opengl’fundamentals - unibzrusso/ap/opengl_presentation.pdf · opengl fundamentals opengl...

1 78 / Radek Oslejsek, Bolzano 2015 OpenGL

Fundamentals

OpenGL Fundamentals

Radek Ošlejšek

Masaryk University, Faculty of Informa<cs Brno, Czech Republic

[email protected]

hGp://www.fi.muni.cz/~oslejsek


Fundamentals

1. Introduc<on to OpenGL


Fundamentals

Literature

• OpenGL Programming Guide (Red Book)

– Basics for 1.1 version

– hGp://www.glprogramming.com/red/

• OpenGL Reference Manual (Blue Book) hGp://www.glprogramming.com/

blue/

• hGp://www.opengl.org/documenta<on/


Fundamentals

What’s OpenGL

• Applica<on programming interface (API) for graphical subsystems • Hardware independent – the only required part is the framebuffer. • Independence on opera<ng system, on graphic drivers and window managers • OpenGL isn’t „pixel exact“. Same sequence of commands can produce slightly different pictures on different pla\orms

5 78 / Radek Oslejsek, Bolzano 2015 Radek Oslejsek, Bolzano 2015 OpenGL

Fundamentals

What OpenGL can/can't do

Can do: l rendering of triangles, lines, polygons (3D) l picture manipula<on (2D) l local illumina<on, i.e. ligh<ng, shading, textures l fog l visibility detec<on l transforma<ons l ...

Can't do: l window creation and management l defining objects, scene representation l NURBS l global illumination (shadows, reflections) l voxels, ...

http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter09.html


Fundamentals

OpenGL libraries

l OpenGL U<lity Library l Extension of basic OpenGL

l Mapping between the global and local coordinate system l Mip-‐mapping l NURBS objects l …

l All func<ons have the glu prefix l Other libraries: hGp://www.opengl.org/resources/libraries/

l Window Management: Different tools for different programming languages

l C/C++: GLUT, SDL l Java: JOGL + Swing/AWT


Fundamentals

OpenGL Architecture: Pipeline

http://www.ntu.edu.sg/home/ehchua/programming/opengl/cg_basicstheory.html


Fundamentals

OpenGL as a state automat

• Set the state, ask for the current state, …


Fundamentals

Syntax Rules


Fundamentals

Data types


Fundamentals

Different version of one func<on

glVertex2i(int1, int2); glVertex2d(double1, double2); glVertex3f(float1, float2, float3); glVertex4f(float1, float2, float3, float4);


Fundamentals

„v“ usage

• Parameters of given func<on are stored in an array: GLfloat color[]={1.0, 0.0, 1.0}; glColor3fv(color);

• Without array: GLfloat r=1.0, g=0.0, b=1.0; glColor3f(r, g, b);

• Both types are leading to the same result, the first op<on is mostly faster


Fundamentals

First example

void render() { glDisable(GL_LIGHTING); // turn off the lights glBegin(GL_TRIANGLES); // drawing triangles glShadeModel(GL_SMOOTH); // Gouraud shading // drawing a triangle with red, blue and green vertices glColor3f(1.0,0.0,0.0); glVertex3f(0.0, 0.0, 0.0); glColor3f(0.0,0.0,1.0); glVertex3f(1.0, 0.0, 0.0); glColor3f(0.0,1.0,0.0); glVertex3f(1.0, 1.0, 0.0); glEnd(); // rendering finished }


Fundamentals

2. Camera and view-‐port sekngs


Fundamentals

Camera position

l Camera can be posi<oned in two manners: l Object manipula<on (model transforma<ons) l Camera manipula<on (view transforma<ons) l Shifting the camera = shifting all objects in opposite direction l Rotating the camera CW = rotating objects CCW

l Initial position: [0,0,0] l Initial viewing direction: (0,0,-1) l How to set initial camera position and orientation: l  glMatrixMode(GL_PROJECTION); l  glLoadIdentity();


Fundamentals

Camera posi<on (cont.)

