MAE152 Computer Graphics for Scientists and Engineers Antialiasing Fall 2003.
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Transcript of MAE152 Computer Graphics for Scientists and Engineers Antialiasing Fall 2003.
MAE152Computer Graphics for Scientists and
Engineers
Antialiasing
Fall 2003
Antialiasing
Contents “Discretization” and consequences What is aliasing and it’s solution (antialiasing) Antialiasing implemented by OpenGL
Samples
most things in the real world are continuous everything in a computer is discrete the process of mapping a continuous function to a
discrete one is called sampling the process of mapping a discrete function to a
continuous one is called reconstruction the process of mapping a continuous variable to a
discrete one is called discretization rendering an image requires sampling and
discretization displaying an image involves reconstruction
Imaging Devices Area Sample
eye : photoreceptors
J. Liang, D. R. Williams and D. Miller, "Supernormal visionand high- resolution retinal imaging through adaptive optics,"J. Opt. Soc. Am. A 14, 2884- 2892 (1997)
Film is similar : irregular array of receptors.
Images
an image is a 2D function I(x, y) that specifies intensity for each point (x, y)
Imaging Devices Area Sample
video camera : CCD array.
Regular array of imagingsensors. y
x
yx
IdxdykV,
Value sensed is an area integral over a pixel.
Intensity, I
Slide © Rosalee Nerheim-Wolfe
Continuous Luminosity Signal
Slide © Rosalee Nerheim-Wolfe
Sampled Luminosity
Slide © Rosalee Nerheim-Wolfe
Reconstructed Luminosity
Slide © Rosalee Nerheim-Wolfe
Reconstruction Artefact
Line Segments
If we tried to sample a line segment so it would map to a 2D raster display…
we would see stair steps, or jaggies … since we “quantized” the pixel values to 0 or 1.
Line Segments
instead, quantize to many shades but what sampling algorithm is used?
Aliasing
The effect created when rasterization is performed over a discrete series of pixels.
In particular, when lines or edges do not necessarily align directly with a row or column of pixels, that line may appear unsmooth and have a stair-step edge appearance.
Antialiasing utilizes blending techniques to blur the edges of the lines and provide the viewer with the illusion of a smoother line.
Points, lines or polygons can be antialiased.
Anti-Aliasing
Two general approaches: Area sampling and super-sampling
Area sampling approaches sample primitives with a box (or Gaussian, or whatever) rather than spikes
Requires primitives that have area (lines with width) Sometimes referred to as pre-filtering
Super-sampling samples at higher resolution, then filters down the resulting image
Sometimes called post-filtering The prevalent form of anti-aliasing in hardware
Unweighted Area Sampling
Consider a line as having thickness (all good drawing programs do this)
Consider pixels as little squares
Fill pixels according to the proportion of their square covered by the line
Other variations weigh the contribution according to where in the square the primitive falls
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Alpha-based Anti-Aliasing
Rather than setting the intensity according to coverage, set the
The pixel gets the line color, but with <=1
This supports the correct drawing of primitives one on top of the other
Draw back to front, and composite each primitive over the existing image
Only some hidden surface removal algorithms support it
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Super-sampling
Sample at a higher resolution than required for display, and filter image down
Issues of which samples to take, and how to average them
4 to 16 samples per pixel is typical
Samples might be on a uniform grid, or randomly positioned, or other variants
Number of samples can be adapted
Unweighted Area Sampling
primitive cannot affect intensity of pixel if it does not intersect the pixel
equal areas cause equal intensity, regardless of distance from pixel center to area
unweighted sampling colors two pixels identically when the primitive cuts the same area through the two pixels
intuitively, pixel cut through the center should be more heavily weighted than one cut along corner
Weighted Area Sampling
weighting function, W(x,y) specifies the contribution of primitive passing through
the point (x, y) from pixel center
x
IntensityW(x,y)
Antialiasing in OpenGL
OpenGL calculates a coverage value for each fragment based on the fraction of the pixel square on the screen that it would cover. In RGBA mode, OpenGL multiplies the
fragment’s alpha value by its coverage. Resulting alpha value is used to blend the
fragment with the corresponding pixel already in the frame buffer.
Antialiasing (Cont.)
Hints
With OpenGL, you can control the behavior of anti-aliasing effects by using the glHint() function:void glHint(GLenum target, GLenum hint);
target: parameter indicates which behavior is to be controlled. Specifies the desired sampling quality of points, lines or polygons during antialiasing operations
target parameter can be GL_POINT_SMOOTH_HINT GL_LINE_SMOOTH_HINT GL_POLYGON_SMOOTH_HINT GL_FOG_HINT GL_PERSPECTIVE_CORRECTION_HINT
glHint(target, hint)
hint: parameter specifies the approach
hint parameter can be GL_FASTEST (the most efficient option) GL_NICEST (the highest-quality option) GL_DONT_CARE (no preference)
Enabling Antialiasing
Anti aliasing is enabled using the glEnable() command, We can enable GL_POINT_SMOOTH or GL_LINE_SMOOTH modes.
With RGBA mode, you must also enable blending to utilize GL_SRC_ALPHA as the source factor and GL_ONE_MINUS_SRC_ALPHA as the destination factor.
Using a destination factor of GL_ONE will make intersection points a little brighter.
Antialiasing Lines
init() {
glEnable (GL_LINE_SMOOTH);
glEnable (GL_BLEND);
glBlendFunc (GL_SRC_ALPHA,
GL_ONE_MINUS_SOURCE_ALPHA);
glHint (GL_LINE_SMOOTH_HINT,
GL_NICEST);
draw_lines_here();
}
Antialiasing Polygons
Study OpenGL Book
We can also use accumulation buffer
Using accumulation buffer is easier to understand than using blending, but computationally more expensive.
Demo Intersecting lines
Fog
General term that describes similar forms of atmospheric effects Haze, mist, smoke, or pollution
Can make an entire image appear more natural by adding fog, which makes objects fade into the distance.
Fog is applied after matrix transformations, lighting, and texturing are performed.
With large simulation programs, fog can improve performance.
Defining Fog in OpenGL
glFogf(GL_FOG_MODE, GL_EXP); GL_EXP2 GL_LINEAR
glFogf(GL_FOG_DENSITY, 0.5); glFogf(GL_FOG_START, 0.0); glFogf(GL_FOG_DENSITY, 1.0);
In RGBA mode, the fog factor f isC = fCi + (1-f)Cf
Ci is incoming fragment’s RGBA value, Cf is fog-color value.
Fog Equations
Sample Code
init() {
Glfloat fogcolor[4] = {0.5, 0.5, 0.5, 1.0};
glEnable(GL_FOG);
glFogi (GL_FOG_MODE, GL_EXP);
glFogfv (GL_FOG_COLOR, fogColor);
glFogf (GL_FOG_DENSITY, 0.35);
glFogf (GL_FOG_HINT, GL_NICEST);
glFogf (GL_FOG_START, 1.0);
glFogf (GL_FOG_END, 5.0);
}
End of Antialiasing and Fog