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Chapter 6Blending, Antialiasing, Fog, and PolygonOffset

Guihua YangJan 14, 2004

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Blending: concepts and the example “alpha.c” Antialiasing: concepts and the example “aargb.c” Fog: concepts and the example “fog.c” Polygon: concepts and the example “polyoff.c”

What we are going to learn in this lecture

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Blending

Blending: combines color values from a source and a destination. The final effect is that parts of your scene appear translucent.

Source: color values of the incoming fragmentDestination: color values of the corresponding

currently stored pixel

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Alpha(opacity): alpha--, transparent or translucent surfacesalpha++, opaque surfacesglColor*(R,G,B,A)

Blending-cont

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The Source and Destination Factors:blended RGBA values are given by

(RsSr+RdDr, GsSg+GdDg, BsSb+BdDb, AsSa+AdDa)

Each component of this quadruplet is eventually clamped to [0,1].

Blending-cont

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Blending Example: alpha.c(a)

………………..static int leftFirst = GL_TRUE;

/* Initialize alpha blending function. */

static void init(void)

{glEnable (GL_BLEND); //to have blending take effect; use glDisable() to disable blending.

glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

// void glBlendFunc(GLenum sfactor, GLenum dfactor);

glShadeModel (GL_FLAT); //select flat or smooth shading

glClearColor (0.0, 0.0, 0.0, 0.0); // specify clear values for the color buffers: black

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alpha.c(b)

static void drawLeftTriangle(void)

{

/* draw yellow triangle on LHS of screen */

glBegin (GL_TRIANGLES);

glColor4f(1.0, 1.0, 0.0, 0.75); //to set the color to yellow, alpha to 0.75

glVertex3f(0.1, 0.9, 0.0); //3 points of the triangle …..

glVertex3f(0.1, 0.1, 0.0);

glVertex3f(0.7, 0.5, 0.0);

glEnd();

}

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alpha.c(c)

static void drawRightTriangle(void)

{

/* draw cyan triangle on RHS of screen */

glBegin (GL_TRIANGLES);

glColor4f(0.0, 1.0, 1.0, 0.75); //to set the color to cyan, alpha to 0.75

glVertex3f(0.9, 0.9, 0.0); //3 points of the triangle …..

glVertex3f(0.3, 0.5, 0.0);

glVertex3f(0.9, 0.1, 0.0);

glEnd();

}

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alpha.c(d)void keyboard(unsigned char key, int x, int y){ switch (key) {case `t':case `T':leftFirst = !leftFirst;glutPostRedisplay(); //marks the current or specified window as

needing to be redisplayed.break;case 27: /* Escape key */ // 27 is ascii code for escape key.exit(0);break;default:break; } }

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alpha.c(e)void display(void){ glClear(GL_COLOR_BUFFER_BIT); // clear buffers within the

viewport . GL_COLOR_BUFFER_BIT : Indicates the buffers currently enabled for color writing.

if (leftFirst) {drawLeftTriangle();drawRightTriangle();}else {drawRightTriangle();drawLeftTriangle();}glFlush(); //force execution of GL commands in finite time}

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alpha.c(f)void reshape(int w, int h){ glViewport(0, 0, (GLsizei) w, (GLsizei) h); //set the viewport, lower

left corner(0,0), viewport’s width and height.glMatrixMode(GL_PROJECTION); //specify which matrix is the current

matrix. GL_PROJECTION: Applies subsequent matrix operations to the projection matrix stack.

glLoadIdentity(); // replace the current matrix with the identity matrix if (w <= h)gluOrtho2D (0.0, 1.0, 0.0, 1.0*(GLfloat)h/(GLfloat)w);elsegluOrtho2D (0.0, 1.0*(GLfloat)w/(GLfloat)h, 0.0, 1.0);}

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alpha.c(g)int main(int argc, char** argv){ // 5 routines perform tasks necessary to initialize a window.glutInit(&argc, argv);glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);glutInitWindowSize (200, 200); // plus glutInitWindwoPosition(…).glutCreateWindow (argv[0]);// until glutMainLoop() is called, the window is not yet displayed.init();glutReshapeFunc (reshape);glutKeyboardFunc (keyboard);glutDisplayFunc (display);glutMainLoop();return 0;}

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Antialiasing aliasing causes images to have a jagged staircase look to

their edges while anti-aliasing smoothes out these

jagged edges.