• Positioning the camera in 3D space is too complex • It can be enhanced by using function of the glu library

gluLookAt( GLdouble eyeX,GLdouble eyeY,GLdouble eyeZ, GLdouble posX,GLdouble posY,GLdouble posZ, GLdouble upX,GLdouble upY,GLdouble upZ)


Fundamentals

Projec<ons

Orthographic Perspective

“focal length” of the camera


Fundamentals

Orthographic projec<on

void glOrtho(GLdouble left,GLdouble right, GLdouble bottom,GLdouble top, GLdouble near, GLdouble far)


Fundamentals

Perspec<ve projec<on

void glFrustum(GLdouble left,GLdouble right, GLdouble bottom,GLdouble top, GLdouble near, GLdouble far)


Fundamentals

Perspec<ve projec<on

void gluPerspective(GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar)


Fundamentals

Viewport sekngs

By default, the viewport is set to cover the en<re applica<on window. We can use the glViewport() func<on to choose a smaller area (e.g., for split-‐screen or mul<-‐screen applica<on) • x and y determine the lower lel viewport corner • w and h determine the size of the viewport rectangle

void glViewport(GLint x,GLint y, GLsizei w, GLsizei h)


Fundamentals

Example – rendering into 2 viewports

int w = 600, h = 300; // window size // ... gluPerspective(60.0, w/(2.0*h), 1, 10.0);//!!! // ... glViewport(0,0,w/2,h); glutSolidTeapot(0.8); glViewport(w/2,0,w/2,h); glRotatef(60, 1, 1, 0); glutSolidTeapot(0.8);


Fundamentals

Transformation matrices

l In general, they enable to express the linear transforma<ons – transla<on, rota<on, scaling. l We can combine the linear transforma<ons – by mul<plying the matrices l ModelView matrix – affects loca<ons of objects in the scene as well as the posi<on of the camera. Func<ons affec<ng transforma<on matrix:

l gluLookAt l glRotate, glTranslate, etc. – will be described later

l Projec=on matrix – affects the projec<on of the camera. Func<ons affec<ng transforma<on matrix:

l glOrtho, glFrustum, gluPerspec9ve l Texture matrix – maps textures of object surface – out of the scope of this lecture.


Fundamentals

Sekng the current transforma<on matrix

We determine the transforma<on matrix to be modified: parameter mode: GL_MODELVIEW GL_PROJECTION GL_TEXTURE … because gluLookAt() can easily modify projec<on matrix, for instance. Advice: Keep the modelview matrix always on, i.e. a code modifying the projec<on, for instance, should switch to projec<on matrix only temporarily and then switch back by glMatrixMode(GL_MODELVIEW).

glMatrixMode(GLenum mode)


Fundamentals

Clearing transforma<on matrix

Matrix opera<ons are “cumula<ve”. To set the current transforma<on matrix do default value: Advice: Always call glLoadIden9ty() before sekng the camera posi<on or projec<on, otherwise the new computed matrix is added (mul<plied with) the current matrix

glLoadIdentity()


Fundamentals

Exercise: Goal 1


Fundamentals

3. Geometry and colors


Fundamentals

Drawing primi<ves

• Any arbitrary complex scene is composed of ver<ces (triangles). • Vertex is a basic part of all primi<ves • Next commands are equivalent

void glVertex{234}{sifd}[v](TYPE coords)

GLfloat foo[]={3.0, 2.0, 0.0}; glVertex3fv(foo); glVertex2i(3,2); glVertex2f(3.0f,2.0f); glVertex3i(3,2,0);


Fundamentals

Drawing state

• Switching to drawing state using glBegin(primi=ve type) • Finishing the drawing state using glEnd() • Example: glBegin(GL_TRIANGLES); // setting the OpenGL state glVertex3f(0.0, 0.0, 0.0); glVertex3f(1.0, 0.0, 0.0); glVertex3f(1.0, 1.0, 0.0); glEnd(); //end of rendering • The following command doesn't exist! It's because of using OpenGL pipeline. glTriangle(x1,y1,z1, x2,y2,z2, x3,y3,z3);


