Placing objects on scrolling background in XNA

Question

How do I place objects that appear only if the background scrolls to a certain point?

Example- I have this long image that keeps scrolling using the technique above. However, after scrolling to part of the image, I want to add a platform there. How would I do that?

Answer 1

In general, you will probably need to save the locations of your objects in a file and then load that file at the beginning of the level (assuming you are making some kind of platformer game). You can do this by creating a class or a struct containing all the relevant information for the platform (position, size, texture, etc) and then using XML serialization to write an array of those classes/structs to a file.

Your level loader would then load and deserialize the level data, which would end up being a list of all the objects in your level (such as platforms). Now that you have the locations of your platforms in memory you have a couple different options on how to get them to the screen.

1. Draw all the objects (platforms) all the time, whether or not they are in the view of the camera. If your levels don't contain a lot of objects, this would be simple to implement.
2. Draw only those in the camera's view. Without knowing how you implemented the horizontal scroller, it's kind of hard to make suggestions for this part. Whatever mechanism you currently have to identify the boundaries of what part of the background to show could be used to determine which objects to draw as well.

I'm working on a game that scrolls vertically right now, and I needed a way to do something similar: place objects in a level and have them appear when the background scrolled to them. I used TorqueX 2D (free engine binaries if you've payed to develop for XNA) and its 2D scene editor to set this up pretty easily. I have my camera scrolling up, the background stays in place. When it gets to an object position defined in the XML level file it spawns the object in the level.