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Antialiasing-cont anti-aliasing is more than just making something slightly

fuzzy so that you can't see the jagged edges: it's a way of fooling the eye into seeing straight lines and smooth curves where there are none.

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Antialiasing in RGBA Mode

2 color modes: RGBA mode, Color index mode Blending is enabled The blending factors are: GL_SRC_ALPHA (source) and

GL_ONE_MINUS_SRC_ALPHA (destination).

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Antialiasing Example: aargb.c(a)static float rotAngle = 0.;

/* Initialize antialiasing for RGBA mode, including alpha blending, hint, and line width. Print out implementation specific info on line width granularity and width.*/

void init(void)

{ GLfloat values[2];

glGetFloatv (GL_LINE_WIDTH_GRANULARITY, values); //returns one value, the width difference between adjacent supported widths for antialiased lines.

printf ("GL_LINE_WIDTH_GRANULARITY value is %3.1f\n", values[0]);

glGetFloatv (GL_LINE_WIDTH_RANGE, values); //returns two values: the smallest and largest supported widths for antialiased lines

printf ("GL_LINE_WIDTH_RANGE values are %3.1f %3.1f\n", values[0], values[1]);

glEnable (GL_LINE_SMOOTH); //indicates that antialiasing of lines is enabled

glEnable (GL_BLEND); // enable blendingglBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

glHint (GL_LINE_SMOOTH_HINT, GL_DONT_CARE);

glLineWidth (1.5); //specify the width of rasterized lines

glClearColor(0.0, 0.0, 0.0, 0.0); }

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aargb.c(b)/* Draw 2 diagonal lines to form an X */void display(void){ glClear(GL_COLOR_BUFFER_BIT);

glColor3f (0.0, 1.0, 0.0); //green colorglPushMatrix(); //pushes the current matrix stack down by one, duplicating the

current matrix. That is, after a glPushMatrix call, the matrix on the top of the stack is identical to the one below it.

glRotatef(-rotAngle, 0.0, 0.0, 0.1); //multiply the current matrix by a rotation matrix glBegin (GL_LINES);

glVertex2f (-0.5, 0.5);glVertex2f (0.5, -0.5);

glEnd ();

glPopMatrix(); //pop the current matrix stack

glColor3f (0.0, 0.0, 1.0); //blue colorglPushMatrix();glRotatef(rotAngle, 0.0, 0.0, 0.1);glBegin (GL_LINES);

glVertex2f (0.5, 0.5);glVertex2f (-0.5, -0.5);

glEnd ();glPopMatrix();glFlush(); }

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aargb.c(c)void keyboard(unsigned char key, int x, int y)

{switch (key) {

case `r':

case `R':

rotAngle += 20.;

if (rotAngle >= 360.) rotAngle = 0.;

glutPostRedisplay();

break;

case 27: /* Escape Key */

exit(0);

break;

default:

break;

}

}

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Fog Fog: a general term that describes similar forms of

atmospheric effects; it can be used to simulate haze, mist, smoke, or pollution.

Make an entire image appear more natural by adding fog, which makes objects fade into the distance.

When fog is enabled, objects that are farther from the viewpoint begin to fade into the fog color.

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Fog Example

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Fog equations

Fog blends a fog color with an incoming fragment's color using a fog blending factor. This factor, f, is computed with one of these three equations and then clamped to the range [0,1].

The equation for GL_LINEAR fog is:

The equation for GL_EXP fog is:

The equation for GL_EXP2 fog is:

Z: the distance in eye coordinates from the origin to the fragment being fogged

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Fog example: fog.c(a)static GLint fogMode;

static void init(void)

{ GLfloat position[] = { 0.5, 0.5, 3.0, 0.0 };glEnable(GL_DEPTH_TEST); //do depth comparisons and update the depth buffer.

glLightfv(GL_LIGHT0, GL_POSITION, position);

glEnable(GL_LIGHTING); // use the current lighting parameters to compute the vertex color or index.

glEnable(GL_LIGHT0); // include light i in the evaluation of the lighting equation. It is always the case that GL_LIGHTi = GL_LIGHT0 + i.