Fundamentals

OpenGL primi<ves

• Basic primi<ves: – glBegin(GL_POINTS) – glBegin(GL_LINES) – glBegin(GL_LINE_STRIP) – glBegin(GL_LINE_LOOP) – glBegin(GL_TRIANGLES) – glBegin(GL_TRIANGLE_FAN) – glBegin(GL_TRIANGLE_STRIP) – glBegin(GL_QUADS) – glBegin(GL_QUAD_STRIP) – glBegin(GL_POLYGON)

http://www.informit.com/articles/article.aspx?p=1377833&seqNum=2

Surfaces must be always planar and convex (triangles are always planar and convex), otherwise:


Fundamentals

Changing aGributes of points

• Color: glColor*(); • Texture mapping: glTexCoord*(); • Size: glPointSize(Glfloat size); • Example: glBegin(GL_TRIANGLES); glColor3i(1, 0, 0); glTexCoord2f(0.1, 0.1); glVertex3f(-2.0, -1.0, 0.0); glTexCoord2f(0.0, 0.9); glVertex3f(-2.0, 1.0, 0.0); glTexCoord2f(1.1, 0.8); glVertex3f(0.0, 1.0, 0.0); glEnd();


Fundamentals

Example 1 – which color is used?

glBegin(GL_TRIANGLES); glColor3f(1.0,0.0,0.0); glVertex2i(0, 0); glVertex2i(0, 10); glVertex2i(5, 0); glColor3f(0.0,1.0,0.0); glVertex2i(5, 10); glColor3f(0.0,0.0,1.0); glVertex2i(10, 0); glColor3f(0.0,0.0,0.0); // black glVertex2i(10, 10); glEnd();

1

2

3

4

5

6


Fundamentals

Example 2 – which color is used?

glBegin(GL_TRIANGLE_STRIP); glColor3f(1.0,0.0,0.0); glVertex2i(0, 0); glVertex2i(0, 10); glVertex2i(5, 0); glColor3f(0.0,1.0,0.0); glVertex2i(5, 10); glColor3f(0.0,0.0,1.0); glVertex2i(10, 0); GlColor3f(0.0,0.0,0.0); // black glVertex2i(10, 10); glEnd();

2 4 6

1 3 5


Fundamentals

Changing the drawing mode

• While the glBegin() affects the defini<on of the geometry, glPolygonMode() affects visualiza<on of the geometry. • glPolygonMode(GL_FRONT_AND_BACK, GL_POINT); • glPolygonMode(GL_FRONT_AND_BACK, GL_LINE); • glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);


Fundamentals

Color of polygons

glShadeModel(GL_SMOOTH) glShadeModel(GL_FLAT)


Fundamentals

4. Object transforma<ons


Fundamentals

Modeling transforma<ons

• Basic affine transforma<ons are predefined in OpenGL transla=on, rota=on, scaling • Using these predefined transforma<on is much faster than mul<plying matrices (it has the hardware support) • Note: we are working with modelview matrix, recall the glMatrixMode(GL_MODELVIEW)


Fundamentals

glTranslate()

Shiling by vector [x, y, z]

void glTranslate{fd}(TYPE x,TYPE y,TYPE z)


Fundamentals

glScale()

• Magnifica<on/minifica<on in all three coordinates x, y, z determines the size change in axes x, y, z • Example glLoadIdentity(); DrawObject(); glScalef(0.8, -2.0, 1.3); DrawObject();

void glScale{fd}(TYPE x,TYPE y,TYPE z)


Fundamentals

glRotate()

• Rota<ng by angle around the axis passing through the (x, y, z) origin angle is in <0, 360> x, y, z determines the axis of rota<on

void glRotate{fd}(TYPE angle,TYPE x,TYPE y,TYPE z)


Fundamentals

glRotate()

• Example glLoadIdentity(); DrawObject(); glRotatef(45,-1,-1.5,0.5); DrawObject();


Fundamentals

Order of transforma<ons

glMatrixMode(GL_MODELVIEW); 1: glLoadIdentity(); 2: glMultMatrix(translate); 3: glMultMatrix(rotate); 4: DrawObject();

x x x

y y y

z z z


Fundamentals

Order of transforma<ons

glMatrixMode(GL_MODELVIEW); 1: glLoadIdentity(); 2: glMultMatrix(rotate); 3: glMultMatrix(translate); 4: DrawObject();

x x x

y y y

z z z


Fundamentals

Matrix stack

• OpenGL enables to store current modelview matrix (and other matrices as well) and to recover this matrix later on (return to stored transforma<on)

