Porting Matrix and Transformation Functions

IRIS GL and OpenGL handle matrices and transformations in a similar manner. But there are several differences to keep in mind when porting code from IRIS GL:

• In OpenGL you are always in double-matrix mode; there is no single-matrix mode.

• Angles are measured in degrees, instead of tenths of degrees.

• Projection matrix calls, like glFrustum and glOrtho, now multiply onto the current matrix, instead of being loaded onto the current matrix.

• The OpenGL function, glRotate, is very different from rotate. You can rotate around any arbitrary axis, instead of being confined to the x-, y-, and z- axes. For example, you can translate:

  rotate(200*(i+1), 'z');
  

  to:

  glRotate(.1*(200*(i+1), 0.0, 0.0, 1.0);
  

  When translating from rotate to glRotate you switch to degrees from tenths of degrees and replace 'z' with a vector for the z-axis.

• OpenGL has no equivalent to the polarview function. You can replace it easily with a translation and three rotations. For example, you can translate:

  polarview(distance, azimuth, incidence, twist);
  
  

  to:

  glTranslatef( 0.0, 0.0, -distance); 
  glRotatef( -twist * 10.0, 0.0, 0.0, 1.0); 
  glRotatef( -incidence * 10.0, 1.0, 0.0, 0.0); 
  glRotatef( -azimuth * 10.0, 0.0, 0.0, 1.0);
  

The following table lists the OpenGL matrix functions and their equivalent IRIS GL functions.

IRIS GL function	OpenGL function	Meaning
mmode	glMatrixMode	Sets current matrix mode.
	glLoadIdentity	Replaces current matrix with the identity matrix.
loadmatrix	glLoadMatrixf,glLoadMatrixd	Replaces current matrix with the specified matrix.
multmatrix	glMultMatrixf,glMultMatrixd	Post-multiplies current matrix with the specified matrix (note that multmatrix pre-multiplied).
mapw, mapw2	gluUnProject	Projects world-space coordinates to object space (see also gluProject).
ortho	glOrtho	Multiplies current matrix by an orthographic projection matrix.
ortho2	gluOrtho2D	Defines a two-dimensional orthographic projection matrix.
perspective	gluPerspective	Defines a perspective projection matrix.
picksize	gluPickMatrix	Defines a picking region.
popmatrix	glPopMatrix	Pops current matrix stack, replacing the current matrix with the one below it.
pushmatrix	glPushMatrix	Pushes current matrix stack down by one, duplicating the current matrix.
rotate,rot	glRotated,glRotatef	Rotates current coordinate system by the given angle about the vector from the origin through the given point. Note that rotate rotated only about the x-, y-, and z-axes.
scale	glScaled,glScalef	Multiplies current matrix by a scaling matrix.
translate	glTranslatef,glTranslated	Moves coordinate-system origin to the point specified, by multiplying the current matrix by a translation matrix.
window	glFrustum	Given coordinates for clipping planes, multiplies the current matrix by a perspective matrix.

OpenGL has three matrix modes, which are set with glMatrixMode. The following table lists the modes available as parameters for glMatrixMode.

IRIS GL matrix mode	OpenGL mode	Meaning	Min stack depth
MTEXTURE	GL_TEXTURE	Operates on the texture matrix stack.	2
MVIEWING	GL_MODELVIEW	Operates on the model view matrix stack.	32
MPROJECTION	GL_PROJECTION	Operates on the projection matrix stack.	2