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but what exactly IS a full screen quad?

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A quadrangle is a four sided shape with four vertices.

A full screen quadrangle has its four vertices at each corner of the screen.

Screen coordinates are as follows:

        public static readonly VertexPositionTexture[] Vertices_NearPlane = new VertexPositionTexture[4]            {               /*                *  (-1,1)-----------(1,1)  [x]: -1 -----> 1                 *      |               |                *      |               |   [y]:    1                *      |     (0,0)     |           |                *      |               |           |                *      |               |           |                *  (-1,-1)__________(1,-1)         -1                */                // Screen X,Y coordinatates go from (-1,1) [Top Left] to (1,-1) [Bottom Right]                // .Z coordinate of 0 = Draw on near clipping plane (as close to camera as possible)                new VertexPositionTexture(new Vector3(-1f, 1f, 0f), new Vector2(0f, 0f)),           // Top Left                new VertexPositionTexture(new Vector3(1f, 1f, 0f), new Vector2(1f, 0f)),            // Top Right                new VertexPositionTexture(new Vector3(-1f, -1f, 0f), new Vector2(0f, 1f)),          // Bottom Left                new VertexPositionTexture(new Vector3(1f, -1f, 0f), new Vector2(1f, 1f))            // Bottom Right            };

Hope that helps. :)

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A quad is a rectangle. A fullscreen quad is a rectangle covering the entire screen. So if you draw a textured fullscreen quad, then the texture image will simply be stretched to cover the entire screen.
If you draw a fullscreen quad with a pixel shader, the pixel shader will be executed for every pixel on the screen.

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I'm just being a little fussy, but the term "quad" is used pretty loosely in the graphics community. "Quad" comes from quadrilaterial which means it has 4 sides (the sides are not necessarily perpendicular to one another), as Sp8nky mentioned. So, strictly speaking, a rectangle is a quadrilaterial but a quadrilateral may or may not be a rectangle.

However, in the graphics community, when someone describes something as a "quad," they quite often mean "rectangle" instead. Because "full-screen" implies a rectangle (most screens are rectangular), "full screen quad" is rectangle that completely fills the screen.

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thanks, that clears things up :)

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