Tutorial 3

Rotation constants and p-fields

This tutorial will show you how to use the glRotation instruction, and show you what score p-fields are and how they are used.

The program starts just the same as the previous tutorials, until instrument 1. Remember to type only the lines in italics.

<CsoundSynthesizer>

<CsOptions>

-+Y

</CsOptions>

<CsInstruments>

#include "OpenGL.h"

sr=100

kr =100

ksmps=1

nchnls=1

GLfps 30

GLpanel "OpenGL panel", 512, 512

GLpanel_end

FLrun

glMatrixMode $GL_PROJECTION

glLoadIdentity

gluPerspective 60,0.1,100

glMatrixMode $GL_MODELVIEW

GLinsert_i $GL_NOT_VALID

glClear $GL_COLOR_BUFFER_BIT + $GL_DEPTH_BUFFER_BIT

GLinsert_i 1.1

instr 1

islant = p4

Upto here you should recognize the code. Now we see a csound variable being defined. Csound works with several types of variables that have different uses and performance. The simplest ones are called i-rate (or instrument-rate) variables. They can be though of as constants within an instrument. An i-rate variable starts always with the character 'i'. Normally the characters following this 'i' give some short description about what the variable will be used for. In this case, we will be using the variable islant. Notice that we assign the value p4 to the variable. This means assign p-field 4 to the variable. P-fields are numbers that are entered in the score section when calling the instrument. See below in the score that apart from the 3 values we've used when calling instruments (instrument number, start time and end time) there are 2 more values or columns. These are p-fields number 4 and 5. All p-fields after 3 have no definite or fixed use. Their use is defined by the composer for every instrument. P-fields are useful because they permit instantiation of an instrument with variations intead of having to write a whole new instrument. P-fields must be separated by at least one space (can be more or a tab).

irotation = p5

The above applies here as well. The fifth value accompanying the instrument call is written to variable irotation. Notice the values usedin the score: for irotation we have multiples of 45, and for islant we have either 0 or 45. Also notice that islant and irotation are local variables, i.e. they are different and independent for every instrument call. Each time the instrument is called it has its own independent islant and irotation.

glLoadIdentity

Remember that glLoadIdentity resets all rotations and transformations.

glTranslate 0,0,-5

First we translate our axis 5 units into the screen so we can see the objects drawn. If we didn't do this, the objects would be drawn at the same place the camera is, and wouldn't be seen.

glRotate irotation,0,0,1

Now we will use the instruction glRotate. The values that must be entered are: first the angle and then a scaling factor the rotaion around each axis. In this first case we will rotate irotation degrees around the z axis. Rotation around the z axis is rotating like the hands of a clock or a propeller. Notice that each time instrument 1 is instantiated, it is rotated 45 degrees more than the previous instrument.

glRotate islant,0,1,0

Now we will use the variable islant to make a rotation around the y axis. Notice that this rotation occurs after the previous rotation and applies to the already modified axis. In other words, we are not rotation around the original y axis, but upon a modified relative y axis. This might take a while to understand specially if you're not good at visualizing 3D geometry, but hopefully with a little practice it'll make sense. Rotation upon the y axis is like the rotation of an LP record. Just imagine that the turntable has first rotated like a propeller and then plays the record.... makes sense?

glBegin $GL_TRIANGLES

Now that we have our axis positioned, we will draw a triangle. This is done just like drawing QUADS like in the previous tutorial, but require only 3 vertices. Notice the way glColor is used to make the centre white and the edges blue.

glColor 1,1,1,1

glVertex3 0,0,0

glColor 0,0,1,1

glVertex3 1,2,0

glVertex3 -1,2,0

glEnd

GLinsert 1.5

Never forget to put GLinsert...

endin

This is the only instrument... It will be called 8 times down in the score with different rotation values and start times.

</CsInstruments>

<CsScore>

i 1 1 12 45 45

i 1 2 12 0 90

i 1 3 12 45 135

i 1 4 12 0 180

i 1 5 12 45 225

i 1 6 12 0 270

i 1 7 12 45 315

i 1 8 12 0 0

</CsScore>

</CsoundSynthesizer>

</CsoundSynthesizer>

That's tutorial 3. Rotation and the use of p-fields will be become clear with use, so don't worry if you can't quite grasp it at first.

The other values that can be used with glBegin are:

For a complete definition of each, refer to the openGL programming guide at:

http://www.parallab.uib.no/SGI_bookshelves/SGI_Developer/books/OpenGL_PG/sgi_html/index.html