# Tutorial 27: Post Processing

Tutorial 27: Post Processing

This tutorial shows how to implement post processing for D3D9 and OpenGL with the engine. In order to do post processing, scene objects are firstly rendered to render target. With the help of screen quad, the render target texture is then drawn on the quad with shader-defined effects applied.

This tutorial shows how to create a screen quad. It also shows how to create a render target texture and associate it with the quad. Effects are defined as shaders which are applied during rendering the quad with the render target texture attached to it.

A simple color inverse example is presented in this tutorial. The effect is written in HLSL and GLSL.

Author
  Boshen Guan

We include all headers and define necessary variables as we have done before.

#include "driverChoice.h"
#include "exampleHelper.h"
#include <irrlicht.h>
using namespace irr;
#ifdef _MSC_VER
#pragma comment(lib, "Irrlicht.lib")
#endif

We write a class derived from IShaderConstantSetCallBack class and implement OnSetConstants callback interface. In this callback, we will set constants used by the shader. In this example, our HLSL shader needs texture size as input in its vertex shader. Therefore, we set texture size in OnSetConstants callback using setVertexShaderConstant function.

IrrlichtDevice* device = 0;
video::ITexture* rt = 0;
class QuadShaderCallBack : public video::IShaderConstantSetCallBack
{
public:
QuadShaderCallBack() : FirstUpdate(true), TextureSizeID(-1), TextureSamplerID(-1)
{ }
virtual void OnSetConstants(video::IMaterialRendererServices* services,
s32 userData)
{
core::dimension2d<u32> size = rt->getSize();
// get texture size array
f32 textureSize[] =
{
(f32)size.Width, (f32)size.Height
};
if ( FirstUpdate )
{
TextureSizeID = services->getVertexShaderConstantID("TextureSize");
TextureSamplerID = services->getPixelShaderConstantID("TextureSampler");
}
// set texture size to vertex shader
services->setVertexShaderConstant(TextureSizeID, reinterpret_cast<f32*>(textureSize), 2);
// set texture for an OpenGL driver
s32 textureLayer = 0;
services->setPixelShaderConstant(TextureSamplerID, &textureLayer, 1);
}
private:
bool FirstUpdate;
s32 TextureSizeID;
s32 TextureSamplerID;
};
class ScreenQuad : public IReferenceCounted
{
public:
ScreenQuad(video::IVideoDriver* driver)
: Driver(driver)
{
// --------------------------------> u
// |[1](-1, 1)----------[2](1, 1)
// | | ( 0, 0) / | (1, 0)
// | | / |
// | | / |
// | | / |
// | | / |
// | | / |
// | | / |
// | | / |
// | | / |
// |[0](-1, -1)---------[3](1, -1)
// | ( 0, 1) (1, 1)
// V
// v

A screen quad is composed of two adjacent triangles with 4 vertices. Vertex [0], [1] and [2] create the first triangle and Vertex [0], [2] and [3] create the second one. To map texture on the quad, UV coordinates are assigned to the vertices. The origin of UV coordinate locates on the top-left corner. And the value of UVs range from 0 to 1.

