--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doodle/tk/pixel_model.d	Sat Aug 14 20:05:55 2010 +0930
@@ -0,0 +1,42 @@
+module doodle.tk.pixel_model;
+
+public {
+    import doodle.tk.geometry;
+}
+
+// FIXME consider using the term Screen instead of Pixel...
+
+class PixelModel {
+    this(in double zoom, in Rectangle canvasBounds, in Rectangle viewBounds) {
+        _zoom = zoom;
+        _viewBounds = viewBounds;
+        _canvasBounds = canvasBounds;
+
+        _viewCentre = _canvasBounds.centre;
+    }
+
+    void consolidateCanvasBounds(in Rectangle requiredCanvasBounds) {
+        Rectangle r = pixelToModel(_viewBounds);
+        _canvasBounds = r | requiredCanvasBounds;
+    }
+
+    // For normalZoom 1.0 -> 100% means the presentation on the screen is
+    // one-to-one with real-life
+    double normalZoom(in double pixelsPerMillimetre) { return _zoom / pixelsPerMillimetre; }
+
+    Point modelToPixel(in Point model) const { return _viewBounds.centre + _zoom * (model - _viewCentre); }
+    Point pixelToModel(in Point pixel) const { return _viewCentre + (pixel - _viewBounds.centre) / _zoom; }
+    Vector modelToPixel(in Vector model) const { return _zoom * model; }
+    Vector pixelToModel(in Vector pixel) const { return pixel / _zoom; }
+    Rectangle modelToPixel(in Rectangle model) const { return Rectangle(modelToPixel(model.position), modelToPixel(model.size)); }
+    Rectangle pixelToModel(in Rectangle model) const { return Rectangle(pixelToModel(model.position), pixelToModel(model.size)); }
+
+    private {
+        // Screen units are pixels
+        // Model units are millimetres
+        double _zoom;               // pixels-per-millimetre
+        Rectangle _viewBounds;      // pixel: bounds of the viewport in pixels
+        Point _viewCentre;          // model: where in the model is the centre of our view
+        Rectangle _canvasBounds;    // model: the bounds of the canvas in millimetres
+    }
+}