2d shadow mapping

Question

I have been wondering about how to implement this with openGL:

example

I have a map, with a flat floor and walls. Every thing here is 2d, there is no 3d geometry, only 2d poligons that compose the map.
Using the vertex of the polygons I cast shadows, to define the viewable area.
The shadows define the field of view, but since the cells with walls obstructi view, they are also darkened. I can draw the walls on top of the shadows, but doing so would show even walls outside the field of view.

I have been suggested to approach this problem with shadow mapping. I should render the 2D scene into 4 different 1D textures that hold the depth of the distance to the first colliding surface. The problem is that I have no idea how to render the projection of the 2d scene into the 1D texture. If I use, for example: gluLookAt (x, y, 0.0, 0.0, x , y+1, 0.0, 0.0, 1.0);

To render the top view, the result is still 2D. Also, nothing would be rendered since all the vertex will be at the same plane, so all surfaces will be ortogonal to the camera.

Do you have any tip or idea of how to do these 2D-to-1D projections? I have been googling for scenarios like this one, but all of them are in 3D enviroments.

Where exactly is the "light source" that interacts with shadow casters? — Sir Digby Chicken Caesar, Mar 29 '13 at 18:20
The character is the light source. The purpose of the shadows is to simulate what the character can see from where he is standing. — Leo, Mar 29 '13 at 18:33
Can't you just drop one of the coordinates to go from 2D to 1D? That would be the equivalent of an orthographic projection, right? — fluffels, Apr 15 '13 at 13:17

Andreas Haferburg · Answer 1 · 2013-05-14T17:36:35.790

Shadow mapping assumes either a directional light, or a spotlight, and you have a point light. But since you only need shadow on the floor, you could model it as a spotlight that hovers e.g. 2 m above the floor and points downwards. All the walls would have to be at least 2 m high. In the first shadow mapping pass, you could render the floor and all the walls into the shadow buffer.

However, I would not go with shadow mapping, but use volumetric shadows instead. If you go from 3D to 2D, a 3D volume becomes a 2D polygon.

Assuming that all the walls are on a regular grid, we can compute view rays from the player's position P to all the corners of walls. For each corner, store the adjacent walls, and ignore all the walls that face away from the player. Then cast rays from P to each corner, convert the rays to polar coordinates, and sort them by their angle, say counter-clockwise. Now go through this sorted list in a sweeping motion, and build the shadow polygon.

The shadow polygon consists either of corner points in this list, or of intersections between a) a line that is parallel to a wall and b) a line that goes through P and a corner. The only thing that makes this a bit complicated is that you have to find the wall that receives the shadow. Since the input is so small, I would probably start with brute force (check the corner against each wall), and see how slow it is. Note that only player-facing walls can cast shadows. Also note that the point closest to the player doesn't need to be visible.

It's probably going to look really cool with a moving character.

2d shadow mapping

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