EDIT: I solved it now, but Noob as I am - I cannot figure out how to delete this post 
I am currently messing around a bit with procedural generation.
Unfortunatly I have two problems ( details below):
First, after moving 11 chunks in the same direction, the offset between chunks does not work anymore.
Second, the overall offset for a 64*64 Chunk is only correct if set to 60. If I set it to 64, there is a gap of (suprise) 4 Tiles.
I am quite sure, that has to do with my matrices, but I cannot figure out, what is going wrong.
I render 9 Chunks at a time to get a 3*3 square rendered.
The center Chunk is the one the player is in, as I move the world and not the player, the player can never exit the center chunk.
Each Chunk is rendered by an instanced draw call to a tile with the top left coordinates of the chunk - I will call that one the ChunkTile.
Furthermore Each Chunk is 6464 Tiles with each Tile being scaled to 1616 by its model matrix.
When generated, the chunks are stored with their world coordinates as key in an unordered map.
For each direction I have a transition vector (northwest, north, northeast etc)
After crossing the border of a chunk into one direction, I redraw the scene by assigning my DrawStorage the correct chunks, e.g.:
move to west
DrawStorage before
123
4c6
789
DrawStorage after:
n12
nc5
n78
The new center is assigned first, and then the new Chunks are either generated or loaded from storage.
Their world coordinates are always found by center-worldcoordinates + transitionVector
The transition vector is basically the offset between the chunks.
To stay with the example transitionwest = (-60,0)
To get the right Position on Screen I keep track of the player World and Screenposition.
Each new Chunk gets its world Position:
newCenterWorld+TransitionVector
and Screen Position:
NewCenterWorld+Transition-PlayerScreenPosition.
The Screenposition is used to get the translation-part in the modelmatrix done.
Apparently after moving 11 chunks west (or any other direction), the offset between chunks is growing and even more weirdly, if I move several times west/east over the same border, the offset grows even further.
I know it has probably something to do with the way I process my movement, but right now I cannot see anything wrong with it.
My movement is basically 8 Screenunits(Pixels) and 0.5 Worldunits.
My whole matrix-stuff works (at least for the first 10 chunks) in the way that I set the offset for the instanced drawing to 1.0f, meaning 1 World Unit is 16px on the screen.
However I draw 64 Tiles per Chunk but the offset between each Chunk is 60 and I did not figured out how that can actually be.
In the shader it is like: glPos = MVPvec4(VertexVector+Offsetvector , 1.0f)
The offset is stored in the instanced buffer and is basically the TL Worldposition +1 on x/y for each tile.
So for the top left of the north Chunk at initialising the world it is:
glPos = MVP[(-0.5f, 0.5f, 0.0f)+(0.0f,-60.0f,0.0f)
The next one right would havfe a Offset.x = 1 instead and so on.
The translation to the correct Screenposition happens in the MVP.
It is all with an ortho matrix. The window, ortho and viewport are set to 1024/768.
Any suggestion why this could happen? I know we won’t solve this directly here, but right now I cannot see where the miscalculation actually happens.
Thanks anyway!