如何在Android屏幕外的psition上的大位图上绘制小位图？

Question

Assuming that i have an android phone with max x=400 and max y=300

I have a 5000 by 5000 px bitmap and i display a 400 by 300 part of it on screen. I scroll it around using touch events. I want to draw a smaller bitmap(30 by 30px) onto that larger bitmap at location (460,370). But i cant statically do it(as my game requires). I want to draw and undraw that smaller image according to the player's input or collision detections.

What i am confused about is that suppose currently (50,50) to (450,350) of the larger image is displayed on the screen and now ,due to a certain requirement of the game at this moment, i have to draw that 30by30 bitmap at (460,370) but this point is in two systems - 1)In the big bitmap's co-ordinate system it's 460,370 but when it will come inside the visible area of screen it will have values something between (0,0) and (400,300) depending upon its position on the screen as per the player's movements...

So how do i track that image- i mean using which co-ordinate system, and how do i go about doing all this?

I know I've put it here in a very confusing way but that's the best way i can put it and i'll be really glad, and grateful to you if somebody could help me on this.....

Answer 1

Terminology:

• Let's call the coordinates on the big map world coordinates

• Let's call the coordinates on the screen view coordinates . You are talking to the drawing system (canvas?) in view coordinates.

Your currently displayed region of the bigger map is 100% defined by an offset, let's call this (offsetX, offsetY) . Formally, this offset maps your world coordinates onto your view coordinates .

If you have the small image displayed at position (x, y) in world coordinates, all you need to do is subtract your offset to get to view coordinates, ie

viewX = worldX - offsetX;
viewY = worldY - offsetY;