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Windows Phone

Windows Phone 7 : Using the Touch Screen (part 5) - Sprite Hit Testing - Retrieving the Objects at a Hit Location, Hit Testing Example Projects

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3.5. Retrieving the Objects at a Hit Location

Another common function will be to identify the sprites that are contained in a specific location or the frontmost sprite at a specific location. Once again, we can add functions for both of these operations to the GameHost class.

The first, GetSpritesAtPoint, loops through all the game objects looking for those that can be found at the specified position. These are added to an array and returned to the calling procedure. The code for this function is shown in Listing 11.

Example 11. Finding all the objects at a specified position
public SpriteObject[] GetSpritesAtPoint(Vector2 testPosition)
    {
        SpriteObject spriteObj;
        SpriteObject[] hits = new SpriteObject[GameObjects.Count];
        int hitCount = 0;

        // Loop for all of the SelectableSpriteObjects
        foreach (GameObjectBase obj in GameObjects)
        {
            // Is this a SpriteObject?
            if (obj is SpriteObject)
            {
                // Yes... Cast it to a SelectableSpriteObject
                spriteObj = (SpriteObject)obj;
                // Is the point in the object?
                if (spriteObj.IsPointInObject(testPosition))
                {
                    // Add to the array
                    hits[hitCount] = spriteObj;
                    hitCount += 1;
                }
            }
        }

        // Trim the empty space from the end of the array
        Array.Resize(ref hits, hitCount);

        return hits;
    }

					  

The second function, GetSpriteAtPoint, returns just a single sprite and attempts to find the frontmost sprite at the specified location. It does this by keeping track of the LayerDepth value for each matching sprite. When subsequent sprites are ready to be checked, they are compared against the LayerDepth of the previous matching sprite and ignored if the value is higher (remember that lower values appear in front of higher values).

If LayerDepth values are found to be equal, the check is still made, and the later sprite will supersede the earlier sprite if it also matches the hit point. Because XNA will normally draw sprites in the order requested when LayerDepth values match, later objects in the GameObjects collection will appear in front of earlier objects with a matching depth. This check therefore allows us to find the frontmost object even if LayerDepths are not being used.

The GetSpriteAtPoint function is shown in Listing 12.

Example 12. Finding the frontmost sprite at a specified position
public SpriteObject GetSpriteAtPoint(Vector2 testPosition)
    {
        SpriteObject spriteObj;
        SpriteObject ret = null;
        float lowestLayerDepth = float.MaxValue;

        // Loop for all of the SelectableSpriteObjects


foreach (GameObjectBase obj in GameObjects)
        {
            // Is this a SpriteObject?
            if (obj is SpriteObject)
            {
                // Yes... Cast it to a SelectableSpriteObject
                spriteObj = (SpriteObject)obj;
                // Is its layerdepth the same or lower than the lowest we have seen so far?
                // If not, previously encountered objects are in front of this one
                // and so we have no need to check it.
                if (spriteObj.LayerDepth <= lowestLayerDepth)
                {
                    // Is the point in the object?
                    if (spriteObj.IsPointInObject(testPosition))
                    {
                        // Mark this as the current frontmost object
                        // and remember its layerdepth for future checks
                        ret = spriteObj;
                        lowestLayerDepth = spriteObj.LayerDepth;
                    }
                }
            }
        }

        return ret;
    }

					  

3.6. Hit Testing Example Projects

The first, HitTesting, provides a demonstration of the accuracy of the hit testing functions that we have added to the SpriteObject class. A screenshot from this project can be seen in Figure 6.

Figure 6. A sample image from the HitTesting project

This example project creates a number of randomly positioned sprites, some of which have a square texture, whereas others have a circular texture. The sprites are rotated and scaled such that they form rectangles and ellipses. The objects can be touched to select them; all the objects that fall under the touch position will be selected and highlighted in red.

The image in Figure 6 is taken from the emulator, and the mouse cursor can be seen selecting some of the shapes that are displayed. The emulator is a great way to accurately explore the edges of the shapes because the mouse cursor is much more precise than a finger on a real device touch screen. You will see that the object really is selected only when it is touched inside its displayed shape and that the algorithms we are using for hit testing work exactly as required.

Inside the HitTestingGame.Update method you will find that there are actually two possible calls to object selection functions, one of which is in use (calling SelectAllMatches); the other is commented out (calling SelectFrontmost). SelectAllMatches finds all the objects at the touch point using the GetSpritesAtPoint function and selects all of them, whereas SelectFrontmost uses GetSpriteAtPoint and selects just the one sprite returned (if there is one).

Try swapping these over so that SelectFrontmost is called instead. You will find now that it is always the object in front that is selected when multiple objects overlap, as described in the previous section.

The project defines a new game object class called SelectableSpriteObject and adds to the basic SpriteObject functionality a new boolean property called Selected. It also overrides the SpriteColor property and returns a red color when the sprite is selected or the underlying sprite color if it is not. This simple class provides a useful mechanism for selecting sprites and visually indicating which are selected. We will use this same approach in the "Initiating Object Motion" section coming up in a moment.

The second example project, Balloons, turns the hit testing into a simple interactive game. Colored balloons gently float up the screen, and the player must pop them by touching them. This can be quite relaxing until too many balloons start to reach the top of the screen, at which point trying to pop them all becomes somewhat more frantic! A screenshot from the project is shown in Figure 7.

Figure 7. A sample image from the Balloons project

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- Microsoft XNA Game Studio 3.0 : Adding Bread to Your Game (part 2) - Improving Programs Using Methods, Handling Collisions
- Microsoft XNA Game Studio 3.0 : Adding Bread to Your Game (part 1) - Using a Structure to Hold Sprite Information, Using the Gamepad Thumbsticks to Control Movement
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- User Interface : Using the Windows Phone 7 Predefined Styles
- Handling Input on Windows Phone 7 : Touch Input (part 3) - Multi-Point Touch
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