3Dc™ is a new
compression technology developed by ATI, and introduced in the new
RADEON X800 series of Visual Processing Units. This technology is
designed to allow game developers to pack more detail into real time
3D images than ever before, making 3Dc a key enabler of the HD Gaming
vision.
Figure1: A
close-up of the Optico character from ATI's Double Cross demo showing
the increase in fine detail made possible by 3Dc compression.
Today’s graphics
processors rely heavily on data and bandwidth compression techniques
to reach ever increasing levels of performance and image quality.
Rendering a scene in a modern 3D application requires many different
kinds of data, all of which must compete for bandwidth and space in
the local memory of the graphics card. For games, texture data tends
to be the largest consumer of these precious resources, making it
one of the most obvious targets for compression.
A set of algorithms known as DXTC (DirectX Texture Compression) has
been widely accepted as the industry standard for texture compression
techniques. Introduced in 1999, along with S3TC (its counterpart for
the OpenGL API), DXTC has been supported by all new graphics hardware
for the past few years, and has seen widespread adoption by game developers.
It has proven particularly effective for compressing two-dimensional
arrays of color data stored in RGB and RGBA formats.
With the appearance of graphics hardware and APIs that support programmable
pixel shaders in recent years, researchers and developers have come
up with a variety of new uses for textures. A variety of material
properties such as surface normals, shininess, roughness, and transparency
can now be stored in textures along with the traditional color information.
Unfortunately, DXTC is often not as effective at compressing textures
carrying these different types of data as it is at compressing color
data. This makes it challenging to fit all of the necessary data into
graphics memory, and also impacts performance because of the increased
memory bandwidth requirements.
3Dc is designed to resolve
these issues by going beyond the capabilities of DXTC, and offering
efficient and effective compression of new types of texture data.
Whereas DXTC is optimized for compressing 3- and 4-channel data formats,
3Dc focuses on 2-channel data formats. It is particularly effective
at two increasingly common tasks – compressing normal maps,
and packing multiple pieces of data into a single compressed texture.
Normal Maps
Normal maps are special textures that are used to add detail to 3D
surfaces. They are an extension of earlier “bump map”
textures, which contained per-pixel height values and were used to
create the appearance of bumpiness on otherwise smooth surfaces. Normal
maps contain more detailed surface information, allowing them to represent
much more complex shapes.
You can think of a normal as an arrow that points outward from a surface,
and is perpendicular to that surface at its point of origin. The usefulness
of normals stems from the fact that, given the location of a light
source, they can be used to accurately determine the direction in
which incoming light will reflect off of a surface at any given point.
Normals are typically stored at each vertex in a 3D model. Depending
on the number of vertices and how far apart they are, there may be
large gaps between vertices in which the direction of the normals
cannot be precisely determined. This prevents the normals from being
used to create fine detail. To address this problem, a look-up into
a normal map can be performed for each pixel, making it possible to
more precisely define the direction of the normal at any location
on a surface.
Normal maps provide a much more efficient way of capturing the fine
details on a surface than using very large numbers of polygons. Their
only limitation is that they cannot be used for large scale features,
because they do not actually affect the shape of an object. The three
dimensional appearance of the details on normal mapped surfaces are
only an illusion, enabled by accurately modeling light and shadow.
This can become apparent when looking at the silhouette of a normal
mapped surface.
The most common way to use normal maps is to use just enough polygons
to give the shape of an object and its silhouette a satisfactory appearance,
and then use normal maps to add in any finer details. These details
may be small in scale, but they can have a large impact on the realism
and image quality of a rendered image.
Normal maps can be generated by creating two versions of each 3D model,
one with a high polygon count, and one with a much lower polygon count.
The low polygon version has just enough detail to capture the general
shape of the object. The high polygon version uses as many polygons
as are required to capture all of the finer details. A simple program
can then be run that takes these two models as inputs, calculates
the differences between them at many points on the surface, and stores
the results in a normal map texture. For improved image quality, this
normal map can be combined with a detail map, which includes details
too small to be captured by even the high polygon version of the model.
Often this can be a simple repeating pattern that adds realistic texture
to a surface.
Figure 2:
Generating normal maps
This normal mapping
technique is one of the key things that makes the new generation of
games like Half-Life 2, Doom 3, and Far Cry look so much better than
previous titles. The only limitation to the effectiveness of this
technique is the size of the textures required. In an ideal case where
every surface had both a color texture map and a normal texture map,
the texture memory and bandwidth
requirements would double compared with just using color maps alone.
