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Rect.offset() not working along negative x axis

I am doing a Pong clone and I've implemented the ball as a Rect() object. Now to move the ball I use the Rect.offset(dx,dy), which offsets the ball by a specefied speed. For bouncing the ball off a wall I multiply the velocity of respective axis by -1. Now for bounce along the Y-axis , it bounces perfectly, but along the X-axis it starts behavioral weirdly. Is this some glitch with the android studio or am I doing something wrong?
package com.nblsoft.pong;
import android.content.Context;
import android.content.res.Configuration;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.view.MotionEvent;
import android.view.View;
public class PongLogic extends View {
//set screen constrains in dip
Configuration configuration = this.getResources().getConfiguration();
int dpHeight = configuration.screenHeightDp; //The current height of the available screen space, in dp units, corresponding to screen height resource qualifier.
int dpWidth = configuration.screenWidthDp; //The current width of the available screen space, in dp units, corresponding to screen width resource qualifier.
//int smallestScreenWidthDp = configuration.smallestScreenWidthDp; //The smallest screen size an application will see in normal operation, corresponding to smallest screen width resource qualifier.
//DisplayMetrics displayMetrics = this.getResources().getDisplayMetrics();
//float dpHeight = displayMetrics.heightPixels / displayMetrics.density;
//float dpWidth = displayMetrics.widthPixels / displayMetrics.density;
private int dptopixel(int DESIRED_DP_VALUE){
final float scale = getResources().getDisplayMetrics().density;
return (int)((DESIRED_DP_VALUE) * scale + 0.5f);
}
private int pixeltodp(int DESIRED_PIXEL_VALUE){
final float scale = getResources().getDisplayMetrics().density;
return (int) ((DESIRED_PIXEL_VALUE) - 0.5f / scale);
}
//set paddle size, speed, position vector
int AI_paddle_pos_x = 4 * (dptopixel(dpWidth)/100); //3 for 320x480, 10 for 1080x1920 etc.
int paddle_width = (dptopixel(dpWidth)/10); //
int AI_paddle_pos_y = (dptopixel(dpHeight)/10); //48 for 320x480, 190 for 1080x1920 etc.
int paddle_height = (dptopixel(dpHeight)/100) + 3; //the paddle is 100% of the total height of phone.
int user_paddle_pos_x = 4 * (dptopixel(dpWidth)/100) ;
int user_paddle_pos_y = dptopixel(dpHeight) - ((dptopixel(dpHeight)/10) + (dptopixel(dpHeight)/100) + 3) ;
//User Paddle
public Rect paddle_user = new Rect(user_paddle_pos_x,
user_paddle_pos_y,
user_paddle_pos_x + paddle_width,
user_paddle_pos_y + paddle_height);
//AI paddle
Rect paddle_AI = new Rect(AI_paddle_pos_x,
AI_paddle_pos_y,
AI_paddle_pos_x + paddle_width,
AI_paddle_pos_y + paddle_height);
//set ball position vector, Velocity vector, acceleration
int ball_pos_x = 0 ;
int ball_pos_y = (dptopixel(dpHeight)/2) ;
int ball_size = dptopixel(dpWidth)/100 ;
int ball_velocity = 3;
// Ball
Rect ball = new Rect(ball_pos_x,
ball_pos_y,
ball_pos_x+ball_size,
ball_pos_y+ball_size);
//Override onDraw method
#Override
protected void onDraw(Canvas canvas){
super.onDraw(canvas);
Paint mytext = new Paint();
mytext.setColor(Color.WHITE);
//mytext.setStyle(Paint.Style.STROKE);
//mytext.setStrokeWidth(2);
// Draw Middle point
canvas.drawRect(0, ((dptopixel(dpHeight)) / 2), (dptopixel(dpWidth)), (((dptopixel(dpHeight)) / 2) + 2), mytext);
//draw both paddles
canvas.drawRect(paddle_user,mytext);
canvas.drawRect(paddle_AI, mytext);
//draw ball
canvas.drawRect(ball,mytext);
//Practise Methods
//canvas.drawText(Integer.toString(dptopixel(dpHeight)),300,300,mytext);
//canvas.drawText(Integer.toString(dptopixel(dpWidth)), 400, 400, mytext);
//canvas.drawText(Integer.toString(dpHeight),500,500,mytext);
//canvas.drawText(Integer.toString(dpWidth),600,600,mytext);
//canvas.drawText("Fuck", 700, 700, mytext);
//canvas.drawRect(0,0,dptopixel(dpWidth),dptopixel(dpHeight),mytext);
//Game Loop Updater
update();
invalidate();
}
private void update() {
if (ball.centerX() > (dptopixel(dpWidth))/2){
ball.offset(-ball_velocity,ball_velocity);
}
else{
ball.offset(ball_velocity,ball_velocity);
}
}
//Override Touch method
#Override
public boolean onTouchEvent(MotionEvent event) {
if (event.getAction() == MotionEvent.ACTION_DOWN) {
paddle_user.offset(10,0);
}
return true;
}
/* #Override
public boolean onTouch(View v, MotionEvent event) {
this.paddle_user.offsetTo(10,10);
return true; //Event Handled
}
*/
public PongLogic(Context context) {
super(context);
setBackgroundColor(Color.BLACK); //to set background
this.setFocusableInTouchMode(true); //to enable touch mode
}
}

