OpenGL Frequently Asked Questions [3/3]
From pho@dilow.asd.sgi.com (Paul Ho)
Organization Silicon Graphics Inc.
Date 14 Jul 1997 00:13:31 GMT
Keywords OpenGL FAQ
Newsgroups comp.graphics.api.opengl
Message-ID <5qbqvb$5eb$4@murrow.corp.sgi.com>
------
Q_3_01: Where can I find OpenGL source code examples? For instance,
where is an example which combines OpenGL with Motif, using
the Motif widget?
A:
You can get the source code examples which are found in the OpenGL
Programming Guide via anonymous, public ftp from
ftp://sgigate.sgi.com/pub/opengl/opengl.tar.Z
Mark Kilgard has created an ftp site for source code, which is part
of his articles in the X Journal magazine. This includes the GLUT
toolkit (version 2.0) and OpenGL with Motif examples. The
directory is ftp://sgigate.sgi.com/pub/opengl/xjournal
Some contributed source code of useful tools for developing OpenGL
code can be found on ftp://sgigate.sgi.com/pub/opengl/contrib
Source code found here includes:
isfast--compares the performance of OpenGL states
samples--more OpenGL program examples
toogl--helps port IRIS GL code to OpenGL
xglinfo--display information on X visuals extended for OpenGL
xscope--examines OpenGL protocol sent to an extended X server
Nate Robins <ndr@pobox.com> has a web page that contains OpenGL
information. In particular, source for the Windows NT/95 version of
GLUT. The site is http://www.cs.utah.edu/~narobins/opengl.html.
------
Q_3_02: How do I contribute OpenGL code examples to a publicly
accessible archive?
A:
To contribute to the public OpenGL archive, send mail to
opengl-contrib@sgi.com. Your mail should contain:
The material to be archived or instructions for obtaining it.
An announcement suitable for posting to comp.graphics.opengl.
SGI will place the material in the opengl/contrib directory on
sgigate.sgi.com and post the announcement to this newsgroup.
To retrieve something from the archive, use anonymous ftp to
sgigate.sgi.com. Once connected, cd to the directory OpenGL.
(Case is significant.) Currently there are two subdirectories:
doc - Manual pages for OpenGL and related libraries.
contrib - Contributions from the public.
Note that all contributions are distributed as-is; neither SGI nor
the other companies on the OpenGL Architecture Review Board make
any legally valid claims about the robustness or usefulness of this
software.
If you do not have access to anonymous ftp, consider using an
"ftp-by-mail" server. For information on one such server, send
mail to ftpmail@decwrl.dec.com with a message body containing only
the word "help".
------
Q_3_03: What is the GLUT toolkit? Where do I get it?
A:
GLUT is a portable toolkit which performs window and event
operations to support OpenGL rendering.
GLUT version 2.0 has:
o window functions, including multiple windows for OpenGL rendering
o callback driven event processing
o sophisticated input devices, including dials and buttons box,
tablet, Spaceball(TM)
o idle routines and timers
o a simple cascading pop-up menu facility
o routines to generate wire and solid objects
o bitmap and stroke fonts
o request and queries for multisample and stereo windows
o OpenGL extension query support
The version 2 functionality is fully backward compatible with the
version 1 functionality.
The specification, source code (including FORTRAN bindings), and
articles for GLUT (Graphics Library Utility Toolkit) is in: URL
ftp://sgigate.sgi.com/pub/opengl/xjournal/GLUT
This distribution of GLUT should compile on:
o DEC Alpha workstation running OSF/1 with Open3D layered product
o IBM RS/6000 workstations running AIX with OpenGL support
o SGI workstation running IRIX 5.2 or higher supporting OpenGL
o Template Graphics Software's OpenGL for Sun workstations
o Mesa 1.1 for Unix workstations.
------
Q_3_04: What is the relationship between IRIS GL and OpenGL?
Is OpenGL source code or binary code compatible with IRIS GL?
A:
IRIS GL is the predecessor to OpenGL. After other implementors had
experience trying to port the IRIS GL to their own machines, it was
learned that the IRIS GL was too tied to a specific window system
or hardware. Based upon consultations with several implementors,
OpenGL is much more platform independent.
IRIS GL is being maintained and bugs will be fixed, but SGI will no
longer add enhancements. OpenGL is now the strategic interface for
3-D computer graphics.