In fact, the problem is much worse because DXTC & S3TC are ineffective
at compressing normal maps. They tend to have trouble capturing the
small edges and subtle curvature that normal maps are designed to
capture, and they also introduce unsightly block artifacts. Because
normal maps are used to capture light reflections and realistic surface
highlights, these problems are amplified relative to their impact
on color textures. The results are sufficiently poor that game artists
and developers would rather not use normal maps at all on most surfaces,
and instead limit themselves to lower resolution maps on selected
parts of the rendered scene.
3Dc provides an ideal solution to the normal map compression problem.
It provides up to 4:1 compression of normal maps, with image quality
that is virtually indistinguishable from the uncompressed version.
The technique is hardware accelerated, so the performance impact is
minimal. Thus, developers are freed to use higher resolution, more
detailed normal maps, and/or use them on all of the objects in a scene
rather than just a select few.
Figure 3: Comparison
of normal maps compressed with 3Dc and DXTC. Note how the 3Dc compressed
normal map looks virtually identical to the original, while the DXTC
version exhibits blocky artefacts and loss of detail.
How 3Dc Works
3Dc is a block-based compression technique. It breaks a texture map
up into 4x4 blocks containing 16 values each. These values must consist
of two components. Each component is compressed separately. A maximum
and minimum value is determined for each block, and these are stored
as 8- bit values. A set of six intermediate values are then calculated,
spaced equally between the minimum
and the maximum. This gives a total of eight values that each component
can take within a block Each component is assigned a 3-bit index corresponding
to whichever of these values is closest to its original value.
The resulting compressed
blocks consist of four 8-bit values and thirty-two 3-bit values, for
a total of 128 bits. Since the original blocks consisted of sixteen
32-bit values, for a total of 512 bits, this represents a compression
ratio of 4:1. If the original values were 16-bit rather than 32-bit,
then a compression ratio of 2:1 can still be achieved.
Using 3Dc to compress normal maps requires an additional step. This
is because each value in a normal map is actually a 3D vector, consisting
of 3 components (x, y & z). These values must be reduced to 2-component
values in order to work with 3Dc. Fortunately, this can be handled
in a simple way by assuming that all of the normal vectors have a
length of 1. Given the values of two components of a vector, the value
of the third component can be found using the following mathematical
relationship:
This formula can be implemented
using just a couple of pixel shader instructions.
Figure 4: Block
data formatting for a standard normal map (upper left) and 3Dc compressed
normal map (lower right). The generation of intermediate values during
the compression process is shown in the upper right.
Benefits of
3Dc
3Dc makes it possible to use much higher resolution and more detailed
normal maps than would otherwise be possible due to memory restrictions.
Since the decompression of 3Dc compressed textures is hardware accelerated,
the performance cost of achieving this higher level of detail is minimal.
Alternatively, 3Dc can be used to reduce the memory footprint of existing
normal maps and allow a wider variety of them to be stored, meaning
they can be used on larger number of different surfaces.
It can also be used to optimize performance in applications that are
strongly limited by memory bandwidth, since the normal maps are read
from memory in compressed format.
Another important benefit of 3Dc is its ease of implementation. In
most cases it can be used with existing art assets. The same compression
tools used for DXTC/S3TC can be used for 3Dc with only minor modifications.
Pixel shader changes consist of only two extra instructions in the
case of compressed normal maps. The simplicity of 3Dc allows it to
work easily with all other image quality enhancement techniques including
anti-aliasing, texture filtering, shadows, HDR, and post-processing
effects.
The new 3Dc texture format is supported in DirectX 9.0 using a FourCC
code, and in OpenGL using a vendor-specific extension. 3Dc is expected
to be incorporated in some form into future versions of these programming
interfaces. To assist in this process, ATI is making all the details
of this format openly available.
Summary
3Dc is an exciting new compression technology designed to bring
out fine details in games while minimizing memory usage. It is
the first compression technique optimized to work with normal
maps, which allow fine per-pixel control over how light reflects
from a textured surface. With up to 4:1 compression possible,
this means game designers can now include up to 4x the detail
without changing the amount of graphics memory required and without
impacting performance.
Next: Advanced
Depth