Ok I've Figured out why the offset method was behaving erratically. First I had the original offset position in the else part of the conditional. Second I was negating the dx component only when the if evaluated to true. Which happened for just one frame because I was evaluating Rect object's X position. Which cause it to become false as soon as the -dx was subtracted from the ball's position.
To resolve it I had to place the offset out of the conditional and pass two separate values for the dx and dy. Then I tweaked the conditional so that it only assigned positive and negative values to the variables which were later passed into offset() function, Rather then calling the offset function itself.
This is the new update() method
private void update() {
if (ball.centerX() > (dptopixel(dpWidth))/2){
ball_velocity_x = -3;
}
else if (ball.centerY() > (dptopixel(dpHeight))) {
ball_velocity_y = -3;
}
ball.offset(ball_velocity_x, ball_velocity_y);
}

Get draw bounds/rect of a view

I'm developing an app where a lot of views can be rotated - it's something like a map of physical objects. I have to detect when 2 objects (all objects are rectangles/squares) are overlapping and if a user has performed a single/double/long tap on an object. For this reason I need to know the drawing bounds of a view.
Let's look at the example image bellow - the green rectangle is rotated 45 degrees. I need to get the coordinates of the 4 corners of the green rectangle. If I use view.getHitRect() it returns the bounding box (marked in red) of the view, which is of no use to me.
Do you know how could I get the coordinates of the edges of a view?
The only solution I could think of is to subclass a View, manually store the initial coordinates of the corners and calculate their new values on every modification to the view - translation, scale and rotation but I was wondering if there is a better method.
P.S. The app should be working on Android 2.3 but 4.0+ solutions are also welcomed.

Thanks to pskink I explored again the Matrix.mapPoints method and managed to get the proper coordinates of the corners of the rectangle.
If you are running on Android 3.0+ you can easily get the view's matrix by calling myView.getMatrix() and map the points of interest. I had to use 0,0 for the upper left corner and getWidth(),getHeight() for the bottom right corner and map these coordinates to the matrix. After that add view's X and Y values to get the real values of the corners.
Something like:
float points[] = new float[2];
points[0] = myView.getWidth();
points[1] = myView.getHeight();
myView.getViewMatrix().mapPoints(points);
Paint p = new Paint();
p.setColor(Color.RED);
//offset the point and draw it on the screen
canvas.drawCircle(center.getX() + points[0], center.getY() + points[1], 5f, p);
If you have to support lower versions of Android you can use NineOldAndroids. Then I've copied and modified one of its internal methods to get the view's matrix:
public Matrix getViewMatrix()
{
Matrix m = new Matrix();
Camera mCamera = new Camera();
final float w = this.getWidth();
final float h = this.getHeight();
final float pX = ViewHelper.getPivotX(this);
final float pY = ViewHelper.getPivotY(this);
final float rX = ViewHelper.getRotationX(this);;
final float rY = ViewHelper.getRotationY(this);
final float rZ = ViewHelper.getRotation(this);
if ((rX != 0) || (rY != 0) || (rZ != 0))
{
final Camera camera = mCamera;
camera.save();
camera.rotateX(rX);
camera.rotateY(rY);
camera.rotateZ(-rZ);
camera.getMatrix(m);
camera.restore();
m.preTranslate(-pX, -pY);
m.postTranslate(pX, pY);
}
final float sX = ViewHelper.getScaleX(this);
final float sY = ViewHelper.getScaleY(this);;
if ((sX != 1.0f) || (sY != 1.0f)) {
m.postScale(sX, sY);
final float sPX = -(pX / w) * ((sX * w) - w);
final float sPY = -(pY / h) * ((sY * h) - h);
m.postTranslate(sPX, sPY);
}
m.postTranslate(ViewHelper.getTranslationX(this), ViewHelper.getTranslationY(this));
return m;
}
I've put this method in an overloaded class of a view (in my case - extending TextView). From there on it's the same as in Android 3.0+ but instead of calling myView.getMatrix() you call myView.getViewMatrix().

