap, 0,0, null);

float lastX;
float lastY;
@Override public boolean onTouchEvent(MotionEvent event) {

float curX = event.getX()/scale;
float curY = event.getY()/scale;
switch (event.getAction()){
case MotionEvent.ACTION_DOWN:{
mCanvas.drawCircle(curX, curY,width/2/scale, mPaint);
case MotionEvent.ACTION_MOVE:{
mCanvas.drawLine(lastX, lastY, curX, curY, mPaint);
mCanvas.drawCircle(curX, curY,width/2/scale, mPaint); //fix for weird jaggies occur between line start and line stop
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:{
mCanvas.drawLine(lastX, lastY, curX, curY, mPaint);
mCanvas.drawCircle(curX, curY,width/2/scale, mPaint);

lastX = curX;
lastY = curY;
invalidate(); //invalidate only modified rect...

return true;

TouchPaint was used extensively to figure out how to do this, along with the standard 2d drawing doc.

For me the most non-obvious part of the whole 2d drawing thing was how canvases work. I originally didn't quite understand how a given canvas was linked to what was actually drawn on the screen. In short you have your own canvas, in this case mCanvas, that operates on your own bitmap, mBitmap, then when the onDraw(Canvas) method is called, you copy your bitmap over to the canvas that method provides. The canvas provided by onDraw is backed by the bitmap rendered to the screen, which is how your modifications actually make it to the screen.