I have a very subtle problem with XNA, specifically the SpriteBatch. In my game I have a Camera class. It can Translate the view (obviously) and also zoom in and out. I apply the Camera to the scene when I call the "Begin" function of my spritebatch instance (the last parameter).
The Problem: When the cameras Zoomfactor is bigger than 1.0f, the spritebatch stops drawing. I tried to debug my scene but I couldn't find the point where it goes wrong.
I tried to just render with "Matrix.CreateScale(2.0f);" as the last parameter for "Begin". All other parameters were null and the first "SpriteSortMode.Immediate", so no custom shader or something. But SpriteBatch still didn't want to draw.
Then I tried to only call "DrawString" and DrawString worked flawlessly with the provided scale (2.0f).
However, through a lot of trial and error, I found out that also multiplying the ScaleMatrix with "Matrix.CreateTranslation(0, 0, -1)" somehow changed the "safe" value to 1.1f. So all Scale values up to 1.1f worked. For everything above SpriteBatch does not render a single pixel in normal "Draw" calls. (DrawString still unaffected and working).
Why is this happening? I did not setup any viewport or other matrices. It appears to me that this could be some kind of strange Near/Farclipping. But I usually only know those parameters from 3d stuff.
If anything is unclear please ask!
It is near/far clipping.
Everything you draw is transformed into and then rasterised in projection space. That space runs from (-1,-1) at the bottom left of the screen, to (1,1) at the top right. But that's just the (X,Y) coordinates. In Z coordinates it goes from 0 to 1 (front to back). Anything outside this volume is clipped. (References: 1, 2, 3.)
When you're working in 3D, the projection matrix you use will compress the Z coordinates down so that the near plane lands at 0 in projection space, and the far plane lands at 1.
When working in 2D you'd normally use Matrix.CreateOrthographic, which has near and far plane parameters that do exactly the same thing. It's just that SpriteBatch specifies its own matrix and leaves the near and far planes at 0 and 1.
The vertices of sprites in a SpriteBatch do, in fact, have a Z-coordinate, even though it's not normally used. It is specified by the layerDepth parameter. So if you set a layer depth greater than 0.5, and then scale up by 2, the Z-coordinate will be outside the valid range of 0 to 1 and won't get rendered.
(The documentation says that 0 to 1 is the valid range, but does not specify what happens when you apply a transformation matrix.)
The solution is pretty simple: Don't scale your Z-coordinate. Use a scaling matrix like:
Matrix.CreateScale(2f, 2f, 1f)