1 Lecture 6 Attributes of graphical primitives Colors Colors in OpenGL Algorithms for scan-conversion

2 Colors  It is difficult to explain colors: they should be experienced.  Color is nothing more that wavelengths of light at a certain frequency.  Actually, the human eye is only said to perceive a small fraction of the total light available.  Certain lights called Gamma Rays, X-rays, and Ultraviolet lights are too low of a frequency for humans to see.  Lights like Infrared, Microwaves, and Radio waves, are too high of a frequency for the human eye to see.  Humans only see a small portion of light between these frequencies and we calls these lights color.

3 Colors  The color of an object is evident by the amount of light it reflects.  For example, a red apple reflects mostly red light, and absorbs mostly all other colors.  The color white is obtained when all colors are reflected off an object (no colors absorbed) giving the object appearance of white.  Black is the absorption of all colors. It is not considered as a color.  Black is considered the absence of color, and therefore is not allowed to join its color family.

4 Colors: RGB Model  There are different ways to choose color for your drawing.  The easiest and most powerful way is to use RGB color.  RGB color stands for Red, Green, and Blue, and is called additive color.  It is given this name because RGB starts with the absence of color, and then adds elements of red, green, and blue to achieve the desired color.

5 Colors: RGB Model  It is not possible to obtain all colors with RGB model, or with any other color model. RGB just matches a color as close as possible.  Usually there are 256 different values for each red, green, and blue. This also means that there are about 16.7 million different colors available (256*256*256 = 2 8 * 2 8 * 2 8 ).  Some systems and languages, such as OpenGL, uses a similar idea with RGB color, except that they may use numbers from 0.0 to 1.0.  Theoretically this means that there is an infinite amount of colors formed from RGB because there is an infinite amount of numbers between 0 and 1.

6 Colors RGB model

7 Colors RGB Color cube

8 Colors CMY model

9 Colors CMY cube

10 Colors HSV model (hue, saturation, value): used by artists Called also HSL model (L-light) Hue: the name that we give to a color: red, yellow, cyan, etc. Saturation: “how pure the color is”, that is how far is a hue from the same hue mixed with white so that the color has more pastel shade Value/Light - how bright the color appear

11 Colors HSV model (hue, saturation, value)

12 Levels of Gray Levels of gray/colors: instead of 0/1 more digits

13 Levels of Gray Reducing the levels of gray: Original scanned picture

14 Levels of Gray Reducing the levels of gray: Three bits per pixel

15 Levels of Gray Reducing the levels of gray: One bit per pixel

16 Color palette Color table (palette)

17 Colors in OpenGL  OpenGL uses RGB color model  The values of red, green, and blue are numbers from 0.0 to 1.0.

18 Colors in OpenGL  The instruction glColor3f( float red, float green, float blue ) sets a color.  The syntax is the same as in the sample program for drawing a triangle.  Problem: Draw 5 triangles with different colors. Write the values of RGB and the color obtained.

19 Colors in OpenGL  Syntax of OpenGL instructions: FunctionName2i or FunctionName3f  2 or 3 means number of parameters: 2 or 3  i - integer values, f- float values, d-double values.  All the following instructions will give the same color - red: glColor1f( 1.0 ); glColor1d( 1.0 ); glColor4i( 1, 0, 0, 0 );

20 Colors in OpenGL  There is an optional fourth value in the color definition called the alpha value.  Alpha values are used for displaying different effects, e.g., blending, transparency, lighting and shadows.  OpenGL may interpolate different colors.  Problem: Use the sample triangle program and assign different colors for each vertex. Describe the result.

21 Colors in OpenGL glBegin( GL_TRIANGLES ); // Begin a triangle glColor3f( 1.0, 0.0, 0.0 );// Red color glVertex3f( 0.25, 0.25, 0.0 ); glColor3f( 0.0, 1.0, 0.0 );// Green color glVertex3f( 0.75, 0.25, 0.0 ); glColor3f( 0.0, 0.0, 1.0 );// Blue color glVertex3f( 0.75, 0.75, 0.0 ); glEnd(); // End the triangle

22 Scan-Conversion

23 Triangle Scan-Conversion

24 Simple Algorithm

25 Inside Triangle Test

26 Inside Triangle Test

27 Triangle Sweep-Line Algorithm

28 Triangle Sweep-Line Algorithm

29 Polygon Scan Conversion

30 Inside Polygon Rule

31 Polygon Sweep-Line Algorithm

32 Polygon Sweep-Line Algorithm

33 FloodFill Algorithm

34 Scan Conversion of Polygons: Hardware Conversion