# GPipe

### From HaskellWiki

(Now working with GPipe 1.2.0) |
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== What is GPipe? == | == What is GPipe? == | ||

− | [http://hackage.haskell.org/package/GPipe GPipe] is a library for programming the GPU (graphics processing unit). It is an alternative to using OpenGl, and has the advantage that it is purely functional, statically typed and operates on immutable data as opposed to OpenGl's inherently imperative style. GPipe uses the same conceptual model as OpenGl, and it is recommended that you have at least a basic understanding of how OpenGl works to be able to use GPipe. | + | [http://hackage.haskell.org/package/GPipe GPipe] is a library for programming the GPU (graphics processing unit). It is an alternative to using OpenGl, and has the advantage that it is purely functional, statically typed and operates on immutable data as opposed to OpenGl's inherently imperative style. Another important difference with OpenGl is that with GPipe you don't need to write shaders in a second shader language such as GLSL or Cg, but instead use regular Haskell functions on the GPU data types. GPipe uses the same conceptual model as OpenGl, and it is recommended that you have at least a basic understanding of how OpenGl works to be able to use GPipe. |

In GPipe, you'll primary work with these four types of data on the GPU: | In GPipe, you'll primary work with these four types of data on the GPU: | ||

− | * <hask>PrimitiveStream</hask> | + | * [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Primitive.html#t:PrimitiveStream <hask>PrimitiveStream</hask>] |

− | * <hask>FragmentStream</hask> | + | * [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Fragment.html#t:FragmentStream <hask>FragmentStream</hask>] |

− | * <hask>FrameBuffer</hask> | + | * [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#t:FrameBuffer <hask>FrameBuffer</hask>] |

− | * <hask>Texture</hask> | + | * [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Texture.html <hask>Texture</hask>] |

+ | |||

Let's walk our way through an simple example as I explain how you work with these types. | Let's walk our way through an simple example as I explain how you work with these types. | ||

This page is formatted as a literate Haskell page, simply save it as "<tt>box.lhs</tt>" and then type | This page is formatted as a literate Haskell page, simply save it as "<tt>box.lhs</tt>" and then type | ||

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</haskell> | </haskell> | ||

− | Every side of the box is created from a | + | 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 [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Primitive.html#t:PrimitiveStream <hask>PrimitiveStream</hask>]s on the GPU by [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Primitive.html#v:toGPUStream <hask>toGPUStream</hask>] 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. |

All six sides are then concatenated together into a cube. We can see that the type of the cube is a <hask>PrimitiveStream</hask> of [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Primitive.html#t:Triangle <hask>Triangle</hask>]s 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 <hask>Vertex Float</hask>s instead of <hask>Float</hask>s since they are now on the GPU. | All six sides are then concatenated together into a cube. We can see that the type of the cube is a <hask>PrimitiveStream</hask> of [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Primitive.html#t:Triangle <hask>Triangle</hask>]s 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 <hask>Vertex Float</hask>s instead of <hask>Float</hask>s since they are now on the GPU. | ||

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</haskell> | </haskell> | ||

− | The [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream.html#v:toGPU <hask>toGPU</hask>] function transforms normal values like <hask>Float</hask>s into GPU-values like <hask>Vertex Float</hask> so it can be used with the vertices of the <hask>PrimitiveStream</hask>. | + | When applying a function on the <hask>PrimitiveStream</hask> using <hask>fmap</hask>, that function will be executed on the GPU using vertex shaders. The [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream.html#v:toGPU <hask>toGPU</hask>] function transforms normal values like <hask>Float</hask>s into GPU-values like <hask>Vertex Float</hask> so it can be used with the vertices of the <hask>PrimitiveStream</hask>. |

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</haskell> | </haskell> | ||

− | The function [http://hackage.haskell.org/packages/archive/GPipe/1.1.3/doc/html/Graphics-GPipe-Texture.html#v:sample <hask>sample</hask>] is used for sampling the texture we have loaded, using the fragment's interpolated uv-coordinates and a sampler state. | + | Using <hask>fmap</hask> on a <hask>FragmentStream</hask> will execute a function on the GPU using fragment shaders. The function [http://hackage.haskell.org/packages/archive/GPipe/1.1.3/doc/html/Graphics-GPipe-Texture.html#v:sample <hask>sample</hask>] is used for sampling the texture we have loaded, using the fragment's interpolated uv-coordinates and a sampler state. |

Once we have a <hask>FragmentStream</hask> of <hask>Color</hask>s, we can paint those fragments onto a <hask>FrameBuffer</hask>. | Once we have a <hask>FragmentStream</hask> of <hask>Color</hask>s, we can paint those fragments onto a <hask>FrameBuffer</hask>. | ||

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If you have any questions or suggestions, feel free to [mailto:tobias_bexelius@hotmail.com mail] me. I'm also interested in seeing some use cases from the community, as complex or trivial they may be. | If you have any questions or suggestions, feel free to [mailto:tobias_bexelius@hotmail.com mail] me. I'm also interested in seeing some use cases from the community, as complex or trivial they may be. | ||

+ | |||

+ | [[Category:3D]] |

## Revision as of 17:41, 12 March 2010

## Contents |

## 1 What is GPipe?

GPipe is a library for programming the GPU (graphics processing unit). It is an alternative to using OpenGl, and has the advantage that it is purely functional, statically typed and operates on immutable data as opposed to OpenGl's inherently imperative style. Another important difference with OpenGl is that with GPipe you don't need to write shaders in a second shader language such as GLSL or Cg, but instead use regular Haskell functions on the GPU data types. GPipe uses the same conceptual model as OpenGl, and it is recommended that you have at least a basic understanding of how OpenGl works to be able to use GPipe.

In GPipe, you'll primary work with these four types of data on the GPU:

- <div class="inline-code"></div>PrimitiveStream
- <div class="inline-code"></div>FragmentStream
- <div class="inline-code"></div>FrameBuffer
- <div class="inline-code"></div>Texture

Let's walk our way through an simple example as I explain how you work with these types.
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.Vec.LinAlg.Transform3D > import Data.Monoid > import Data.IORef > import Graphics.UI.GLUT > (Window, > mainLoop, > postRedisplay, > idleCallback, > getArgsAndInitialize, > ($=))

Besides GPipe, this example also uses the Vec-Transform package for the transformation matrices, and the GPipe-TextureLoad package for loading textures from disc. GLUT is used in GPipe for window management and the main loop.

## 2 Creating a window

We 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 + 0.001) `mod'` (2*pi)) > return $ cubeFrameBuffer tex angle size > initWindow :: Window -> IO () > initWindow win = idleCallback $= Just (postRedisplay (Just win))

## 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:.()]

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 > transformedNorm = toGPU (Vec.map (Vec.take n3) $ Vec.take n3 $ modelMat) `multmv` norm

## 4 FragmentStreams

To render the primitives on the screen, we must first turn them into pixel fragments. This called rasterization and in our example done by the function <div class="inline-code">> 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

## 5 FrameBuffers

A <div class="inline-code">> 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)

## 6 Screenshot

## 7 Questions and feedback

If you have any questions or suggestions, feel free to mail me. I'm also interested in seeing some use cases from the community, as complex or trivial they may be.