– Storing the currently selected matrix to the top of the stack

– Removing the matrix from the top of the stack

void glPushMatrix()

void glPopMatrix()


Fundamentals

Matrix stack

https://www.cosc.brocku.ca/Offerings/3P98/course/lectures/2d_3d_xforms/

http://what-when-how.com/opengl-programming-guide/manipulating-the-matrix-stacks-viewing-opengl-programming/

push pop


Fundamentals

Example – matrix stack

void Tea(GLfloat i){ glPushMatrix(); if (i>0.01) Tea(i/2.0); glPopMatrix(); glPushMatrix(); glTranslatef(-i,-i,0);glutWireTeapot(i/2.0); glPopMatrix(); glPushMatrix(); glTranslatef(-i,i,0);glutWireTeapot(i/2.0); glPopMatrix(); glPushMatrix(); glTranslatef(i,-i,0);glutSolidTeapot(i/2.0); glPopMatrix(); glPushMatrix(); glTranslatef(i,i,0);glutSolidTeapot(i/2.0); glPopMatrix(); }


Fundamentals

Exercise: Goal 2


Fundamentals

5. Ligh<ng


Fundamentals

Shading in OpenGL

• OpenGL supports two types of basic shadings ... – Constant (flat) shading:

glShadeModel(GL_FLAT) – Gouraud shading (color interpola<on)

glShadeModel(GL_SMO-OTH) • … and Phong shading, which is more complicated.

http://148.204.81.206/matlab/visualize/selecting-a-lighting-method.html


Fundamentals

Phong shading

• OpenGL works with colors (examples so far) or with lights (Phong shading model)

– Using colors: Specifying color of each vertex – Using lights: Color of each vertex is computed from

l incoming light – we have to define the light proper<es l material proper<es of objects

• Switching between these modes: l glEnable(GL_LIG-HTING) l glDisa-ble(GL_LIGHTIN-G)

• Enabling/disabling individual lights: glEnable(GL_LIGHT?), glDisable(GL_LIG-HT?)


Fundamentals

Ambient light

• Omnidirec<onal light – independent on the posi<on and orienta<on of sources of lights • Objects are flat – losing the spa<al percep<on


Fundamentals

Diffuse light

• Ligh<ng the object surface from given light source • The light reflects from the surface to all direc<ons with the same intensity – opaque materials


Fundamentals

Specular light

• Ray of light reflects from the object surface according to the law of reflec<on • Reflec<ng is not ideal – we have to take into account the coefficient of the change of intensity of the reflected light


Fundamentals

Combina<on


Fundamentals

Sekng light proper<es

• where light is GL_LIGHTn • p is one of the parameters (next slide) • val is the parameter value

void glLight{if}[v](GLenum light, GLenum p, GLint val);


Fundamentals

Parameter p

GL_AMBIENT (0.0, 0.0, 0.0, 1.0) ambient light intensity GL_DIFFUSE (1.0, 1.0, 1.0, 1.0) diffuse light intensity

or (0.0, 0.0, 0.0, 1.0) GL_SPECULAR (1.0 ,1.0 ,1.0, 1.0) specular light intensity

or (0.0, 0.0 ,0.0 ,1.0) GL_POSITION (0.0 ,0.0, 1.0, 0.0) (x,y,z,w) light position note: position (x,y,z,1) means local light position (x,y,z,0) means infinity light, which is the directional light GL_SPOT_DIRECTION (0.0, 0.0, -1.0) (x,y,z) direction of the spot light GL_SPOT_EXPONENT 0.0 spot exponent GL_SPOT_CUTOFF 180.0 cutting angle for the spot light GL_CONSTANT_ATTENUATION 1.0 constant attenuation factor GL_LINEAR_ATTENUATION 0.0 linear attenuation factor GL_QUADRATIC_ATTENUATION 0.0 quadratic attenuation factor