// define vertices array
Vertices[0] = irr::video::S3DVertex(-1.0f, -1.0f, 0.0f, 1, 1, 0, irr::video::SColor(0,255,255,255), 0.0f, 1.0f);
Vertices[1] = irr::video::S3DVertex(-1.0f, 1.0f, 0.0f, 1, 1, 0, irr::video::SColor(0,255,255,255), 0.0f, 0.0f);
Vertices[2] = irr::video::S3DVertex( 1.0f, 1.0f, 0.0f, 1, 1, 0, irr::video::SColor(0,255,255,255), 1.0f, 0.0f);
Vertices[3] = irr::video::S3DVertex( 1.0f, -1.0f, 0.0f, 1, 1, 0, irr::video::SColor(0,255,255,255), 1.0f, 1.0f);
// define indices for triangles
Indices[0] = 0;
Indices[1] = 1;
Indices[2] = 2;
Indices[3] = 0;
Indices[4] = 2;
Indices[5] = 3;
// turn off lighting as default
Material.setFlag(video::EMF_LIGHTING, false);
// set texture warp settings to clamp to edge pixel
for (u32 i = 0; i < video::MATERIAL_MAX_TEXTURES; i++)
{
Material.TextureLayer[i].TextureWrapU = video::ETC_CLAMP_TO_EDGE;
Material.TextureLayer[i].TextureWrapV = video::ETC_CLAMP_TO_EDGE;
}
}
virtual ~ScreenQuad() {}
virtual void render()
{
// set the material of screen quad
Driver->setMaterial(Material);
// set matrices to fit the quad to full viewport
Driver->setTransform(video::ETS_WORLD, core::IdentityMatrix);
Driver->setTransform(video::ETS_VIEW, core::IdentityMatrix);
Driver->setTransform(video::ETS_PROJECTION, core::IdentityMatrix);
// draw screen quad
Driver->drawVertexPrimitiveList(Vertices, 4, Indices, 2);
}
virtual void setMaterialFlag(video::E_MATERIAL_FLAG flag, bool newvalue)
{
Material.setFlag(flag, newvalue);
}
void setMaterialTexture(u32 textureLayer, video::ITexture* texture)
{
Material.setTexture(textureLayer, texture);
}
virtual void setMaterialType(video::E_MATERIAL_TYPE newType)
{
Material.MaterialType = newType;
}
private:
video::IVideoDriver *Driver;
video::S3DVertex Vertices[4];
u16 Indices[6];
video::SMaterial Material;
};

We start up the engine just like before. Then shader programs are selected according to the driver type.

int main()
{
// ask user for driver
video::E_DRIVER_TYPE driverType=driverChoiceConsole();
if (driverType==video::EDT_COUNT)
return 1;
// create device
device = createDevice(driverType, core::dimension2d<u32>(640, 480));
if (device == 0)
return 1; // could not create selected driver.
video::IVideoDriver* driver = device->getVideoDriver();
scene::ISceneManager* smgr = device->getSceneManager();

In this example, high level post processing shaders are loaded for both Direct3D and OpenGL drivers. File pp_d3d9.hlsl is for Direct3D 9, and pp_opengl.frag/pp_opengl.vert are for OpenGL.

const io::path mediaPath = getExampleMediaPath();
io::path vsFileName; // filename for the vertex shader
io::path psFileName; // filename for the pixel shader
switch(driverType)
{
case video::EDT_DIRECT3D9:
psFileName = mediaPath + "pp_d3d9.hlsl";
vsFileName = psFileName; // both shaders are in the same file
break;
case video::EDT_OPENGL:
psFileName = mediaPath + "pp_opengl.frag";
vsFileName = mediaPath + "pp_opengl.vert";
break;
}

Check for hardware capability of executing the corresponding shaders on selected renderer. This is not necessary though.

if (!driver->queryFeature(video::EVDF_PIXEL_SHADER_1_1) &&
!driver->queryFeature(video::EVDF_ARB_FRAGMENT_PROGRAM_1))
{
device->getLogger()->log("WARNING: Pixel shaders disabled "\
"because of missing driver/hardware support.");
psFileName = "";
}
if (!driver->queryFeature(video::EVDF_VERTEX_SHADER_1_1) &&
!driver->queryFeature(video::EVDF_ARB_VERTEX_PROGRAM_1))
{
device->getLogger()->log("WARNING: Vertex shaders disabled "\
"because of missing driver/hardware support.");
vsFileName = "";
}

An animated mesh is loaded to be displayed. As in most examples, we'll take the fairy md2 model.

// load and display animated fairy mesh
scene::IAnimatedMeshSceneNode* fairy = smgr->addAnimatedMeshSceneNode(
smgr->getMesh(mediaPath + "faerie.md2"));
if (fairy)
{
fairy->setMaterialTexture(0,
driver->getTexture(mediaPath + "faerie2.bmp")); // set diffuse texture
fairy->setMaterialFlag(video::EMF_LIGHTING, false); // disable dynamic lighting
fairy->setPosition(core::vector3df(-10,0,-100));
fairy->setMD2Animation ( scene::EMAT_STAND );
}
// add scene camera
smgr->addCameraSceneNode(0, core::vector3df(10,10,-80),
core::vector3df(-10,10,-100));

We create a render target texture (RTT) with the same size as frame buffer. Instead of rendering the scene directly to the frame buffer, we firstly render it to this RTT. Post processing is then applied based on this RTT. RTT size needs not to be the same with frame buffer though. However in this example, we expect the result of rendering to RTT to be consistent with the result of rendering directly to the frame buffer. Therefore, the size of RTT keeps the same with frame buffer.

