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Latest revision as of 08:50, 19 April 2014
 1 Introduction
In GPipe, you'll primarily work with these four types of data on the GPU:
Let's walk our way through an simple example as I explain how you work with these types. This example requires GPipe version 1.4.1 or later. This page is formatted as a literate Haskell page, simply save it as "box.lhs" and then type
ghc --make –O box.lhs box
at the prompt to see a spinning box. You’ll also need an image named "myPicture.jpg" in the same directory (I used a picture of some wooden planks).
> module Main where > import Graphics.GPipe > import Graphics.GPipe.Texture.Load > import qualified Data.Vec as Vec > import Data.Vec.Nat > import Data.Monoid > import Data.IORef > import Graphics.UI.GLUT > (Window, > mainLoop, > postRedisplay, > idleCallback, > getArgsAndInitialize, > ($=))
 2 Creating a windowWe start by defining the
> main :: IO () > main = do > getArgsAndInitialize > tex <- loadTexture RGB8 "myPicture.jpg" > angleRef <- newIORef 0.0 > newWindow "Spinning box" (100:.100:.()) (800:.600:.()) (renderFrame tex angleRef) initWindow > mainLoop > renderFrame :: Texture2D RGBFormat -> IORef Float -> Vec2 Int -> IO (FrameBuffer RGBFormat () ()) > renderFrame tex angleRef size = do > angle <- readIORef angleRef > writeIORef angleRef ((angle + 0.005) `mod'` (2*pi)) > return $ cubeFrameBuffer tex angle size > initWindow :: Window -> IO () > initWindow win = idleCallback $= Just (postRedisplay (Just win))
newWindow. When the window is created,
 3 PrimitiveStreams
The graphics pipeline starts with creating primitives such as triangles on the GPU.Let's create a box with six sides, each made up of two triangles each.
> cube :: PrimitiveStream Triangle (Vec3 (Vertex Float), Vec3 (Vertex Float), Vec2 (Vertex Float)) > cube = mconcat [sidePosX, sideNegX, sidePosY, sideNegY, sidePosZ, sideNegZ] > sidePosX = toGPUStream TriangleStrip $ zip3 [1:.0:.0:.(), 1:.1:.0:.(), 1:.0:.1:.(), 1:.1:.1:.()] (repeat (1:.0:.0:.())) uvCoords > sideNegX = toGPUStream TriangleStrip $ zip3 [0:.0:.1:.(), 0:.1:.1:.(), 0:.0:.0:.(), 0:.1:.0:.()] (repeat ((-1):.0:.0:.())) uvCoords > sidePosY = toGPUStream TriangleStrip $ zip3 [0:.1:.1:.(), 1:.1:.1:.(), 0:.1:.0:.(), 1:.1:.0:.()] (repeat (0:.1:.0:.())) uvCoords > sideNegY = toGPUStream TriangleStrip $ zip3 [0:.0:.0:.(), 1:.0:.0:.(), 0:.0:.1:.(), 1:.0:.1:.()] (repeat (0:.(-1):.0:.())) uvCoords > sidePosZ = toGPUStream TriangleStrip $ zip3 [1:.0:.1:.(), 1:.1:.1:.(), 0:.0:.1:.(), 0:.1:.1:.()] (repeat (0:.0:.1:.())) uvCoords > sideNegZ = toGPUStream TriangleStrip $ zip3 [0:.0:.0:.(), 0:.1:.0:.(), 1:.0:.0:.(), 1:.1:.0:.()] (repeat (0:.0:.(-1):.())) uvCoords > uvCoords = [0:.0:.(), 0:.1:.(), 1:.0:.(), 1:.1:.()]
Every side of the box is created from a regular list of four elements each, where each element is a tuple with three vectors: a position, a normal and an uv-coordinate. These lists of vertices are then turned into
PrimitiveStreams on the GPU by
toGPUStream that in our case creates triangle strips from the vertices, i.e 2 triangles from 4 vertices. Refer to the OpenGl specification on how triangle strips and the other topologies works.
Triangles where each vertex is a tuple of three vectors, just as the lists we started with. One big difference is that those vectors now are made up of
The cube is defined in model-space, i.e where positions and normals are relative the cube. We now want to rotate that cube using a variable angle and project the whole thing with a perspective projection, as it is seen through a camera 2 units down the z-axis.
> transformedCube :: Float -> Vec2 Int -> PrimitiveStream Triangle (Vec4 (Vertex Float), (Vec3 (Vertex Float), Vec2 (Vertex Float))) > transformedCube angle size = fmap (transform angle size) cube > transform angle (width:.height:.()) (pos, norm, uv) = (transformedPos, (transformedNorm, uv)) > where > modelMat = rotationVec (normalize (1:.0.5:.0.3:.())) angle `multmm` translation (-0.5) > viewMat = translation (-(0:.0:.2:.())) > projMat = perspective 1 100 (pi/3) (fromIntegral width / fromIntegral height) > viewProjMat = projMat `multmm` viewMat > transformedPos = toGPU (viewProjMat `multmm` modelMat) `multmv` (homPoint pos :: Vec4 (Vertex Float)) > transformedNorm = toGPU (Vec.map (Vec.take n3) $ Vec.take n3 modelMat) `multmv` norm
toGPUfunction transforms normal values like
 4 FragmentStreamsTo render the primitives on the screen, we must first turn them into pixel fragments. This is called rasterization and in our example done by the function
rasterizeFront, which transforms
> rasterizedCube :: Float -> Vec2 Int -> FragmentStream (Vec3 (Fragment Float), Vec2 (Fragment Float)) > rasterizedCube angle size = rasterizeFront $ transformedCube angle size
For each fragment, we now want to give it a color from the texture we initially loaded, as well as light it with a directional light coming from the camera.
> litCube :: Texture2D RGBFormat -> Float -> Vec2 Int -> FragmentStream (Color RGBFormat (Fragment Float)) > litCube tex angle size = fmap (enlight tex) $ rasterizedCube angle size > enlight tex (norm, uv) = RGB (c * Vec.vec (norm `dot` toGPU (0:.0:.1:.()))) > where RGB c = sample (Sampler Linear Wrap) tex uv
sampleis used for sampling the texture we have loaded, using the fragment's interpolated uv-coordinates and a sampler state. Once we have a
 5 FrameBuffersA
FrameBufferis a 2D image in which fragments from
paintColorwithout any blending or color masking.
> cubeFrameBuffer :: Texture2D RGBFormat -> Float -> Vec2 Int -> FrameBuffer RGBFormat () () > cubeFrameBuffer tex angle size = paintSolid (litCube tex angle size) emptyFrameBuffer > paintSolid = paintColor NoBlending (RGB $ Vec.vec True) > emptyFrameBuffer = newFrameBufferColor (RGB 0)