Moving and rotating a triangle simultaneously around a circle: Android [closed]

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Closed 9 years ago.
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I have a triangle image whose one edge is always in the same direction as the circle.
This image has to be moved around the circle based on user swipe/drag. So, it has to both rotate (so that it's edge is in same direction as the circle) and at the same time revolve around the circle.
How to implement this feature?
UPDATE: My custom View is as follows:
public class ThermoView extends FrameLayout{
private ImageView mThermoBgrd;
private ImageView mCurTempArrow;
public static final int THEMROSTAT_BACKGROUND = 0;
public static final int THEMROSTAT_CURR_TEMP = 1;
public ThermostatView(Context context, AttributeSet attrs) {
super(context, attrs);
mThermoBgrd = new ImageView(context);
mThermoBgrd.setImageResource(R.drawable.circle_icon);
addView(mThermoBgrd, ThermostatView.THEMROSTAT_BACKGROUND);
mCurTempArrow = new ImageView(context);
mCurTempArrow.setImageResource(R.drawable.ruler_triangle_icon);
mCurTempArrow.setScaleType(ImageView.ScaleType.MATRIX);
addView(mCurTempArrow, ThermostatView.THEMROSTAT_CURR_TEMP, new LayoutParams(50, 50));
}
#Override
protected void onLayout(boolean changed, int left, int top, int right,
int bottom) {
super.onLayout(changed, left, top, right, bottom);
int currTempHeight = mCurTempArrow.getMeasuredHeight();
int currTempWidth = mCurTempArrow.getMeasuredWidth();
int parentWidth = right - left;
int parentHeight = bottom - top;
int padding = currTempHeight;
//We need square container for the circle.
int containerLeft = padding;
int containerTop = parentHeight - parentWidth + padding;
int containerRight = parentWidth - padding;
int containerBottom = parentHeight - padding;
int containerWidth = containerRight - containerLeft;
int containerHeight = containerBottom - containerTop;
//place the arrow indicating current temperature
int curTempLeft = containerRight - ((containerWidth/2) + currTempWidth/2);
int curTempTop = containerTop - (currTempHeight/2);
int curTempRight = curTempLeft + currTempWidth;
int curTempBottom = curTempTop + currTempHeight;
mCurTempArrow.layout(curTempLeft, curTempTop, curTempRight, curTempBottom);
}

try this (it uses Paths instead of Bitmaps but the idea is the same):
public class MyView extends View {
private Paint mPaint;
private Path mTriangle;
private Path mCircle;
private Matrix mMatrix;
private float mAngle;
public MyView(Context context) {
super(context);
mPaint = new Paint();
mPaint.setStrokeWidth(10);
mPaint.setStyle(Style.STROKE);
mTriangle = new Path();
mTriangle.moveTo(0, -21);
mTriangle.lineTo(0, 21);
mTriangle.lineTo(36, 0);
mTriangle.close();
mCircle = new Path();
mCircle.addCircle(0, 0, 50, Direction.CW);
mMatrix = new Matrix();
}
#Override
public boolean onTouchEvent(MotionEvent event) {
float w2 = getWidth() / 2f;
float h2 = getHeight() / 2f;
mAngle = (float) (180 * Math.atan2(event.getY() - h2, event.getX() - w2) / Math.PI);
invalidate();
return true;
}
#Override
protected void onDraw(Canvas canvas) {
float w2 = getWidth() / 2f;
float h2 = getHeight() / 2f;
mMatrix.reset();
mMatrix.postTranslate(w2, h2);
canvas.concat(mMatrix);
mPaint.setColor(0xaaff0000);
canvas.drawPath(mCircle, mPaint);
mMatrix.reset();
mMatrix.postTranslate(60, 0);
mMatrix.postRotate(mAngle);
canvas.concat(mMatrix);
mPaint.setColor(0xaa00ff00);
canvas.drawPath(mTriangle, mPaint);
}
}

