Intermediate Tutorial 6         Projective Decals

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Introduction

This tutorial will cover projecting decals on to a mesh. This is a common technique used to highlight units in certain strategy games by painting a circle on the ground around their feet. It could also be used to put a gun's laser sight on to its target.

The full source for this tutorial is here.

Note: There is also source available that uses the BaseApplication framework and Ogre 1.7 here.

projective_visual.png

Prerequisites

This tutorial assumes that you already know how to set up an Ogre project and compile it successfully. Knowledge of the topics from previous tutorials is also assumed.

The base code for this tutorial is here.

These two images are required for this tutorial:

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Put these files into your media/materials/textures folder.

Note: The tutorial uses lower case names for these files (i.e. 'decal.png' and 'decal_filter.png'). Either save the images with lower case names or change the names when you add them to your code.

This tutorial also uses a material that is not from the Samples folder. You can use whatever material you want, but to get the look from the tutorial you can place this material into one of the material scripts. They are stored in '/media/materials/scripts'. Of course, you also have to make sure the 'spacesky.jpg' image is in your media folder as well.

material Tutorial/SpaceFloor
{
	technique
	{
		pass
		{
			texture_unit
			{
				texture spacesky.jpg
			}
		}
	}
}

Setting Up the Scene

First, comment out the setAmbientLight call at the beginning of createScene. Now, create a light and position the camera:

Ogre::Light* light = mSceneMgr->createLight("MainLight");
light->setPosition(0, 40, 0);

mCamera->setPosition(60, 100, 80);
mCamera->lookAt(0, 70, 0);

Now we'll add a floor plane.

Ogre::Plane plane;
plane.normal = Ogre::Vector3::UNIT_Y;
plane.d = 0;

Ogre::MeshManager::getSingleton().createPlane(
  "floor",
  Ogre::ResourceGroupManager::DEFAULT_RESOURCE_GROUP_NAME,
  plane,
  450.0, 450.0,
  10, 10, true, 1,
  50.0, 50.0,
  Ogre::Vector3::UNIT_Z);

Ogre::Entity* planeEntity = mSceneMgr->createEntity("floor");
planeEntity->setMaterialName("Tutorial/SpaceFloor");
planeEntity->setCastShadows(false);
mSceneMgr->getRootSceneNode()->createChildSceneNode()->attachObject(planeEntity);

This should all be familiar from the Basic Tutorial series. Next we are going to build a ring of Ogre heads to project our decals on to.

int numOfHeads = 14;
double radius = 180;
Ogre::Radian curAngle(0);
Ogre::Entity* ogreEntity;

for (int i = 0; i < numOfHeads; ++i)
{
  Ogre::SceneNode* headNode = mSceneMgr->getRootSceneNode()->createChildSceneNode();
  ogreEntity = mSceneMgr->createEntity("ogrehead.mesh");
  headNode->setScale(2.2, 2.8, 2.2);
  headNode->attachObject(ogreEntity);

  curAngle = curAngle + Ogre::Radian(Ogre::Math::TWO_PI / numOfHeads);

  headNode->setPosition(
    -radius * Ogre::Math::Sin(curAngle),
    20,
    -radius * Ogre::Math::Cos(curAngle));

  headNode->rotate(Ogre::Vector3::UNIT_Y, curAngle);
}

You can play around with the number of heads and the radius. Our code will automatically place them evenly around the circle. We won't go into the details here, but it's probably becomming clear that a good understanding of mathematics will be extremely useful for a graphics programmer. This is why you learned trigonometry in school. It was so you could build righteous Ogre head structures someday.

Projecting Decals

You create a frustum by chopping off the top of something like a pyramid or a cone. Here's a nice royalty free image of one:
frustum_visual.png
Ogre uses a frustum to represent the camera. In fact, the Camera class inherits directly from the Frustum class. This is why Ogre's representation of the camera doesn't behave as if it were a point-like object. When we called getCameraToViewportRay in previous tutorials it projected the rays from the "cap" of the Camera frustum. In this tutorial, we will be using a Frustum to project our decal onto the Ogre heads.

The first step will be creating the projector. Add the following to our BasicApp class:

BasicApp.h
void createProjector();
void addDecalToMaterial(const Ogre::String& matName);
BasicApp.cpp
void BasicApp::createProjector()
{
}

void BasicApp::addDecalToMaterial(const Ogre::String& matName);
{
}


Also add these to BasicApp class and initialize them to 0 in the constructor:

BasicApp.cpp
Ogre::SceneNode* mProjectorNode;
Ogre::Frustum* mDecalFrustum;
Ogre::Frustum* mFilterFrustum;


You might also need to use the foolowing inclusions:

BasicApp.cpp
#include <OgreMaterialManager.h>
#include <OgreTechnique.h>


We will get to the addDecalToMaterial method shortly. To start building our projector, we will first create the frustum. Add the following to createProjector:

mDecalFrustum = new Ogre::Frustum();

mProjectorNode = mSceneMgr->getRootSceneNode()->createChildSceneNode();
mProjectorNode->attachObject(mDecalFrustum);
mProjectorNode->setPosition(0, 30, 0);

The projector is placed in the center of the ring of Ogre heads. The image produced by this projector will grow larger as it moves farther away from the surface it is projecting onto. This is like how a real film projector would behave. If you wanted a projector which maintains a constant size image, then you would use code like this:

// Do not add to your project.
mDecalFrustum->setProjectionType(Ogre::PT_ORTHOGRAPHIC);
mDecalFrustum->setOrthoWindowHeight(100);