Fundamentals

Example

GLfloat light_position[] = {0,0,-4,1.0}; // w=0:infinite GLfloat light_color[] = {1,1,1,1}; // set the lights glLightfv(GL_LIGHT0, GL_POSITION, light_position); glLightfv(GL_LIGHT0, GL_DIFFUSE, light_color); glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); // set the spheres glTranslatef(0,0,-10); glutSolidSphere(0.2, 100, 100);//middle glLoadIdentity();glTranslatef(0.5,0,-5); glutSolidSphere(0.2, 100, 100); //left glLoadIdentity();glTranslatef(-0.5,0,-5); glutSolidSphere(0.2, 100, 100); //right


Fundamentals

Example – cont. GLfloat specular[] = {1, 1, 1, 1}; GLfloat diffuse[] = {1, 0, 0, 1}; glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse); glLightfv(GL_LIGHT0, GL_SPECULAR, specular); GLfloat specular[] = {1, 1, 0, 1}; GLfloat diffuse[] = {1, 0, 0, 1}; GLfloat specular[] = {0, 0, 1, 1}; GLfloat diffuse[] = {0.5, 0.5, 0.5, 1}; GLfloat specular[] = {1, 0, 0, 1}; GLfloat diffuse[] = {1, 1, 1, 1};


Fundamentals

Point lights

• Ligth bulb in space emikng the light in all direc<ons with the same intensity

– Parameter posi9on – pointer to the array of 4 values. First three values represent the x, y, z posi<on of light, the last value is 1

glLightfv(GL_LIGHT?, GL_POSITION, position); glLightiv(GL_LIGHT?, GL_POSITION, position);


Fundamentals

Direc<onal lights

• Light it emiGed from one direc<on independently on the mutual posi<on of objects in the scene • E.g. sun – sunrays are almost parallel

glLightfv(GL_LIGHT?, GL_POSITION, direction); glLightiv(GL_LIGHT?, GL_POSITION, direction);


Fundamentals

Reflector lights

• The most complex light in OpenGL • Light is emmited from one point only in given cone • We have to define more parameters:

– Light posi<ons – Direc<on of rays – Angle in which the light intensity decreases (to 0)


Fundamentals

Reflector lights

GL_SPOT_DIRECTION direction of refl. light (0,0,-1) GL_SPOT_CUTOFF angle cutoff for refl. Light 180.0 GL_SPOT_CUTOFF sekng to 180o means non-‐reflec<on light


Fundamentals

Reflec<on lights

• Changing GL_SPOT_CUTOFF glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse); glLightfv(GL_LIGHT0, GL_SPECULAR, specular); glLightfv(GL_LIGHT0, GL_POSITION, light_position); glLightf(GL_LIGHT0,GL_SPOT_CUTOFF,7.0); glLightfv(GL_LIGHT0,GL_SPOT_DIRECTION,spot_dir); glEnable(GL_LIGHTING); glEnable(GL_LIGHT0);


Fundamentals

Lights -‐ aGenua<on

GL_CONSTANT ATTENUATION 1 GL_LINEAR_ATTENUATION 0 GL_QUADRATIC_ATTENUATION 0 • These coefficients are combined into equa<on: • d -‐ distance • AGenua<on is not implicitly used

( )2321

1dcdcc

nattenuatio++

=


Fundamentals

Sekng material proper<es

face is GL_FRONT, GL_BACK or GL_FRONT_AND_BACK p one of the parameters (next slide) val parameter value

void glMaterial{if}[v](GLenum face, GLenum p, TYPE val);


Fundamentals

Sekng material proper<es

GL_AMBIENT ambient part of light (0.2,0.2,0.2,1) GL_DIFFUSE diffuse part of light (0.8,0.8,0.8,1) GL_AMBIENT_AND_DIFFUSE GL_SPECULAR specular part of light (0,0,0,1) GL_SHININESS exponent 0.0 [0.0 – 128.0] GL_EMISSION self-lighting (0,0,0,1)


Fundamentals

Material GL AMBIENT GL DIFFUSE GL SPECULAR GL SHININESS Brass 0.329412 0.780392 0.992157 27.8974