Since I don't know if you are using open GL or a standard canevas, I can't really give you some working code. But the general idea is (assuming you have the current position of the triangle (x, y) stored, and the center of your circle (cx, cy) stored.
Do the following:
v = (cx-x, cy-y) // v is the normal vector of your triangle: it faces the center of the circle
triangle.translate(v) // translate to the center of the circle
triangle.rotate(angle) // rotate the triangle on itself
v.rotate(angle) // apply the same rotation on the normal vector
triangle.translate(-v) // translate back on the circle, but since v is rotated, the position will be updated
I hope it is clear enough, good luck
EDIT:
First, you should really try to be more accurate: in your first post, you didn't say that the triangle was an image (that's important). Then you don't say what is your current rendering, what works, what doesn't. I would be easier to help you to know what your program currently display.
From your code, I assume you place the triangle image properly , but it is not rotated. So first, try to add
//place the arrow indicating current temperature
int curTempLeft = containerRight - ((containerWidth/2) + currTempWidth/2);
int curTempTop = containerTop - (currTempHeight/2);
int curTempRight = curTempLeft + currTempWidth;
int curTempBottom = curTempTop + currTempHeight;
mCurTempArrow.setRotate(angle); // rotate the image. angle is in degrees
mCurTempArrow.layout(curTempLeft, curTempTop, curTempRight, curTempBottom);
If you don't know the angle, you might have to compute it from the previous position of the triangle

How to set a high res Image in xml?

I just need to set a high res image, but wants to know if its possible to set it in xml. I want to be able to set it even if it will be bigger than the mobile screen.
EDIT: I want to add a huge image, so the user will have to scroll it. Something like a map, but need to add clickable areas on it and I only know how to do this in xml. That's why I asked to add the image using xml not in java. But if someone can tell me how to add clickable areas in java. That will do it better, since I already have the image but using java and its able to scroll.
EDIT: Heres where I got the code from : Anddev.org I'm using exactly the same code, just with other image.

What are you trying for? In android the resolutions are based on like below -
Screen Support
MultipleResolution
Just read out this. Hope these two links enough for your query.

Have a look at this sample code to see how to scroll an image larger than the screen. It also does bitmap caching to speed up drawing of a complex image, and shows how to respond to long-taps and double-taps at any point on the image.