OpenGL code is neither binary nor source code compatible with IRIS
GL code. It was decided to bite the bullet at this time to make
OpenGL incompatible with IRIS GL and fix EVERYTHING that made IRIS
GL difficult to port or use. For example, the gl prefix has been
added to every command: glVertex(), glColor(), etc.
------
Q_3_05: Why should I port my IRIS GL application to OpenGL?
A:
SGI will be maintaining the old IRIS GL, but not enhancing it.
OpenGL is the API of choice on all new SGI machines.
OpenGL has no subsets. You can use the same functionality on all
machines from SGI or from other vendors.
OpenGL is better integrated with the X Window System than the old
IRIS GL. For example, you can mix OpenGL and X or Display
PostScript drawing operations in the same window.
The OpenGL naming scheme, argument list conventions, and rendering
semantics are cleaner than those of IRIS GL. This should make
OpenGL code easier to understand and maintain.
------
Q_3_06: How much work is it to convert an IRIS GL program to OpenGL?
What are the major differences between them?
A: from Mason Woo (woo@sgi.com) and Debbie Herrington (debbie@portable.com)
There is a fair amount of work, most of which is in substituting
for window management or input handling routines, for which the
equivalents are not OpenGL, but the local window system, such as
the X Windows System or Windows NT.
To help ease the way, port to "mixed model" right away, mixing the
X Window System calls to open and manage windows, cursors, and
color maps and read events of the window system, mouse and
keyboard. You can do that now with IRIS GL, if you are running
IRIX 4.0.
In the X Window System, display mode choices (such as single or
double buffering, color index or RGBA mode) must be declared before
the window is initially opened. You may also substitute for other
IRIS GL routines, such as using a OSF/Motif menu system, in place
of the IRIS GL pop-up menus. You should use glXUseXFont(),
whenever you were using the font manager with IRIS GL.
OpenGL uses standard, predictable naming conventions, which
required that all names have been changed from IRIS GL. The OpenGL
naming scheme uses unique prefixes, suffixes, and capitalization to
help prevent potential conflicts among application, system, and
library routine names.
And all routine names have changed, at least, minimally; for
example: ortho() is now glOrtho().
Tables for states such as lighting or line and polygon stipples
will be gone. Instead of using a def/set or def/bind sequence to
load a table, you turn on the state with glEnable() and also
declare the current values for that state.
Colors are best stored as floating point values, scaled from 0.0 to
1.0 (0% to 100%). Alpha is fully integrated in the RGBA mode and
at least source alpha will be available on all OpenGL
implementations. OpenGL will not arbitrarily limit the number of
bits per color to 8. Clearing the contents of buffers no longer
uses the current color, but a special "clearing" color for each
buffer (color, depth, stencil, and accumulation).
The transformation matrix has changed. In OpenGL, there is no
single matrix mode. Matrices are now column-major and are
post-multiplied, although that does NOT change the calling order of
these routines from IRIS GL to OpenGL. OpenGL's glRotate*() now
allows for a rotation around an arbitrary axis, not just the x, y,
and z axes. lookat() of IRIS GL is now gluLookAt(), which takes an
up vector value, not merely a twist. There is no polarview() in
OpenGL, but a series of glRotate*()s and glTranslate*()s can do the
same thing.
There are no separate depth cueing routines in OpenGL. Use linear fog.
Feedback and selection (picking) return values, which are different
from those found on any IRIS GL implementation. For selection and
picking, depth values will be returned for each hit. In OpenGL,
feedback and selection will now be standardized on all hardware
platforms.
------
Q_3_07: When using Xlib, how do I create a borderless window?
A: from blythe@sgi.com (David Blythe)
Essentially you can create the window with override-redirect (see
man xcreatewindow) which is the sledgehammer approach or you can
change the _MOTIF_WM_HINTS property to tell the window manager to
leave your windows undecorated.
from alex@eagle.hd.hac.com (Alex Madarasz)
Also of note is that the window manager decorations of any client
can be turned off by putting something like the following in the
.Xdefaults file in your home directory - assuming you aren't
overriding them in your app:
4Dwm*ClientAppOrClassName*clientDecoration: none
( see the 4Dwm / mwm man pages for a full description of this resource )
( you must restart the window manager or logout/login for 4Dwm
resources to take effect )
"none" will remove all of the window manager decorations - border,
title bar etc.
