Parallax Background

[DivFord]

Parallax backgrounds are on my list of graphical improvements. I now have all the art I need for it, but I'm going to need to set up the code.

@Fyll, could you explain how your view-scrolling works, and where it is?

[Fyll]

In Screen::draw(), the Window::draw() takes 3 parameters. The first is the texture id (no need to touch that), the second is the position on the screen, and the third is the dimensions (on the screen) of the image. At the moment, it's drawn at (0, 0), and goes all the way across to (1, 1) (aka, it's drawn over the full screen).

There are two ways you could do a parallax effect:

• You could draw the whole image (as it does at the moment), but draw it with a negative position according to how scrolled across you want it to be. E.g. If the BG is twice the dimension of the screen, you'd draw it at (0, 0) when you're in the top-left corner, and at (-1, -1) when you're in the bottom-right corner. You'd also need to change the third parameter to be (2, 2), as the BG is twice as big as the screen in both x and y.
• Alternatively, you could leave the 3 parameters as they are, but also pass in the texture pos and dim as the fourth and fifth parameters. E.g. For the twice as big BG, you'd pass in (0, 0) and (0.5, 0.5) (as you're only showing half of the BG at any time in each axis) when you're in the top-left corner. When you're in the bottom-right corner, you'd pass in (0.5, 0.5) and (0.5, 0.5), as you want the middle point of the texture to be in the top-left corner.

Of the two, I think the second one is probably better (I can't say how intuitive they are though), as I'm a bit iffy on the idea of drawing the whole BG, but having a sizeable chunk of it off screen.

The pos_ or pos() variable can be used to determine the position of the top-left corner of the level. It's at (0, 0) when the screen is the top-left chunk of the level, and, if the level is two screen widths wide and two screen heights high, it'll be at (1, 1) when it's in the bottom-right corner. Basically, the position of the screen is given in absolute coordinates in the level, with units of screen dimensions.

EDIT: As an example, if the level has n times the dimensions of the screen, but the BG has m times the dimensions of the screen, a simple parallax effect could be done by (I haven't tested these, but I think they should work):

Window::instance().draw(tex_, -1.0f * pos_ * (m - 1) / (n - 1), Vector(m, m));

or,

Window::instance().draw(tex_, Vector::Zeros(), Vector::Ones(), Vector::Ones() * pos_ / (m * (n - 1)), Vector::Ones() / m);

[DivFord]

@Fyll PATH_BG in FilePaths.hpp seems to be a vector. Is the idea that the vector contains both layers of the background, or should I be making a new variable?

[Fyll]

That was expected to contain the BG for each of the worlds. I hadn't planned parallax BGs that far ahead. That's probably going to be redone when more worlds are created (probably shifted into a WorldList.hpp like everything else), so do whatever's easiest for now.

[DivFord]

Where can I get the size of the level relative to the screen? (the m in your description above). This is from Screen.hpp.

[Fyll]

It's the variable dim_ (see the comment above the constructor).

[DivFord]

So it is. Perfect.