Ok I'm posting this as an answer, just because it will be easier to post this. (It's not working how I want it to, but I got something) I want to share this so anyone else, who knows how to, can help me out here to find a solution for this one. This is the code
package com.example.largeimagescroller;
import android.app.Activity;
import android.os.Bundle;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.view.Display;
import android.view.MotionEvent;
import android.view.View;
import android.view.WindowManager;
import android.widget.ImageView;
import android.widget.Toast;
public class LargeImageScroller extends Activity {
// Physical display width and height.
private static int displayWidth = 0;
private static int displayHeight = 0;
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// displayWidth and displayHeight will change depending on screen
// orientation. To get these dynamically, we should hook
// onSizeChanged().
// This simple example uses only landscape mode, so it's ok to get them
// once on startup and use those values throughout.
Display display = ((WindowManager) getSystemService(Context.WINDOW_SERVICE))
.getDefaultDisplay();
displayWidth = display.getWidth();
displayHeight = display.getHeight();
// SampleView constructor must be constructed last as it needs the
// displayWidth and displayHeight we just got.
setContentView(new SampleView(this));
}
private static class SampleView extends View {
private static Bitmap bmLargeImage; // bitmap large enough to be
// scrolled
private static Rect displayRect = null; // rect we display to
private Rect scrollRect = null; // rect we scroll over our bitmap with
private int scrollRectX = 0; // current left location of scroll rect
private int scrollRectY = 0; // current top location of scroll rect
private float scrollByX = 0; // x amount to scroll by
private float scrollByY = 0; // y amount to scroll by
private float startX = 0; // track x from one ACTION_MOVE to the next
private float startY = 0; // track y from one ACTION_MOVE to the next
public SampleView(Context context) {
super(context);
// Destination rect for our main canvas draw. It never changes.
displayRect = new Rect(0, 0, displayWidth, displayHeight);
// Scroll rect: this will be used to 'scroll around' over the
// bitmap in memory. Initialize as above.
scrollRect = new Rect(0, 0, displayWidth, displayHeight);
// Load a large bitmap into an offscreen area of memory.
bmLargeImage = BitmapFactory.decodeResource(getResources(),
R.drawable.alienware);
}
#Override
public boolean onTouchEvent(MotionEvent event) {
//This code define what to do if you touch the x and y coordinates.
float x1 = event.getX();
float y1 = event.getY();
switch (event.getActionMasked()){
case MotionEvent.ACTION_DOWN:
if (x1>150 & x1<200 & y1>400 & y1<500){
Toast.makeText(getContext(), "Touched the coordinates.", Toast.LENGTH_SHORT).show();
}
}
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
// Remember our initial down event location.
startX = event.getRawX();
startY = event.getRawY();
break;
case MotionEvent.ACTION_MOVE:
float x = event.getRawX();
float y = event.getRawY();
// Calculate move update. This will happen many times
// during the course of a single movement gesture.
scrollByX = x - startX; // move update x increment
scrollByY = y - startY; // move update y increment
startX = x; // reset initial values to latest
startY = y;
invalidate(); // force a redraw
break;
}
return true; // done with this event so consume it
}
#Override
protected void onDraw(Canvas canvas) {
// Our move updates are calculated in ACTION_MOVE in the opposite
// direction
// from how we want to move the scroll rect. Think of this as
// dragging to
// the left being the same as sliding the scroll rect to the right.
int newScrollRectX = scrollRectX - (int) scrollByX;
int newScrollRectY = scrollRectY - (int) scrollByY;
// Don't scroll off the left or right edges of the bitmap.
if (newScrollRectX < 0)
newScrollRectX = 0;
else if (newScrollRectX > (bmLargeImage.getWidth() - displayWidth))
newScrollRectX = (bmLargeImage.getWidth() - displayWidth);
// Don't scroll off the top or bottom edges of the bitmap.
if (newScrollRectY < 0)
newScrollRectY = 0;
else if (newScrollRectY > (bmLargeImage.getHeight() - displayHeight))
newScrollRectY = (bmLargeImage.getHeight() - displayHeight);
// We have our updated scroll rect coordinates, set them and draw.
scrollRect.set(newScrollRectX, newScrollRectY, newScrollRectX
+ displayWidth, newScrollRectY + displayHeight);
Paint paint = new Paint();
canvas.drawBitmap(bmLargeImage, scrollRect, displayRect, paint);
// Reset current scroll coordinates to reflect the latest updates,
// so we can repeat this update process.
scrollRectX = newScrollRectX;
scrollRectY = newScrollRectY;
}
}
}
I rec the application in use. (Since touch events don't work with cpu emulators I did it on my phone)
http://www.youtube.com/watch?v=NrszZoDenXE&feature=youtube_gdata
As you can see its taking the touch, but it moves with the scroll rect, so the question will be: How can I do the touch to stay in the image, so it doesn't move with the scrolling rect?
Thanks

Android : Stop image scaling down

I'm trying to draw an image in a view but having problems trying to maintain the scale of the original image. Basically, I have a small view and I would like to show part of the image in the view. The intention then is to perform a translation on the image so that a different part appears in the view.
No matter what I try, either the image is down-scaled automatically to fit the view or the whole image is viewable. I've tried playing about with the settings on BitmapDrawable, ImageView and Layout to no avail.
Anyone know a good way to achieve this?

