Density plot
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plot::Density(f
,x = x_{min} .. x_{max}
,y = y_{min} .. y_{max}
, <a = a_{min} .. a_{max}
>,options
) plot::Density(A
, <x = x_{min} .. x_{max}, y = y_{min} .. y_{max}
>, <a = a_{min} .. a_{max}
>,options
) plot::Density(L
, <x = x_{min} .. x_{max}, y = y_{min} .. y_{max}
>, <a = a_{min} .. a_{max}
>,options
)
plot::Density(f(x, y), x = `x_{min}`..`x_{max}` , y
= `y_{min}`..`y_{max}` )
generates a regular 2D mesh of
rectangles extending from the lower left corner (x_{min}
, y_{min}
)
to the upper right corner (x_{max}
, y_{max}
).
The rectangle with midpoint (x, y) is
colored according to a color scheme based on the "density"
value f(x, y).
plot::Density
serves for the visualization
of 3D data (x, y, f(x, y)) by
a 2D plot. Roughly speaking, it corresponds to a colored 3D function
graph of the density function f(x, y) viewed
from above. However, in contrast to the 3D function graph, plot::Density
does
not use smooth interpolation ("shading") of the color
between adjacent rectangles.
If the density data are provided by an array or matrix A
or
by a list L
, the number of rectangles in the density
plot is given automatically by the format of A
or L
,
respectively.
If the density data are given by an expression or function f
,
the attribute Mesh
= [m,
n]
serves for advising plot::Density
to
create a grid of m×n rectangles.
Alternatively, one may set XMesh
= m
, YMesh
= n
.
With the default FillColorType
= Dichromatic
,
the rectangle with density value f(x, y) at
the midpoint (x, y) is
colored with the color
,
where
are
the minimal/maximal density values in the graphics and fillcolor
, fillcolor2
are
the RGB
values
of the attributes FillColor
and FillColor2
, respectively.
Thus, fillcolor
indicates high density values whereas fillcolor2
indicates
low density values.
If f_{min} = f_{max},
a flat coloring with fillcolor
is used.
With FillColorType
= Monochrome
,
the rectangle with density value f(x, y) at
the midpoint (x, y) is
colored with the color
.
The user may specify a fill color function via FillColorFunction
= mycolorfunction
to override the density coloring described
above. The procedure mycolorfunction
will be called
with the arguments
mycolorfunction(x, y, f(x, y, a ) a )
,
where (x, y) are
the midpoints of the rectangles and a is
the animation parameter. The color function must return an RGB
or RGBa
color
value.
When density values are specified by an array or a matrix A, the low indices correspond to the lower left corner of the graphics. The high indices correspond to the upper right corner.
Arrays/matrices do not need to be indexed from 1. E.g.,
A = array( `i_{min}` .. `i_{max}` , `j_{min}` .. `j_{max}`
, [..density values..])
yields a graphical array with
XMesh = j_{max}  j_{min} +
1
, YMesh = i_{max}  i_{min} +
1
.
If no plot range `x_{min}` .. `x_{max}`
, `y_{min}`
.. `y_{max}`
is specified,
x_{min} = j_{min} 
1
, x_{max} = j_{max}
, y_{min} =
i_{min}  1
, y_{max} =
i_{max}
is used.
When density values are specified by a list of lists L, the first entries in the list correspond to the lower left corner of the graphics. The last entries correspond to the upper right corner.
If no plot range `x_{min}` .. `x_{max}`
, `y_{min}`
.. `y_{max}`
is specified,
x_{min} = 0
, x_{max} =
m
, y_{min} = 0
, y_{max} =
n
is used, where n is the length of L and m is the (common) length of the sublists in L. All sublists ("rows") must have the same length.
Animations are triggered by specifying a range a =
`a_{min}` .. `a_{max}`
for a parameter a
that
is different from the variables x
, y
.
Thus, in animations, both the ranges x = `x_{min}` .. `x_{max}`
, y
= `y_{min}` .. `y_{max}`
as well as the animation range a
= `a_{min}` .. `a_{max}`
must be specified.
The related plot routine plot::Raster
provides
a similar functionality. However, plot::Raster
does not
use an automatic color scheme based on density values. The user must
provide RGB
or RGBa
values
instead.