------
Q_3_08: How do I switch between single buffer and double buffer mode?
A: from mjk@sgi.com (Mark Kilgard) and blythe@sgi.com (David Blythe)
When using OpenGL with X, switching between a double buffered and
single buffered window can be accomplished by creating a
"container" X window and creating two subwindows, one with a double
buffer visual, the other with a single buffer visual. Make sure
the subwindows are each the full size of their parent window.
You can then use XRaiseWindow or XLowerWindow to change the
stacking order of the two subwindows to switch between double
buffering and single buffering.
You will need to create a separate context for each of the two
windows since they have different visual types. You will need to
make the appropriate window/context pair current when you switch
modes.
IRIS GL made it easy to switch between double buffering and single
buffering. But essentially, IRIS GL implemented the above process
internally.
------
Q_3_09: On my machine, it appears that glXChooseVisual is only able
to match double-buffered visuals. I want to have more bits
of color resolution, so how do I render in single buffer mode?
A: from mjk@sgi.com (Mark Kilgard)
On mid- to low-end machines with double buffer hardware, you'll
probably find you get twice as much color resolution using a single
buffer visual. But if there is no hardware double buffering
support, the double buffered and single buffered visuals are
generally the same depth (the back buffer is "carved" out of
software).
Search again for a double buffered visual. If you find one, use it
instead. Call glDrawBuffer(GL_FRONT) though to make sure you are
drawing to the front buffer (the default for a double buffered
visual is to draw into the back buffer).
------
Q_3_10: I've got a 24-bit machine, but my OpenGL windows are not using
the full color resolution. What's wrong? My program looks
fine on one machine, but the depth buffer doesn't work on
another. What's wrong?
A: from mjk@sgi.com (Mark Kilgard) and woo@sgi.com (Mason Woo)
An unfortunate (but documented) semantic of glXChooseVisual is that
if you don't request GLX_RED_SIZE, GLX_GREEN_SIZE, or
GLX_BLUE_SIZE, glXChooseVisual assumes zero for these parameters
which means pick the visual with the _smallest_ amount of red,
green, and blue that matches the other visual attributes. Make
sure you ask for at least 1 bit of GLX_RED_SIZE, GLX_GREEN_SIZE,
and GLX_BLUE_SIZE. If these configuration parameters are non-zero,
it matches the visual with the _largest_ amount of red, green, and
blue with at least 1 bit of each (probably what you want).
Similarly, if you don't request GLX_DEPTH_SIZE, you may get a
visual with zero bits of depth buffer. Some systems may have few
visuals available, and those visuals all have at least 1 bit of
depth buffer. On other systems, there may be dozens of visuals
available, some with zero bits for the depth buffer. In short, if
hidden surface removal appears to fail, check to see if you have
explicitly specified any bits of depth buffer you have requested.
Also check to see what visual you have received.
------
Q_3_11: What information is available about OpenGL extensions?
A:
Examples of extensions include vertex arrays (calling several
vertexes or related data, such as normals or colors, with a single
function call), blending of constant colors, polygon offset
(multiple coplanar polygons can be rendered without interaction),
Procedure names and tokens for OpenGL extensions are either
suffixed with EXT or a vendor-specific acronym: such as SGI for
Silicon Graphics machines, or INGR for Intergraph. Also note that
Silicon Graphics extensions to OpenGL are suffixed to indicate
whether they will be available on all machines (SGI), on just a
subset of machines (SGIS), or are very experimental and may become
unavailable or completely changed (SGIX).
Vendors are encouraged to add extension information to their
documentation. For Silicon Graphics, extension information is
summarized on the glIntro man page.
------
Q_3_12: How do I make shadows in OpenGL?
A:
There are no individual routines to control shadows nor an OpenGL
state for shadows. However, code can be written to render
shadows.
from woo@sgi.com (Mason Woo)
To project a shadow onto a flat plane (such as in the insect
demo), draw the stippled object, flattened using matrix
transformations. The easiest way to flatten an object is to
use the scale function. For example, use glScalef(1., 0., 1.)
to create from an infinite light shining straight down the y
axis. A transformation matrix can be used to cast a shadow
from an infinite or local light source in an arbitrary
direction. See the article:
Thant Tessman, "Casting Shadows on Flat Surfaces,"
IRIS Universe, Winter, 1989.
from shreiner@sgi.com (Dave Shreiner)
Check out the fast shadow and projective texture multi-pass
algorithms for producing realistic shadows using texture
mapping. See the SIGGRAPH paper:
Mark Segal, Carl Korobkin, et al. "Fast Shadows and
Lighting Effects using Texture Mapping" 1992 SIGGRAPH
Proceedings
------
Q_3_13: How can I use 16 bit X fonts?
