Romanyk O. N., Doctor of Science., professor, VNTU
Melnyk O. V., PhD candidate, VNTU
Vinnytsia National Technical University, Ukraine
ANTI-ALIASING
METHODS. AN OVERVIEW.
Anti-Aliasing is a combination of technical and algorithmic tools used to remove the stair step effect appearing on the boundaries of two-dimensional or three-dimensional images
when reproducing them on a display device.
In Super-Sample Anti-Aliasing method, SSAA, the image
to be displayed is calculated with higher resolution than the resolution of the
display device. To display the
image it must be reduced by averaging. This is the simplest and the most CPU intensive anti-aliasing technology. Sampling
may have different physical locations in pixels.
By the way of sample placement, sampling methods are divided into OGSS -
Ordered Grid Super-Sampling and Rotated Grid Super-Sampling [1]. The name Ordered Grid specifies the position of calculations relative to
a single pixel, where selected excess points
are located in the form of an ordered lattice.
In Rotated Grid Super-Sampling method excess samples are placed
on the turned around grid.
This method gives good results especially for lines close
to horizontal or vertical. Another
method of excess sampling is Sparse Grid Super-Sampling. In Sparse Grid Super-Sampling additional
points are also located on the orderly grid,
but the sample takes place only on
certain nodes of the grid [1].
This method is a compromise between speed
and image quality.
The main advantage of Super-Sampling method is that it has simple hardware implementation and does not depend on character and the specific objects
in the scene displayed.
The main disadvantages of these methods lie in the large computational complexity, which significantly affect the performance of
imaging, and in the use of large arrays of memory. Therefore, the actual problem is
to reduce the number of samples by image analysis only
on the edges of the graphics.
Multisampling
Method, MSAA, was created as an
alternative to supersampling [2]. The basic principle was to find
a compromise between quality and loading, to minimize the computational load, and
at the same time, to smooth the effect of "steps".
MSAA follows this principle with two basic techniques. The first technique
is called edge anti-aliasing, where the smoothing effect is applied only to the edges of objects. A graphic output
device displays the biggest possible scene space with no
anti-aliasing, but then performs additional processing of the samples of pixels located at
the edges of the object that
would benefit most from anti-aliasing. Before any smoothing is done, Z-test is performed: the
difference in depth within a single
pixel indicates that
it contains the edge of the
object, and therefore requires imposition of MSAA.
The second technology MSAA lies in reducing the load on the sample. Some
calculations are performed only once per pixel, such as pixel shaders, textures and request sampling color [2]. Only the meanings of depth and templates are calculated with the total number of samples. The
computer uses this information to
determine the optimal mix of color between the object and the background.
Smoothing MSAA
was accepted de facto as a standard
after its release. It provides an
excellent balance of image
quality and speed compared to
super-sampling. However, the smoothing MSAA can
still be very hard for middle and lower graphics levels. In addition, the
regional smoothing technique,
such as MSAA, does
not improve the texture quality as
effectively as super-sampling method [2]. This means that the textures (especially objects with
transparent textures) will continue to have edges of the "ladder"
that this method does not remove.
Subpixel
Reconstruction Anti-Aliasing Method, SRAA, is
a mechanism of per pixel interpolation
that is applied to the final
image [3]. Anti-aliasing is applied to an image that has been elaborated by the shader
by applying super-sweep and depth buffers
for normals. It is actually a post-processing image filter. Comparing with the super-sampling
methods, where smoothing is
performed at the stage of shaders,
in SRAA minimal loss of
productivity can be observed, because SRAA is applied to the
entire image as a whole and is
characterized by high processing
speed [3].
The developers of the method present their studies, which show SRAA’s significant advantage over methods
of super-sampling. For
example, to smooth the
image with a resolution of 1280
× 720 redundant sampling methods spend
5-10 minutes. SRAA performs smoothing in
1.8 minutes. and
the quality of the resulting image
is similar to the results of the traditional anti-aliasing modes 4-16x. Also,
the method has a fixed SRAA Runtime filtering, which also simplifies its use [3].
Temporal Anti-Aliasing Method, TXAA, is a method of smoothing the cinematic image quality. It is designed to reduce the
time aliasing (drift and shimmer with movement) [4].
The technology used
in the method is a combination of
temporal filter, anti-aliasing
hardware and special computer graphics
of high quality. For filtration of pixels on the screen TXAA uses sample
samples both inside and outside of the pixel, combined with
samples from previous frames, to ensure the highest quality
of filtration. TXAA has improved spatial
filtering compared to MSAA, such as when the
image displays leaf edges or objects in motion [4].
REFERENCES:
1. Analysis of smoothing methods based on super-sampling - [The electronic resource] http://www.ixbt.com/video/fsaa-an-1.html
2. Don Woligroski. Anti-Aliasing in the games: Part I, the theoretical - [The electronic resource] http://cheklab.ru/archives/857
3. SRAA — a new method of smoothing from NVIDIA - [The
electronic resource] http://ru.hw-lab.com/sraa-new-antialiasing-method-from-nvidia.html
4. TXAA (temporal smoothing): games cinematic quality to become a reality - [The electronic resource] http://www.nvidia.ru/object/txaa-anti-aliasing-technology-ru.html