Improving the appearance of 3D OpenGL scenes Brian Farrimond Robina Hetherington

What we shall do Hidden line removal Lighting and shading Materials Enable nearer objects to hide distant objects Lighting and shading Improve the sense of depth with shading Materials Add colour Importing 3DSMax models Add realistic models

Hidden line removal Unless told otherwise, OpenGL draws on top of other objects regardless of how far away they are Suppose we want to draw this -----------

First attempt

The effect Because OpenGL draws the cube then the sphere, the sphere is drawn on top even though it is further away Ex07

Hidden line removal OpenGL uses depth-buffering (also known as z-buffering) Depth buffer is created It records for each pixel, the distance from the viewer Initially all set to a very large value

Hidden line removal As each object drawn each pixel is generated its distance from the viewer is compared with the corresponding value in the depth buffer If smaller than the value already there then Pixel is updated Its distance is recorded in the depth buffer Otherwise Pixel is not drawn

Coding in OpenGL 1. In main – create a depth buffer

Coding in OpenGL 2. In reshape – enable depth testing

Coding in OpenGL 3. In display – set the depth buffer to high values

Results – Ex08

Lighting 3D drawing aims to look realistic Realism includes realistic lighting effects Light in the real world is very complicated Optics is a whole branch of Physics Light involves quantum mechanics! OpenGL uses simplified light calculations Results are acceptable

Default lighting OpenGL default is to have no lighting Switch on the lighting with: glEnable(GL_LIGHTING); OpenGL has 8 lights available named GL_LIGHT0, GL_LIGHT1, .., GL_LIGHT7 Switch on a light like this: glEnable(GL_LIGHT0);

Ex09 Here is init modified to switch on GL_LIGHT0

Ex09 Ex08 – no lighting Ex09 – with lighting

Ex09 Sphere has its polygons visible No colours – glColor3f is ignored in a lit scene

Setting the shade model We can make the sphere show its polygons with glShadeModel(GL_FLAT); When drawing a polygon, OpenGL chooses one of the polygons vertices and colours all the polygon's pixels the same colour as this vertex Put the command into init

Setting the shade model We can make the sphere look smoother with glShadeModel(GL_SMOOTH); When drawing a polygon, OpenGL computes a colour for each vertex then colours the polygon's interior pixels by interpolating the vertex values to provide the smooth effect. This is the OpenGL default. Put the command into init

Defining materials to get colour When we use lights we need to specify an object’s colour in a more sophisticated way Interaction of a surface with light is scary physics In OpenGL we simplify by using the concept of material properties

Lights with colour

OpenGL material properties Material property Description Ambient Light scattered by the rest of the scene. It is omnidirectional. Diffuse Reflected light from light sources. The more directly the surface faces a light, the more light is reflected. Emissiveness Light generated by the object. Used mainly to simulate lamps or the Sun in a scene Specular Specular reflection produces highlights which are brightest at the angle of reflection between the light and the viewpoint. Shininess The brightness and sharpness of the highlight.

Specifying diffuse colour Defining colours as 4 element arrays of floating point numbers: red, green, blue, alpha

Specifying diffuse colour Each array element is a floating point number

Specifying diffuse colour Indicates the variable is an array instead of a single value

Specifying diffuse colour Puts values into the array

Specifying diffuse colour Setting the colour for the red cube

Specifying diffuse colour Setting the colour for the green sphere

Notes We need to use arrays to define material colours GL_FRONT specifies that the colour should be applied to the front of the object’s polygons Alternatives are GL_FRONT and GL_FRONT_AND_BACK

Defining the light Default colour is white Default position is (0, 0, 1) Change the position like this …

Array containing x, y, z coords of light position Defining the light Default colour is white Default position is (0, 0, 1) Change the position like this … Array containing x, y, z coords of light position plus w value w = 0 : light at specified point w = 1 : light is at infinity in direction from origin through (x, y, z)

Light is moved to the new position Defining the light Default colour is white Default position is (0, 0, 1) Change the position like this … Light is moved to the new position

Nate Robbins tutorial LightMaterial

ExLoad3DS Illustrates changing the light position interactively

Using 3DS Models in OpenGL Complex models can be imported into OpenGL if the file format is understood 3DSMax has a facility for exporting models as .3ds files. Web sites contain clues as to the structure of this file format In this module we shall use the vertex and polygon information found there

Using 3DS Models in OpenGL We need to Load the 3DS data into a suitable data structure Use the data structure to draw OpenGL polygons

Procedure: adding files Put a copy of the files load3dsbtf.cpp load3dsbtf.h into your project folder Add load3dsbtf.cpp to your project tree

Procedure: programming the OpenGL Add a variable that forms the data structure Use the call loadBTF3DS to load a model from the .3ds file Use the call display3DSObject to display the model in OpenGL

Include file with definitions needed for 3DS ExLoad3ds Include file with definitions needed for 3DS

3DS file may contain more than one object ExLoad3ds 3DS file may contain more than one object

Array of pointers to 3ds object data structures ExLoad3ds Array of pointers to 3ds object data structures

Count of the number of 3ds objects ExLoad3ds Count of the number of 3ds objects

A function to set the colour

Here we load the 3DS object from its file init Here we load the 3DS object from its file

display

Display each of the objects read from the 3DS file