A: from James A. ("Jim") Miller
Here is some code that will load any font into the server and use
glXUseXFont to build the display lists for you (this does work with
16 bit fonts, it has been tested on IBM, DEC and SGI machines at an
OpenGL interop testing). Once your display lists are created for
each character, you can use the same basic logic to figure out
which characters are valid (using first, last, firstrow and lastrow
in the sample code to get : firstchar = 256 * firstrow + first
lastchar = 256 * lastrow + last ) and use glCallList with those
characters to print them out.
static int LoadFont(char *fontName)
{
Font id;
int first, last, firstbitmap, i;
GLuint base;
Display *display=0;
int firstrow, lastrow;
int maxchars;
tkGetSystem(TK_X_DISPLAY, &display);
fontInfo = XLoadQueryFont(display, fontName);
if (fontInfo == NULL) {
return 0;
}
id = fontInfo->fid;
/*
* First and Last char in a row of chars
*/
first = (int)fontInfo->min_char_or_byte2;
last = (int)fontInfo->max_char_or_byte2;
/*
* First and Last row of chars, important for multibyte charset's
*/
firstrow = (int)fontInfo->min_byte1;
lastrow = (int)fontInfo->max_byte1;
/*
* How many chars in the charset
*/
maxchars = 256 * lastrow + last;
base = glGenLists(maxchars+1);
if (base == 0) {
return 0;
}
/*
* Get offset to first char in the charset
*/
firstbitmap = 256 * firstrow + first;
/*
* for each row of chars, call glXUseXFont to build the bitmaps.
*/
for(i=firstrow; i<=lastrow; i++)
{
glXUseXFont(id, firstbitmap, last-first+1, base+firstbitmap);
firstbitmap += 256;
}
return base;
}
------
Q_3_14: What's in the new GLU 1.2 tesselator?
A: from Mark Kilgard
Our friends at Digital have the answers:
http://www.digital.com:80/pub/doc/opengl/opengl_new_glu.html
------
Q_3_15: Why is my glDrawPixels (or glCopyPixels or glReadPixels) slow?
A: from Allen Akin
It's not, for the most common cases. After all, similar microcode
and the same hardware are used for both commands. However, there
are three issues to keep in mind.
First, some midrange and low-end SGI graphics adaptors
(particularly XS, XZ, Elan, and Extreme) transfer ABGR-ordered
images much faster than they transfer RGBA-ordered images. The
normal image format in IrisGL was ABGR, while in OpenGL it's RGBA.
So to achieve the same performance in OpenGL that you did in IrisGL
on those machines, you need to use ABGR-format images in OpenGL.
The ABGR extension available in Irix 5.3 and later releases allows
you to do this. See ``man glintro'' for background information on
using OpenGL extensions, and ``man gldrawpixels'' for details on
ABGR. Note that RealityEngine, IMPACT, and all future machines
will process RGBA data at least as fast as ABGR, so RGBA is the way
to go for new code.
Second, some OpenGL pixel data types are faster than others. For
most machines, unsigned byte RGBA (or ABGR) is the fastest
full-color type. Unsigned byte and unsigned short are usually the
fastest gray-scale types. Signed integer types are slower.
Third, OpenGL pixel operations have a much richer set of features
than IrisGL, and if any of those features are enabled, then image
transfer can be significantly slower. Always disable the features
that you don't need. The following code fragment disables features
that are likely to make glDrawPixels slow:
/*
* Disable stuff that's likely to slow down glDrawPixels.
* (Omit as much of this as possible, when you know in advance
* that the OpenGL state will already be set correctly.)
* Note that all of these are the default settings, except
* for GL_DITHER!
