1. CS 325 Introduction to Computer Graphics 04 / 12 / 2010 Instructor: Michael Eckmann

2. Michael Eckmann - Skidmore College - CS 325 - Spring 2010 Today’s Topics Questions? Comments about programming style, efficiency, etc. PPM file format Adding surface detail –Texture mapping –Environment mapping –Bump mapping

3. Simple Image format PPM PPM is an image file format that is easy to write to from a program and easy to read into a program. It comes in two forms –Raw and plain The first line of this image file is a “magic number” consisting of two characters –P3 for plain –P6 for raw The second line contains two integers separated by whitespace representing the width and height (in that order) in pixels of this image. The next line contains one number which represents the maximum value for a color (e.g. 255 if you wish to describe each RGB value with a number between 0 and 255 (1 byte).)‏ Although I describe that these things should all be on separate lines, all that is really required is whitespace between the parts that I say should be on separate lines.

4. Simple Image format PPM Next, the file contains height number of lines, each line of which contains width number of R, G, B values which are –3 ASCII decimal values between 0 and the maxvalue, each separated by whitespace for the plain format file –3 binary values (1 byte each if maxvalue < 256, 2 bytes each otherwise) If you wish to write raw format files using one byte per color value, then use type byte in Java. If you wish to write plain format, write out the color values to the file as plain text (e.g. A Red color value of 210 would appear in the file as 3 characters 2 1 and 0.)‏ Let's look at example PPM files. They can be displayed in most image viewers and can be converted to other formats.

5. Suggestions for ray tracer prog. You will be required to save your ray traced images in a file (ppm is suggested since it is easy to write to)‏ You will also be required to show your ray traced image on screen in openGL. Since you are computing the color of one pixel at a time, you can simply draw the pixel as a polygon with four vertices.

6. Adding Surface Detail Until now, we've only discussed surfaces to have a color (and some properties such as how diffuse and specular it is.)‏ That limits drastically the kinds of details we can reproduce that are found on real objects in the real world (at least without going through tons of work). Texture mapping is a technique that can provide detail of a surface without a big hit in performance. For instance, a brick wall can be modelled as one big rectangle, but instead of saying it has one color, we specify its “color” as some pre-stored image of a brick wall. The image of the brick wall is the texture map.

7. Texture mapping Texture mapping can be done in n-dimensions (usually 1, 2 or 3).(the example of a brick wall image is 2d.)‏ An n-dimensional texture is usually described by n coordinates ranging from 0 to 1.0. Each value at a particular set of coordinates is a color. For a linear pattern (1-dimensional texture) we could store a 1d array of colors. Let's say our texture is a list of 6 colors. Our coordinates range from s=0 to 1.0, but since there's only 6 colors, the 0 th color would be s=0, the next would be s=0.2, next s=0.4, and so on until last would be s=1.0. To map this linear pattern into our scene somewhere, we would specify an s value for some point in our scene and another s value for a different point in our scene and linearly interpolate between them to get a multicolored line between them.

8. Texture mapping The length of the line in the scene, determines how many pixels get each color. Example on the board. For 2-dimensional texture we would define a texture with the coordinates s,t each in the range 0 to 1.0. 4 points on the surface of an object are specified as the points where s,t coordinates are ( 0, 0), ( 0, 1.0), ( 1.0, 0), ( 1.0, 1.0)‏ The area in between these four corners is linearly interpolated to map the texture to the object. (like Gouraud shading.)‏

9. Texture mapping We have the option of –starting in texture-space and mapping that to a surface in object-space and then project the textured object into the image space. –or –start in image-space and map to the surface, then determine the texture that that piece of the surface maps to in the texture space. –We typically use the second method because starting with mapping from texture space we'll often end up with a piece of texture not mapping to exactly all one pixel (either it'll be part of a pixel, or a more than one pixel.)‏ –It's generally easier to start mapping from pixels to the surface then the texture --- due to fewer computations and the added possibility of adding in antialiasing.

10. Texture mapping So if we use this method: –start in image-space and map to the surface, then determine the texture that that piece of the surface maps to in the texture space. There are a few issues –we have a finite number of colors in our texture (these are called texels)‏ for example, the 6 colors stored in our linear array example were 6 texels. a 2d checkerboard might contain 64 texels. –a pixel is most certainly not always going to map exactly to the center of a texel, so we have the option of just taking the nearest texel color or averaging the texels (e.g. for a 2d texture, average the 4 nearest texels into one) –The averaging will reduce aliasing

12. Texture mapping From the diagram on the last slide that rotated square (texture mapped pixel area) in the texture space will certainly fall either on –more than one texel or –part of one texel or –parts of more than one texel A solution is to sum the color values of all the texels that that texture mapped pixel area covers (to the appropriate proportions of texels that it overlaps.)‏ If the texture mapped pixel area falls (fully/partly) outside the texture array then either treat the outside area of the texture as –some background color or –more usually as being the texture area repeated (example on the board)‏