Hope This piece of code helps. I have googled it a month ago. It Scrolling performance for the Larger Images.Here whole display size is set as the Height and width of the view. You can change you know. and also can maintain the zoom controls too.
public class LargeImageScroller extends Activity {
// Physical display width and height.
private static int displayWidth = 0;
private static int displayHeight = 0;
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// displayWidth and displayHeight will change depending on screen
// orientation. To get these dynamically, we should hook onSizeChanged().
// This simple example uses only landscape mode, so it's ok to get them
// once on startup and use those values throughout.
Display display = ((WindowManager)
getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();
displayWidth = display.getWidth();
displayHeight = display.getHeight();
// SampleView constructor must be constructed last as it needs the
// displayWidth and displayHeight we just got.
setContentView(new SampleView(this));
}
private static class SampleView extends View {
private static Bitmap bmLargeImage; //bitmap large enough to be scrolled
private static Rect displayRect = null; //rect we display to
private Rect scrollRect = null; //rect we scroll over our bitmap with
private int scrollRectX = 0; //current left location of scroll rect
private int scrollRectY = 0; //current top location of scroll rect
private float scrollByX = 0; //x amount to scroll by
private float scrollByY = 0; //y amount to scroll by
private float startX = 0; //track x from one ACTION_MOVE to the next
private float startY = 0; //track y from one ACTION_MOVE to the next
public SampleView(Context context) {
super(context);
// Destination rect for our main canvas draw. It never changes.
displayRect = new Rect(0, 0, displayWidth, displayHeight);
// Scroll rect: this will be used to 'scroll around' over the
// bitmap in memory. Initialize as above.
scrollRect = new Rect(0, 0, displayWidth, displayHeight);
// Load a large bitmap into an offscreen area of memory.
bmLargeImage = BitmapFactory.decodeResource(getResources(),
R.drawable.testlargeimage);
}
#Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
// Remember our initial down event location.
startX = event.getRawX();
startY = event.getRawY();
break;
case MotionEvent.ACTION_MOVE:
float x = event.getRawX();
float y = event.getRawY();
// Calculate move update. This will happen many times
// during the course of a single movement gesture.
scrollByX = x - startX; //move update x increment
scrollByY = y - startY; //move update y increment
startX = x; //reset initial values to latest
startY = y;
invalidate(); //force a redraw
break;
}
return true; //done with this event so consume it
}
#Override
protected void onDraw(Canvas canvas) {
// Our move updates are calculated in ACTION_MOVE in the opposite direction
// from how we want to move the scroll rect. Think of this as dragging to
// the left being the same as sliding the scroll rect to the right.
int newScrollRectX = scrollRectX - (int)scrollByX;
int newScrollRectY = scrollRectY - (int)scrollByY;
// Don't scroll off the left or right edges of the bitmap.
if (newScrollRectX < 0)
newScrollRectX = 0;
else if (newScrollRectX > (bmLargeImage.getWidth() - displayWidth))
newScrollRectX = (bmLargeImage.getWidth() - displayWidth);
// Don't scroll off the top or bottom edges of the bitmap.
if (newScrollRectY < 0)
newScrollRectY = 0;
else if (newScrollRectY > (bmLargeImage.getHeight() - displayHeight))
newScrollRectY = (bmLargeImage.getHeight() - displayHeight);
// We have our updated scroll rect coordinates, set them and draw.
scrollRect.set(newScrollRectX, newScrollRectY,
newScrollRectX + displayWidth, newScrollRectY + displayHeight);
Paint paint = new Paint();
canvas.drawBitmap(bmLargeImage, scrollRect, displayRect, paint);
// Reset current scroll coordinates to reflect the latest updates,
// so we can repeat this update process.
scrollRectX = newScrollRectX;
scrollRectY = newScrollRectY;
}
}
}

This probably won't be the most efficient way, but if you're not going to be moving it too much, it'll do.
Treat it like a spritesheet, use Bitmap.createBitmap(Bitmap source, int x, int y, int width, int height) to get the section you want from the original bitmap as its own bitmap, then pass that in instead. Then pass that in rather than the whole bitmap, and it'll only be working with the part you want it to show.

Sounds like you want to have a big ImageView control inside a smaller View (acting like a window), and then move the ImageView control around inside that, so only part of it is visible at a time.
Not exactly sure how you'd do that though, but pretty sure it's possible with AbsoluteLayout and a bit of tinkering.