*/
glDisable(GL_ALPHA_TEST);
glDisable(GL_BLEND);
glDisable(GL_DEPTH_TEST);
glDisable(GL_DITHER);
glDisable(GL_FOG);
glDisable(GL_LIGHTING);
glDisable(GL_LOGIC_OP);
glDisable(GL_STENCIL_TEST);
glDisable(GL_TEXTURE_1D);
glDisable(GL_TEXTURE_2D);
glPixelTransferi(GL_MAP_COLOR, GL_FALSE);
glPixelTransferi(GL_RED_SCALE, 1);
glPixelTransferi(GL_RED_BIAS, 0);
glPixelTransferi(GL_GREEN_SCALE, 1);
glPixelTransferi(GL_GREEN_BIAS, 0);
glPixelTransferi(GL_BLUE_SCALE, 1);
glPixelTransferi(GL_BLUE_BIAS, 0);
glPixelTransferi(GL_ALPHA_SCALE, 1);
glPixelTransferi(GL_ALPHA_BIAS, 0);
/*
* Disable extensions that could slow down glDrawPixels.
* (Actually, you should check for the presence of the proper
* extension before making these calls. I've omitted that
* code for simplicity.)
*/
#ifdef GL_EXT_convolution
glDisable(GL_CONVOLUTION_1D_EXT);
glDisable(GL_CONVOLUTION_2D_EXT);
glDisable(GL_SEPARABLE_2D_EXT);
#endif
#ifdef GL_EXT_histogram
glDisable(GL_HISTOGRAM_EXT);
glDisable(GL_MINMAX_EXT);
#endif
#ifdef GL_EXT_texture3D
glDisable(GL_TEXTURE_3D_EXT);
#endif
------
Q_3_16: How can I used OpenGL with Tcl/Tk?
A:
TIGER 1.2: ftp://metallica.prakinf.tu-ilmenau.de/pub/PROJECTS/TIGER1.2/
Tix: http://www.cis.upenn.edu/~ioi/tix/sgi.html
------
Q_3_17: OpenGL pixel-exact rasterization.
A: from Kurt Akeley
1) Why is the center for points, lines and bitmaps different than for
polygon vertices and pixel image positions?
Let's just consider lines and polygons. Polygons have real area,
lines have area only because they would otherwise not be visible.
A polygon is sampled by determining if its area intersects a
pixel. OpenGL makes this determination by testing a single point
within each pixel (this way only a single polygon in a 2D mesh can
contain the sample point). It is symmetric and obvious for this
point to be in the middle of the pixel, so that's what we chose.
There were lots of considerations for our choice of line
rasterization algorithms. We ended up with what we call the
"diamond-exit rule". Each pixel center is surrounded by a square
rotated 45 degrees, whose vertexes just touch the 1x1 pixel
boundaries. If a line segment *exits* this diamond, the pixel is
rasterized; otherwise it is not. This algorithm generates
Bresenham lines, does not multiply rasterize pixels at the shared
vertexes of line segments, and guarantees to generate pixels, no
matter how convoluted and intricate the path of a sequence of
segments is. (It has other properties as well, I don't remember
all the considerations.)
2) How does translationg 0.375 ensure predicatable rasterization for all
primitives?
To generate lines reliably the vertexes must be within the
diamonds, and to fill polygons reliably, the vertexes must *not* be
too near the pixel centers. I chose 0.375 as the best compromise,
as seen in the diagram below ('x' is the center of the pixel, 'O'
is the point 0.375, 0.375, and the diamond is shown with
asterisks).
+------*------+
| * * |
| * * |
| * * |
| * * |
| * * |
|* *|
* x *
|* *|
| * O * |
| * * |
| * * |
| * * |
| * * |
+------*------+
------
Q_3_18: Saving OpenGL screen output.
A: from Reto Koradi
/* OpenGL image dump, written by Reto Koradi (kor@spectrospin.ch) */
/* This file contains code for doing OpenGL off-screen rendering and
saving the result in a TIFF file. It requires Sam Leffler's libtiff
library which is available from ftp.sgi.com.
The code is used by calling the function StartDump(..), drawing the
scene, and then calling EndDump(..).
Please note that StartDump creates a new context, so all attributes
stored in the current context (colors, lighting parameters, etc.)
have to be set again beforing performing the actual redraw. This
can be rather painful, but unfortunately GLX does not allow
sharing/copying of attributes between direct and nondirect
rendering contexts. */
#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Intrinsic.h>
#include <GL/gl.h>
#include <GL/glx.h>
#include <tiffio.h>
/* X servers often grow bigger and bigger when allocating/freeing
many pixmaps, so it's better to keep and reuse them if possible.
