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Improving the appearance of 3D OpenGL scenes

Brian FarrimondRobina Hetherington

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What we shall do

• Hidden line removal– 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

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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 -----------

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First attempt

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The effect

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

• Ex07

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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

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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

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Coding in OpenGL

• 1. In main – create a depth buffer

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Coding in OpenGL

• 2. In reshape – enable depth testing

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

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Results – Ex08

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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

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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);

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Ex09

• Here is init modified to switch on GL_LIGHT0

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Ex09

Ex09 – with lightingEx08 – no lighting

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Ex09

• Sphere has its polygons visible

• No colours – glColor3f is ignored in a lit scene

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Setting the shade model

• We can make the sphere show its polygons withglShadeModel(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

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Setting the shade model• We can make the sphere look

smoother withglShadeModel(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

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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

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Lights with colour

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OpenGL material propertiesMaterial 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.

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Specifying diffuse colour

Defining coloursas 4 element arrays of floating point numbers:

red,green,blue,alpha

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Specifying diffuse colour

Each array element is a floating point

number

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Specifying diffuse colour

Indicates the variable is an

array instead of a single value

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Specifying diffuse colour

Puts values into the array

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Specifying diffuse colour

Setting the colour for the red

cube

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Specifying diffuse colour

Setting the colour for the green sphere

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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

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Defining the light

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

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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 valuew = 0 : light at specified pointw = 1 : light is at infinity in direction from origin through (x, y, z)

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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

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Nate Robbins tutorial

• LightMaterial

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ExLoad3DS

• Illustrates changing the light position interactively

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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

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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

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Procedure: adding files

• Put a copy of the filesload3dsbtf.cppload3dsbtf.h

into your project folder• Add load3dsbtf.cpp to your project tree

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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

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ExLoad3ds

Include file with definitions needed for 3DS

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ExLoad3ds

3DS file may contain more than one object

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ExLoad3ds

Array of pointers to 3ds object data structures

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ExLoad3ds

Count of the number of 3ds objects

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A function to set the colour

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init

Here we load the 3DS object from its

file

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display

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display

Display each of the objects read from the

3DS file