# Difference between revisions of "GPipe/Tutorial"

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

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

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

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

Let's walk our way through an simple example as I explain how you work with these types. This example requires GPipe version 1.2.1 or later. |
Let's walk our way through an simple example as I explain how you work with these types. This example requires GPipe version 1.2.1 or later. |
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</haskell> |
</haskell> |
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− | First we set up GLUT, and load a texture from disc via the [http://hackage.haskell.org/package/GPipe-TextureLoad GPipe-TextureLoad package] function <hask>loadTexture</hask>. In this example we're going to animate a spinning box, and for that we put an angle in an <hask>IORef</hask> so that we can update it between frames. We then create a window with [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#v:newWindow < |
+ | First we set up GLUT, and load a texture from disc via the [http://hackage.haskell.org/package/GPipe-TextureLoad GPipe-TextureLoad package] function <hask>loadTexture</hask>. In this example we're going to animate a spinning box, and for that we put an angle in an <hask>IORef</hask> so that we can update it between frames. We then create a window with [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#v:newWindow <code>newWindow</code>]. When the window is created, <hask>initWindow</hask> registers this window as being continously redisplayed in the idle loop. At each frame, the <hask>IO</hask> action <hask>renderFrame tex angleRef size</hask> is run. In this function the angle is incremented with 0.005 (reseted each lap), and a <hask>FrameBuffer</hask> is created and returned to be displayed in the window. But before I explain <hask>FrameBuffer</hask>s, let's jump to the start of the graphics pipeline instead. |

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</haskell> |
</haskell> |
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− | 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 < |
+ | 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 <code>PrimitiveStream</code>]s on the GPU by [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Primitive.html#v:toGPUStream <code>toGPUStream</code>] 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 < |
+ | 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 <code>Triangle</code>]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. |

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. |
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. |
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</haskell> |
</haskell> |
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− | 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 < |
+ | 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 <code>toGPU</code>] 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>. |

== FragmentStreams == |
== FragmentStreams == |
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− | 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 [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Fragment.html#v:rasterizeFront < |
+ | 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 [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Fragment.html#v:rasterizeFront <code>rasterizeFront</code>], which transforms <hask>PrimitiveStream</hask>s into [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-Stream-Fragment.html#t:FragmentStream <code>FragmentStream</code>]s. |

<haskell> |
<haskell> |
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</haskell> |
</haskell> |
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− | 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 < |
+ | 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 <code>sample</code>] 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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== FrameBuffers == |
== FrameBuffers == |
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− | A [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#t:FrameBuffer < |
+ | A [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#t:FrameBuffer <code>FrameBuffer</code>] is a 2D image in which fragments from <hask>FragmentStream</hask>s are painted. A <hask>FrameBuffer</hask> may contain any combination of a color buffer, a depth buffer and a stencil buffer. Besides being shown in windows, <hask>FrameBuffer</hask>s may also be saved to memory or converted to textures, thus enabling multi pass rendering. A <hask>FrameBuffer</hask> has no defined size, but take the size of the window when shown, or are given a size when saved to memory or converted to a texture. |

− | And so finally, we paint the fragments we have created onto a black <hask>FrameBuffer</hask>. By this we use [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#v:paintColor < |
+ | And so finally, we paint the fragments we have created onto a black <hask>FrameBuffer</hask>. By this we use [http://hackage.haskell.org/packages/archive/GPipe/latest/doc/html/Graphics-GPipe-FrameBuffer.html#v:paintColor <code>paintColor</code>] without any blending or color masking. |

<haskell> |
<haskell> |

## Revision as of 19:44, 30 May 2011

## Introduction

In GPipe, you'll primary 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 1.2.1 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.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.

## Creating a window

We start by defining the `main`

function.

```
> 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))
```

First we set up GLUT, and load a texture from disc via the GPipe-TextureLoad package function `loadTexture`

. In this example we're going to animate a spinning box, and for that we put an angle in an `IORef`

so that we can update it between frames. We then create a window with `newWindow`

. When the window is created, `initWindow`

registers this window as being continously redisplayed in the idle loop. At each frame, the `IO`

action `renderFrame tex angleRef size`

is run. In this function the angle is incremented with 0.005 (reseted each lap), and a `FrameBuffer`

is created and returned to be displayed in the window. But before I explain `FrameBuffer`

s, let's jump to the start of the graphics pipeline instead.

## 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 `PrimitiveStream`

s 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.

All six sides are then concatenated together into a cube. We can see that the type of the cube is a `PrimitiveStream`

of `Triangle`

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 `Vertex Float`

s instead of `Float`

s since they are now on the GPU.

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
```

When applying a function on the `PrimitiveStream`

using `fmap`

, that function will be executed on the GPU using vertex shaders. The `toGPU`

function transforms normal values like `Float`

s into GPU-values like `Vertex Float`

so it can be used with the vertices of the `PrimitiveStream`

.

## 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 `rasterizeFront`

, which transforms `PrimitiveStream`

s into `FragmentStream`

s.

```
> rasterizedCube :: Float -> Vec2 Int -> FragmentStream (Vec3 (Fragment Float), Vec2 (Fragment Float))
> rasterizedCube angle size = rasterizeFront $ transformedCube angle size
```

In the rasterization process, values of type `Vertex Float`

are turned into values of type `Fragment Float`

.

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
```

Using `fmap`

on a `FragmentStream`

will execute a function on the GPU using fragment shaders. The function `sample`

is used for sampling the texture we have loaded, using the fragment's interpolated uv-coordinates and a sampler state.

Once we have a `FragmentStream`

of `Color`

s, we can paint those fragments onto a `FrameBuffer`

.

## FrameBuffers

A `FrameBuffer`

is a 2D image in which fragments from `FragmentStream`

s are painted. A `FrameBuffer`

may contain any combination of a color buffer, a depth buffer and a stencil buffer. Besides being shown in windows, `FrameBuffer`

s may also be saved to memory or converted to textures, thus enabling multi pass rendering. A `FrameBuffer`

has no defined size, but take the size of the window when shown, or are given a size when saved to memory or converted to a texture.

And so finally, we paint the fragments we have created onto a black `FrameBuffer`

. By this we use `paintColor`

without 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)
```

This `FrameBuffer`

is the one we return from the `renderFrame`

action we defined at the top.