Set this to 0 if you don't want to use that. */
#define KEEP_PIXMAP 1
static FILE *TiffFileP;
static int Orient;
static int ImgW, ImgH;
static Bool OutOfMemory;
static Display *Dpy;
static Pixmap XPix = 0;
static GLXPixmap GPix = 0;
static GLXContext OldCtx, Ctx;
static float OldVpX, OldVpY, OldVpW, OldVpH;
static void
destroyPixmap(void)
{
glXDestroyGLXPixmap(Dpy, GPix);
GPix = 0;
XFreePixmap(Dpy, XPix);
XPix = 0;
}
static int
xErrorHandler(Display *dpy, XErrorEvent *evtP)
{
OutOfMemory = True;
return 0;
}
int
StartDump(char *fileName, int orient, int w, int h)
/* Prepare for image dump. fileName is the name of the file the image
will be written to. If orient is 0, the image is written in the
normal orientation, if it is 1, it will be rotated by 90 degrees.
w and h give the width and height (in pixels) of the desired image.
Returns 0 on success, calls RaiseError(..) and returns 1 on error. */
{
Widget drawW = GetDrawW(); /* the GLwMDrawA widget used */
XErrorHandler oldHandler;
int attrList[10];
XVisualInfo *visP;
int n, i;
TiffFileP = fopen(fileName, "w");
if (TiffFileP == NULL) {
RaiseError("could not open output file");
return 1;
}
#if KEEP_PIXMAP
if (GPix != 0 && (w != ImgW || h != ImgH))
destroyPixmap();
#endif
Orient = orient;
ImgW = w;
ImgH = h;
Dpy = XtDisplay(drawW);
n = 0;
attrList[n++] = GLX_RGBA;
attrList[n++] = GLX_RED_SIZE; attrList[n++] = 8;
attrList[n++] = GLX_GREEN_SIZE; attrList[n++] = 8;
attrList[n++] = GLX_BLUE_SIZE; attrList[n++] = 8;
attrList[n++] = GLX_DEPTH_SIZE; attrList[n++] = 1;
attrList[n++] = None;
visP = glXChooseVisual(Dpy,
XScreenNumberOfScreen(XtScreen(drawW)), attrList);
if (visP == NULL) {
RaiseError("no 24-bit true color visual available");
return 1;
}
/* catch BadAlloc error */
OutOfMemory = False;
oldHandler = XSetErrorHandler(xErrorHandler);
if (XPix == 0) {
XPix = XCreatePixmap(Dpy, XtWindow(drawW), w, h, 24);
XSync(Dpy, False); /* error comes too late otherwise */
if (OutOfMemory) {
XPix = 0;
XSetErrorHandler(oldHandler);
RaiseError("could not allocate Pixmap");
return 1;
}
}
if (GPix == 0) {
GPix = glXCreateGLXPixmap(Dpy, visP, XPix);
XSync(Dpy, False);
XSetErrorHandler(oldHandler);
if (OutOfMemory) {
GPix = 0;
XFreePixmap(Dpy, XPix);
XPix = 0;
RaiseError("could not allocate Pixmap");
return 1;
}
}
Ctx = glXCreateContext(Dpy, visP, NULL, False);
if (Ctx == NULL) {
destroyPixmap();
RaiseError("could not create rendering context");
return 1;
}
OldCtx = glXGetCurrentContext();
(void) glXMakeCurrent(Dpy, GPix, Ctx);
return 0;
}
static int
writeTiff(void)
{
TIFF *tif;
int tiffW, tiffH;
int rowsPerStrip, bufSize, rowI;
unsigned char *buf;
int res;
tif = TIFFFdOpen(fileno(TiffFileP), "output file", "w");
if (tif == NULL) {
RaiseError("could not create TIFF file");
return 1;
}
if (Orient == 0) {
tiffW = ImgW;
tiffH = ImgH;
bufSize = 4 * ((3 * tiffW + 3) / 4);
glPixelStorei(GL_PACK_ALIGNMENT, 4);
} else {
tiffW = ImgH;
tiffH = ImgW;
bufSize = 3 * tiffW;
glPixelStorei(GL_PACK_ALIGNMENT, 1);
}
rowsPerStrip = (8 * 1024) / (3 * tiffW);
if (rowsPerStrip == 0)
rowsPerStrip = 1;
TIFFSetField(tif, TIFFTAG_IMAGEWIDTH, tiffW);
TIFFSetField(tif, TIFFTAG_IMAGELENGTH, tiffH);
TIFFSetField(tif, TIFFTAG_BITSPERSAMPLE, 8);
TIFFSetField(tif, TIFFTAG_COMPRESSION, COMPRESSION_LZW);
TIFFSetField(tif, TIFFTAG_PHOTOMETRIC, PHOTOMETRIC_RGB);
TIFFSetField(tif, TIFFTAG_FILLORDER, FILLORDER_MSB2LSB);
TIFFSetField(tif, TIFFTAG_DOCUMENTNAME, "My Name");
