Question

I’m trying to make a small game based on the canvas in Delphi. Basically, I’d like to make a fairly large bitmap ( 3000×3000, for example ), then load it into the canvas, and being able to scroll right/left/up/down just like an ordinary image viewer, however I can’t seem to find what I’m looking for. Any ideas?

Answer 1

Load the image to an off-screen TBitmap object. Then, OnPaint, or whenever is suitable in your particular application, use BitBlt or Canvas.Draw to draw a rectangular subimage of the TBitmap onto the canvas. The subpart should start at (X, Y) on the TBitmap and have a width and height equal to ClientWidth and ClientHeight of the form, respectively.

Now, respond to keyboard events. Write a FormKeyDown event handler, and listen to Key = VK_LEFT, Key = VK_RIGHT, Key = VK_UP, and Key = VK_DOWN (use a case statement). When you detect such a key being pressed, increase/decrease X or Y, as appropriate, and paint the scene again using this starting point.

You can also respond to the MouseDown, MouseMove, and MouseUp events to scroll using the mouse. Either you can use the middle one only (MouseMove): You can check if the cursor is near an edge of the form, and if so, scroll in this direction smoothly (using a TTimer, for instance). Alternatively, you can set a FMouseDown flag to true in MouseDown, and reset it to false in MouseUp. Then, in MouseMove, scroll the bitmap by a delta X-XOld in the x direction if FMouseDown is true, and a delta Y-YOld in the y direction. (Here, X and Y are parameters of the MouseMove event handler; (X, Y) is the current position of the cursor.) The MouseMove procedure should end with

XOld := X;
YOld := Y;

no matter if FMouseDown is on or off.