0.223529 0.568627 0.941176 0.027451 0.113725 0.807843 1.0 1.0 1.0

Bronze 0.2125 0.714 0.393548 25.6

0.1275 0.4284 0.271906 0.054 0.18144 0.166721 1.0 1.0 1.0

Polished 0.25 0.4 0.774597 76.8 Bronze 0.148 0.2368 0.458561

0.06475 0.1036 0.200621 1.0 1.0 1.0

Chrome 0.25 0.4 0.774597 76.8

0.25 0.4 0.774597 0.25 0.4 0.774597 1.0 1.0 1.0

Copper 0.19125 0.7038 0.256777 12.8

0.0735 0.27048 0.137622 0.0225 0.0828 0.086014 1.0 1.0 1.0


Fundamentals

Material GL AMBIENT GL DIFFUSE GL SPECULAR GL SHININESS Polished 0.2295 0.5508 0.580594 51.2 Copper 0.08825 0.2118 0.223257

0.0275 0.066 0.0695701 1.0 1.0 1.0

Gold 0.24725 0.75164 0.628281 51.2

0.1995 0.60648 0.555802 0.0745 0.22648 0.366065 1.0 1.0 1.0

Polished 0.24725 0.34615 0.797357 83.2 Gold 0.2245 0.3143 0.723991

0.0645 0.0903 0.208006 1.0 1.0 1.0

Pewter 0.105882 0.427451 0.333333 9.84615

0.058824 0.470588 0.333333 0.113725 0.541176 0.521569 1.0 1.0 1.0


Fundamentals

5. Buffers


Fundamentals

Buffers

• Framebuffer can contain:

– Color buffer – color of fragments (pixels) – Z-‐buffer – depth memory – Stencil buffer – stencil memory – Accumula=on buffer – mo<on blur, an<aliasing


Fundamentals

Color buffer

• Memory for storing color of individual fragments (pixels) • Stereoskopic displaying:

– min. two color buffers • Double-‐buffering:

– min. two color buffers • Each OpenGL implemen<on has to contain at least one color buffer • You can u<lize other buffers -‐ auxiliary color buffers. OpenGL does not specify their func<on, it can be set arbitrarily (e.g. storing the picture which will be used repeatedly. When drawing, the picture is copied to the color buffer.) • Default color buffer need not to be in<alized.


Fundamentals

Z-‐buffer

• Storing informa<on about the distance between the fragment and the projec<on plane. • Using for hidden parts removal • Depth test has to be enabled, it is called only once: glEnable(GL_DEPTH_TEST) • Each <me when used, the Z-‐buffer has to be cleared: glClear(GL_DEPTH_BUFFER_BIT)

http://bio.gsnu.ac.kr/~youknow/graphic/3DMAX_2.htm

https://lva.cg.tuwien.ac.at/ecg/wiki/doku.php?id=students:task4


Fundamentals

Stencil buffer

• Memory for stencil • Determines which fragments will be drawn


Fundamentals

Accumula<on buffer

• Merging more scenes or more views to one scene • Enables an<aliasing, mo<on blur, blur of close or distant objects

www.root.cz www.sgi.com


Fundamentals

Clearing of buffers

• Step 1: specifying values which should be used aler clearing void glClearColor(GLclampf r,GLclampf g, GLclampf b,GLclampf alpha) void glClearDepth(GLclampd depth) void glClearStencil(GLint s) void glClearAccum(GLfloat r, GLfloat g, GLfloat b,GLfloat alpha)


Fundamentals

Clearing of buffers

Step 2: clearing of buffers where mask is logical OR consis<ng of: color GL_COLOR_BUFFER_BIT depth GL_DEPTH_BUFFER_BIT stencil GL_STENCIL_BUFFER_BIT accumula<on GL_ACCUM_BUFFER_BIT

void glClear(GLbitfield mask)


Fundamentals

Example

glClearColor(0.0,0.0,0.0,0.0); glClearDepth(1.0); glClear(GL_COLOR_BUFFER_BIT| GL_DEPTH_BUFFER_BIT); This sequence of commands sets the color of window to black and the value in the Z-‐buffer to 1


Fundamentals

Exercise: Goals 3 and 4


Fundamentals

Questions?

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