Applying the Decal

Even though we've already set up our projector, we still need to apply the projection to our entity's materials. We do this by creating a new Pass object which renders the decal on to our materials. In this tutorial, we will be modifying the material directly, but you should generally modify a copy of your materials so that you can return to the original material. Add the following to addDecalToMaterial:

Ogre::MaterialPtr mat =
  (Ogre::MaterialPtr)Ogre::MaterialManager::getSingleton().getByName(matName);
Ogre::Pass* pass = mat->getTechnique(0)->createPass();

Now that we have a Pass object, we need to set up blending and lighting. We will be adding a new texture which must be blended with the object's current texture. To do this, we will set the scene blending to be transparent alpha and the depth bias to one. This will make sure there is no transparency in the decal. Finally, we need to disable lighting for the material so that it will always show, like other GUI elements.

pass->setSceneBlending(Ogre::SBT_TRANSPARENT_ALPHA);
pass->setDepthBias(1);
pass->setLightingEnabled(false);

Now we'll create a new texture unit state using our decal.png file. We then turn on projective texturing and set some options.

Ogre::TextureUnitState* texState = pass->createTextureUnitState("decal.png");
texState->setProjectiveTexturing(true, mDecalFrustum);
texState->setTextureAddressingMode(Ogre::TextureUnitState::TAM_CLAMP);
texState->setTextureFiltering(Ogre::FO_POINT, Ogre::FO_LINEAR, Ogre::FO_NONE);

We have set the texture addressing mode to clamp so that the decal doesn't repeat on the object. We have set the magnification of the object to linear, and we have basically turned off minification (FO_POINT), and we've completely disabled mipmapping. This prevents the transparent edges of the decal from being blurred into our material.

That's the entire setup for the material.

Calling the Functions

We need to call the functions we've just built. Add the following to createScene right after the code to build the ogre heads:

createProjector();

for (unsigned int i = 0; i < planeEntity->getNumSubEntities(); ++i)
  addDecalToMaterial(planeEntity->getSubEntity(i)->getMaterialName());

for (unsigned int i = 0; i < ogreEntity->getNumSubEntities(); ++i)
  addDecalToMaterial(ogreEntity->getSubEntity(i)->getMaterialName());

The ent variable still has one of the Ogre head entities stored in it. Since all the heads use the same material, we can use any of them to grab the material name.

You might need to include:

#include <OgreSubEntity.h>

Compile and run the application, you should see the decal projected on to the heads.

Getting Rid of the Back Projection

As you've probably noticed, the decal is being projected forwards and backwards. Ogre projects the decal in both directions by default. For our current purposes, this is not what we want. To fix this, we will introduce a filter that will remove the back projection.

To filter the back projection, we need a new frustum for the filter which points in the direction we wish to filter. Add the following code to the createProjector method:

mFilterFrustum = new Ogre::Frustum();
mFilterFrustum->setProjectionType(Ogre::PT_ORTHOGRAPHIC);

Ogre::SceneNode* filterNode = mProjectorNode->createChildSceneNode();
filterNode->attachObject(mFilterFrustum);
filterNode->setOrientation(Ogre::Quaternion(Ogre::Degree(90),Ogre::Vector3::UNIT_Y));

This should all be familiar. The only difference is that we have rotated the node by 90 degrees.

Modifying the Material

We are going to add our filter as another texture unit state for the Pass we added to our material. Add the following to addDecalToMaterial:

texState = pass->createTextureUnitState("decal_filter.png");
texState->setProjectiveTexturing(true, mFilterFrustum);
texState->setTextureAddressingMode(Ogre::TextureUnitState::TAM_CLAMP);
texState->setTextureFiltering(Ogre::TFO_NONE);

We are using the 'decal_filter.png' texture and the filter frustum. We have also turned off filtering. This may seem strange, but it is just two different uses of the term filter. Compile and run the application. You should now only see the forward projection.

Rotating the Projection

Lastly, let's rotate the projector to help demonstrate the projection. Add the following to frameRenderingQueued:

mProjectorNode->rotate(Ogre::Vector3::UNIT_Y, Ogre::Degree(fe.timeSinceLastFrame * 10));

Compile and run the application. The projected decal should rotate around the circle of heads.

One Final Note

If you create decals for your own application, be sure they have a completely transparent border, otherwise they will be smeared because of the texture clamping process. The border only has to be one pixel wide.

Ogre can automatically pad your image with the necessary buffer if you set the texture address mode to TAM_BORDER (just {MONO()border{MONO} if set from a script). Also set the texture border colour to (0, 0, 0, 0). An implementation can be seen in Projective Decals.

Conclusion

In this tutorial, we projected a decal texture on to a series of meshes. This is a common technique used in a number of different applications, often to highlight objects in the scene. To do this, we used a frustum to represent the projection.

Ater constructing the projector, we still needed to apply the projection to the materials of our scene. Finally, we demonstrated that the projector can be moved, causing the projection to move through the scene. This is a relatively simple process that has an enormous number of applications.

More information is available in Projective Decals.

Exercises

Easy

  1. Experiment with the settings we've used to create and position the projector. You can create a lot of surprising effects by messing with them. For instance, if you just change the orientation we set for the filterNode to 180 degrees, you will project half of the decal forward and half of it backwards.

Intermediate

  1. Figure out how to generate the Ogre head meshes facing outwards from the circle instead of inward.

Difficult

  1. Right now we have the projector rotating. Change your application so that the projector moves up and down in a sinusoidal fashion while it rotates.

Advanced

  1. Attach the projector to our camera so that you can fly around the scene projecting the decal anywhere you look.

Full Source

The full source for this tutorial is here.

Next

Intermediate Tutorial 7


Alias: Intermediate_Tutorial_6