// create render target
if (driver->queryFeature(video::EVDF_RENDER_TO_TARGET))
{
rt = driver->addRenderTargetTexture(core::dimension2d<u32>(640, 480), "RTT1");
}
else
{
device->getLogger()->log("Your hardware or this renderer is not able to use the "\
"render to texture feature. RTT Disabled.");
}

Post processing is achieved by rendering a screen quad with this RTT (with previously rendered result) as a texture on the quad. A screen quad is geometry of flat plane composed of two adjacent triangles covering the entire area of viewport. In this pass of rendering, RTT works just like a normal texture and is drawn on the quad during rendering. We can then take control of this rendering process by applying various shader-defined materials to the quad. In other words, we can achieve different effect by writing different shaders. This process is called post processing because it normally does not rely on scene geometry. The inputs of this process are just textures, or in other words, just images. With the help of screen quad, we can draw these images on the screen with different effects. For example, we can adjust contrast, make grayscale, add noise, do more fancy effect such as blur, bloom, ghost, or just like in this example, we invert the color to produce negative image. Note that post processing is not limited to use only one texture. It can take multiple textures as shader inputs to provide desired result. In addition, post processing can also be chained to produce compound result.

// we create a screen quad
ScreenQuad *screenQuad = new ScreenQuad(driver);
// turn off mip maps and bilinear filter since we do not want interpolated result
screenQuad->setMaterialFlag(video::EMF_USE_MIP_MAPS, false);
screenQuad->setMaterialFlag(video::EMF_BILINEAR_FILTER, false);
// set quad texture to RTT we just create
screenQuad->setMaterialTexture(0, rt);

Let's create material for the quad. Like in other example, we create material using IGPUProgrammingServices and call addShaderMaterialFromFiles, which returns a material type identifier.

// create materials
video::IGPUProgrammingServices* gpu = driver->getGPUProgrammingServices();
s32 ppMaterialType = 0;
if (gpu)
{
// We write a QuadShaderCallBack class that implements OnSetConstants
// callback of IShaderConstantSetCallBack class at the beginning of
// this tutorial. We set shader constants in this callback.
// create an instance of callback class
QuadShaderCallBack* mc = new QuadShaderCallBack();
// create material from post processing shaders
ppMaterialType = gpu->addHighLevelShaderMaterialFromFiles(
vsFileName, "vertexMain", video::EVST_VS_1_1,
psFileName, "pixelMain", video::EPST_PS_1_1, mc);
mc->drop();
}
// set post processing material type to the quad
screenQuad->setMaterialType((video::E_MATERIAL_TYPE)ppMaterialType);

Now draw everything. That's all.

int lastFPS = -1;
while(device->run())
{
if (device->isWindowActive())
{
driver->beginScene(true, true, video::SColor(255,0,0,0));
if (rt)
{
// draw scene into render target
// set render target to RTT
driver->setRenderTarget(rt, true, true, video::SColor(255,0,0,0));
// draw scene to RTT just like normal rendering
smgr->drawAll();
// after rendering to RTT, we change render target back
driver->setRenderTarget(0, true, true, video::SColor(255,0,0,0));
// render screen quad to apply post processing
screenQuad->render();
}
else
{
// draw scene normally
smgr->drawAll();
}
driver->endScene();
int fps = driver->getFPS();
if (lastFPS != fps)
{
core::stringw str = L"Irrlicht Engine - Post processing example [";
str += driver->getName();
str += "] FPS:";
str += fps;
device->setWindowCaption(str.c_str());
lastFPS = fps;
}
}
}
// do not forget to manually drop the screen quad
screenQuad->drop();
device->drop();
return 0;
}