TIFFSetField(tif, TIFFTAG_IMAGEDESCRIPTION, "My Description");
TIFFSetField(tif, TIFFTAG_SAMPLESPERPIXEL, 3);
TIFFSetField(tif, TIFFTAG_ROWSPERSTRIP, rowsPerStrip);
TIFFSetField(tif, TIFFTAG_PLANARCONFIG, PLANARCONFIG_CONTIG);
buf = malloc(bufSize * sizeof(*buf));
res = 0;
for (rowI = 0; rowI < tiffH; rowI++) {
if (Orient == 0)
glReadPixels(0, ImgH - 1 - rowI, ImgW, 1,
GL_RGB, GL_UNSIGNED_BYTE, buf);
else
glReadPixels(rowI, 0, 1, ImgH,
GL_RGB, GL_UNSIGNED_BYTE, buf);
if (TIFFWriteScanline(tif, buf, rowI, 0) < 0) {
RaiseError("error while writing TIFF file");
res = 1;
break;
}
}
free(buf);
TIFFFlushData(tif);
TIFFClose(tif);
return res;
}
int
EndDump(void)
/* Write current image to file. May only be called after StartDump(..).
Returns 0 on success, calls RaiseError(..) and returns 1 on error. */
{
int res;
res = writeTiff();
(void) fclose(TiffFileP);
(void) glXMakeCurrent(Dpy, XtWindow(GetDrawW()), OldCtx);
#if KEEP_PIXMAP
#else
destroyPixmap();
#endif
glXDestroyContext(Dpy, Ctx);
return res;
}
------
Q_3_19: No Logicop in RGB for OpenGL?
A:
- from Kurt Akeley
The best solution that I know of is to save and restore the
portion of the 3D image that will be damaged by the 2D
rendering. If the machine supports double buffering, you can
use the back buffer for this purpose. Use glCopyPixels to do
the transfers. Otherwise use glReadPixels to save the region
and glDrawPixels to restore it.
Note that an advantage to the save/restore approach is that you
have control of the colors of your 2D rendering. Using logical
operations provides much less control.
- From Brian Paul (brianp@ssec.wisc.edu)
There is an extension EXT_blend_logic_op, available in many
implementations which allows you to do what you need. Use
#ifdef GL_EXT_blend_logic_op / #endif in your source code to
test for the extension at compile time and
glGetString(GL_EXTENSIONS) to test for it at run-time. Then
the call to glBlendEquationEXT(GL_LOGIC_OP) will tell GL that
blending is to be done with the logic op specified by
glLogicOp().
If neither overlay planes nor this extension are available, you
may have to resort to your window system's XOR drawing
facility. If you're using X, you can set your GC's function to
GXxor and use the Xlib drawing functions. If you do this be
sure to synchronize the GL and X drawing with glXWaitX() and
glXWaitGL().
------
Q_3_20: Why does the raster position get clipped by the viewing volume?
A: from Kurt Akeley
A vertex on the Z=0 plane in clip coordinates is projected to
infinity if not clipped. Since OpenGL is a 3D library, such a
vertex is always a possibility, even in a 2D application. Thus the
raster position is always clipped.
------
Q_3_21: Is texturing really enabled?
A:
One thing to point on (in Section 3.8.1 "Texture Minification,
Mipmapping" in the OpenGL 1.1 spec) is "If texturing is enabled at the
time a primitive is rasterized and if the set of arrays 0 through p is
incomplete, based on the dimensions of array 0, then it is as if
texture mapping were disabled. The set of arrays 0 through p is
incomplete if the internal formats of all the mipmap arrays were not
specified with the same symbolic constant, or if the border widths of
the mipmap arrays are not the same, or if the dimensions of the mipmap
arrays do not follow the sequence described above. Arrays indexed
greater than p are insignificant."
------
end of file