/usr/share/gem/manual/Texture.html

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   <meta name="Author" content="Mark Danks">
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   <title>Texture mapping</title>
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<h2>
<u>Texture Mapping</u></h2></center>
<a href="Gloss.html#Texture">Texture mapping</a> is the act of applying
pixel data to a geometric object.  In GEM, this is achieved with the
<i>[pix_texture]</i>
object.  It is important to understand that the
<i>[pix_texture]</i>
object merely sets the pix as the current texture.  It does not do
any rendering!  You need to use a geo object which does texture mapping. 
All of the basic geo objects can texture map, such as <i>[square]</i> or
<i>[sphere]</i>.
<p><img SRC="tribar.gif" height=13 width=561>
<p>A simple example of texture mapping is the following patch:
<center>
<p><img SRC="texture.jpg" BORDER=1 height=182 width=160></center>

<p>This patch can be found at 07.texture/01.texture.pd.  Change
the number box connected to the rotate object to see what a texture map
on a cube looks like.
<p>The <i>[pix_image]</i> object loads in the fractal image file.  The
<i>[pix_texture]</i>
object says that the pix data should be used as a texture map.  Notice
that this is different than the previous manual section when we used the
<i>[pix_draw]</i> object.  The final object in the chain is the <i>[cube]</i>
object.  Because we have enabled texture mapping with the <i>[pix_texture]</i>
object, the cube takes the pix data and applies it to the geometry.
<p><img SRC="tribar.gif" height=13 width=561>
<p>Texture mapping can be used with any GEM object.  In the previous
manual section, you saw how to load in pix data with a variety of objects,
including <i>[pix_multiimage]</i> and <i>[pix_video]</i>.  All of these
objects can be used with the <i>[pix_texture]</i> object.
<p>Because the pix data is applied to geometry, you can move, rotate, and
scale the image.  This is extremely useful on the <i>[square]</i> object. 
Instead of doing a one-to-one pixel mapping as occurs with the <i>[pix_draw]</i>
object, you can resize and reshape the image.
<p>OpenGL originally required that images must have dimensions that are power-of-2, such as 64, 128, or 256. This restriction has been released with recent gfx-cards
(like some radeon/nvidia products).
However, if the width or height of an image is not a power of two,
then the <i>[pix_texture]</i> object will take care of this, 
and still render it (depending on you hardware with some tricks).
You can thus texture images of any size, but since this is based on tricking 
the texture-coordinates, <i>[pix_coordinate]</i> might not give the wanted result any more.
<p><img SRC="tribar.gif" height=13 width=561>
<p>The example patch 07.texture/02.moveImages.pd is a much more complex
patch which uses alpha blending to create a transparent object, in this
case, the dancer.  Make sure to turn on the rotation with the <i>[metro]</i>
object.
<p><img SRC="tribar.gif" height=13 width=561><a href="index.html"></a>
<p>People have been asking how textures are handled in GEM.  Here
is a long explanation from an email which I wrote.
<p><tt>  Here is how textures are dealt with under OpenGL and hardware
accelerators.  This can obviously change in the future, but right
now, I am fairly certain that the info is correct (I make games in my day
job, so I have vested interest in this :-)</tt><tt></tt>
<p><tt>  The amount of memory (VRAM) on the card (12mb for Voodoo2,
16mb for TNT, 64mb for GeForce2, etc) is used for both textures (TRAM)
and frame buffer space.  If you have a large rendering window, like
1600x1200, it will take up 1600x1200x4x3 in 32-bit mode with double buffering
and a Z buffer (or 23mb).  Most people run at TV resolution, like
NTSC, so it takes 640x480x4x3 = 3.7mb   All of the space left
is for textures onboard the card (FYI, if you have heard that people are
having problems with the PlayStation2, notice that it only has 4mb of VRAM...not
much onboard texture space, huh? :-)  Thankfully it has an <i>extremely</i>
fast DMA bus)</tt><tt></tt>
<p><tt>  Sooo, when GEM "creates" a texture, it immediately tries
to send the texture to the card, which uses some of the left over space
in the VRAM.  If you had a 640x480 window on a Voodoo2, you have ~8mb
of texture space left over.  On a GeForce2, ~60mb.  The problem
is what happens if you want more textures than can fit into TRAM. 
OpenGL requires that the video drivers deal with the problem, so GEM doesn't
care too much (more about this later).</tt><tt></tt>
<p><tt>  In most cases, the drivers cache the textures in main memory
and if a texture is requested for rendering and it isn't resident on the
card, it will download it.  If you have AGP, then this is pretty quick,
although none of 3dfx cards really take advantage of this (ie, those cards
are about the same speed as the PCI bus).  So depending on the number
of textures, and how complex the scene is, you might be able to display
more textures than you have TRAM.</tt><tt></tt>
<p><tt>  One slowdown that can happen with GEM is that it makes a
copy of the image before sending it down the chain of objects.  If
you are constantly changing images with a pix_multiimage, this can be a
performance hit, but you can modify the actual pixel data with the pix
objects.  The pixels aren't sent to the graphics card until the pix_texture
object is reached.</tt><tt></tt>
<p><tt>  GEM tries to help with this with a few objects.  pix_imageInPlace
acts much the same as pix_multiimage, but it downloads _every_ image in
the sequence to the card when a download message is recieved.  It
also immediately turns on texturing, instead of making a copy (ie, you
don't need a pix_texture object).  Much faster, but not as flexible. 
pix_movie does much the same thing.  It sends the pixel data without
copying it if there is a new frame to display.</tt><tt></tt>
<p><tt>  The entire pix system uses a caching system so that the copying
and processing only occurs if something actually changes.  For example,
if you had a pix_threshold object, it would only process when rendering
started...and every time that the values actually changed.  You can
use pix_buf to isolate parts which don't change from those that do, but
it involves another copy.</tt><tt></tt>
<p><tt>  On the Voodoo2, the hardware itself limits textures to 256x256...this
will never change.  The newest Voodoo5 boards have a higher texture
size.</tt><tt></tt>
<p><tt>  If you load the _exact_ same image (this means the exact
same file/path name), then the pix_image has a cache system which means
that it is only loaded into the</tt>
<br><tt>computers memory once.  However, each pix_image still sends
its own copy down to the gfx card.</tt><tt></tt>
<p><tt>  You could use a single [pix_image]/[pix_texture] with [separator]
to do this...I have done it a lot in the past.</tt><tt></tt>
<p><tt>  The reason that [pix_image] doesn't share the actual texture
data is that you can modify the pixel data with other pix objects...[pix_image]
doesn't actually send the texture data to the gfx card, [pix_texture] does.</tt>
<p><img SRC="tribar.gif" height=13 width=561><a href="index.html"></a>
<p><a href="index.html">[return]</a>
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gem-doc 1:0.93.3-5ubuntu4 / usr / share / gem / manual / Texture.html