{GLfloat mat[3] = {0.1745, 0.01175, 0.01175};

glMaterialfv (GL_FRONT, GL_AMBIENT, mat); //specify material parameters for the lighting model

mat[0] = 0.61424; mat[1] = 0.04136; mat[2] = 0.04136;

glMaterialfv (GL_FRONT, GL_DIFFUSE, mat);

mat[0] = 0.727811; mat[1] = 0.626959; mat[2] = 0.626959;

glMaterialfv (GL_FRONT, GL_SPECULAR, mat);

glMaterialf (GL_FRONT, GL_SHININESS, 0.6*128.0);

}

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fog.c(b)glEnable(GL_FOG);

{

GLfloat fogColor[4] = {0.5, 0.5, 0.5, 1.0};

fogMode = GL_EXP;

glFogi (GL_FOG_MODE, fogMode); //specify fog parameters. The equation for GL_EXP fog is f is blending factor

glFogfv (GL_FOG_COLOR, fogColor);

glFogf (GL_FOG_DENSITY, 0.35); //the fog density used in both exponential fog equations. Only nonnegative densities are accepted. The default fog density is 1.0.

glHint (GL_FOG_HINT, GL_DONT_CARE);

glFogf (GL_FOG_START, 1.0); //the near distance used in the linear fog equation.

glFogf (GL_FOG_END, 5.0); //the far distance used in the linear fog equation.

}

glClearColor(0.5, 0.5, 0.5, 1.0); /* fog color */

}

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fog.c( c)void keyboard(unsigned char key, int x, int y){ switch (key) {

case `f':case `F':if (fogMode == GL_EXP) {fogMode = GL_EXP2;printf ("Fog mode is GL_EXP2\n"); }else if (fogMode == GL_EXP2) {fogMode = GL_LINEAR;printf ("Fog mode is GL_LINEAR\n"); }else if (fogMode == GL_LINEAR) {fogMode = GL_EXP;printf ("Fog mode is GL_EXP\n"); }glFogi (GL_FOG_MODE, fogMode);glutPostRedisplay();break;…………………}

}

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Polygon Offset Raster(from merriam-webster dictionary):a scan pattern (as

of the electron beam in a cathode-ray tube) in which an area is scanned from side to side in lines from top to bottom; also : a pattern of closely spaced rows of dots that form the image on a cathode-ray tube (as of a television or computer display)

Rasterize: The process of converting a vector image into a bitmap image. This is the rasterize dialog box you will see when importing a vector-based image into Photoshop.

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Rasterization: Rasterization is the process by which a primitive is converted to a two-dimensional image. Each point of this image contains such information as color and depth.

http://www.opengl.org/documentation/specs/version1.1/glspec1.1/node41.html

Polygon Offset-cont.

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Polygon Offset-cont.

GL_LINE: Boundary edges of the polygon are drawn as line segments.

GL_FILL: The interior of the polygon is filled.

Polygon offset: adds an appropriate offset to force coincident z values apart to cleanly separate a polygon edge from its highlighting line.

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Polygon offset example:polyoff.cPolygon Offset to Eliminate Visual Artifacts: polyoff.c

glEnable(GL_LIGHTING);

glEnable(GL_LIGHT0);

glEnable(GL_POLYGON_OFFSET_FILL);

glPolygonOffset(1.0, 1.0);

glCallList (list);

glDisable(GL_POLYGON_OFFSET_FILL);

glDisable(GL_LIGHTING);

glDisable(GL_LIGHT0);

glColor3f (1.0, 1.0, 1.0);

glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

glCallList (list);

glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

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References Red book, OpenGL Programming Guide (Addison-Wesley

Publishing Company) Blue book, The OpenGL "Bluebook" HTML format

http://www.parallab.uib.no/SGI_bookshelves/SGI_Developer/books/OpenGL_RM/sgi_html/bk02.html

http://www.sgi.com/software/opengl/glandx/intro/subsection3_2_2.html

http://users.easystreet.com/jkirwan/dda.html http://www.opengl.org/documentation/specs/version1.1/glspec1.1/

node41.html