Attribute  Purpose  Default Value 

AffectViewingBox  influence of objects on the ViewingBox of
a scene  TRUE 
AntiAliased  antialiased lines and points?  FALSE 
Color  the main color  RGB::Red 
DensityData  density values for a density plot  
DensityFunction  density function for a density plot  
FillColor  color of areas and surfaces  RGB::Red 
FillColor2  second color of areas and surfaces for color blends  RGB::CornflowerBlue 
FillColorType  surface filling types  Dichromatic 
FillColorFunction  functional area/surface coloring  
Frames  the number of frames in an animation  50 
Legend  makes a legend entry  
LegendText  short explanatory text for legend  
LegendEntry  add this object to the legend?  FALSE 
LineColor  color of lines  RGB::Blue 
LineWidth  width of lines  0.35 
LineStyle  solid, dashed or dotted lines?  Solid 
LinesVisible  visibility of lines  FALSE 
Mesh  number of sample points  [25 , 25 ] 
Name  the name of a plot object (for browser and legend)  
ParameterEnd  end value of the animation parameter  
ParameterName  name of the animation parameter  
ParameterBegin  initial value of the animation parameter  
ParameterRange  range of the animation parameter  
TimeEnd  end time of the animation  10.0 
TimeBegin  start time of the animation  0.0 
TimeRange  the real time span of an animation  0.0 .. 10.0 
Title  object title  
TitleFont  font of object titles  [" sansserif " , 11 ] 
TitlePosition  position of object titles  
TitleAlignment  horizontal alignment of titles w.r.t. their coordinates  Center 
TitlePositionX  position of object titles, x component  
TitlePositionY  position of object titles, y component  
Visible  visibility  TRUE 
VisibleAfter  object visible after this time value  
VisibleBefore  object visible until this time value  
VisibleFromTo  object visible during this time range  
VisibleAfterEnd  object visible after its animation time ended?  TRUE 
VisibleBeforeBegin  object visible before its animation time starts?  TRUE 
XMax  final value of parameter "x"  
XMesh  number of sample points for parameter "x"  25 
XMin  initial value of parameter "x"  
XName  name of parameter "x"  
XRange  range of parameter "x"  
YMax  final value of parameter "y"  
YMesh  number of sample points for parameter "y"  25 
YMin  initial value of parameter "y"  
YName  name of parameter "y"  
YRange  range of parameter "y" 
We generate a density plot:
p := plot::Density(cos(x^2 + y^2), x = 3..3, y = 2..2, Mesh = [60, 40]):
The plot object is rendered:
plot(p, Axes = Frame):
This turns into a black and white graphics when suitable colors are specified:
plot(plot::Scene2d(p, FillColor = RGB::White, FillColor2 = RGB::Black), plot::Scene2d(p, FillColor = RGB::Black, FillColor2 = RGB::White), Width = 120*unit::mm, Height = 45*unit::mm, Layout = Horizontal, Axes = Frame):
delete p:
We demonstrate the use of a userdefined color function:
mycolor := proc(x, y, f) begin if f >= 2/3 then RGB::Red elif f >= 1/3 then RGB::Orange; elif f >= 0 then RGB::Yellow; elif f >= 1/3 then RGB::BlueLight; elif f >= 2/3 then RGB::Blue; else RGB::SlateBlueDark; end_if; end_proc: plot(plot::Density(cos(x^2 + y^2), x = 3..3, y = 2..2, Mesh = [60, 40], FillColorFunction = mycolor), Axes = Frame):
delete mycolor:
In this example, we demonstrate how plot::Density
can
be used to plot gray data from an external source. Assume, there is
an external PortableGrayMap text file Norton.pgm
containing
data such as
P2 240 180 255 249 237 228 231 245 218 229 195 ...
P2
indicating that this is a PGM
text file. The second line contains the pixel width and pixel height
of the picture. The number 255
in the third line
is the scale of the following gray values.
The remaining data consist of integers between 0 (black) and 255 (white), each representing the gray value of a pixel (row by row).
We import the text data via import::readdata
:
graydata := import::readdata("Norton.pgm", NonNested):
This is a long list of all data items in the file. We extract the 4 items in the first three lines:
[magicvalue, xmesh, ymesh, maxgray] := graydata[1..4]
We delete the header from the pixel data. (If there are comments in the PGM file, they must be deleted, too).
for i from 1 to 4 do delete graydata[1]; end_for:
We transform the plain data list to a nested list containing
the gray data of the rows as sublists. (The call to level
is not really
necessary, but it speeds up the conversion considerably on the interactive
level.)
L := level([graydata[(i  1)*xmesh + 1 .. i*xmesh] $ i=1..ymesh], 1):
This list can be passed to plot::Density
:
plot(plot::Density(L, FillColor = RGB::White, FillColor2 = RGB::Black), Width = 80*unit::mm, Height = 60*unit::mm):
The image is upside down, because the PGM files stores the pixel
data row by row in the usual reading order starting with the upper
left corner of the image. The MuPAD^{®} routine plot::Density
,
however, follows the mathematical orientation of the coordinate axes,
i.e., the first pixel value is interpreted as the lower left corner
of the image. We have to reorder the rows in the graydata
list
via revert
:
plot(plot::Density(revert(L), FillColor = RGB::White, FillColor2= RGB::Black), Width = 80*unit::mm, Height = 60*unit::mm):
The routines import::readbitmap
and plot::Raster
provide
an alternative way to import and display the bitmap image. See the help page of plot::Raster for examples.
This, however, takes more memory, because the bitmap data are imported
as RGB
color
values, whereas only density values (gray data) are needed for plot::Density
.
delete graydata, magicvalue, xmesh, ymesh, maxgray, i, L:
The Mandelbrot set is one of the bestknown fractals. It arises when considering the iteration z_{n + 1} = z_{n}^{2} + c, z_{0} = 0 in the complex plane. For sufficiently large values c of the complex parameter c, the sequence z_{n} diverges to infinity; it converges for sufficiently small values of c. The boundary of the region of those c values that lead to divergence of z_{n} is of particular interest: this border is highly complicated and of a fractal nature.
In particular, it is known that the series z_{n} diverges
to infinity, whenever one of the iterates satisfies z_{n}
> 2. This fact is used by the following procedure f
as
stopping criterion. The return value provides information, how many
iterates z_{0},
…, z_{n} it
takes to escape from the region z
≤ 2 of (potential) convergence. These
data are to be used to color the complex c plane
(i.e., the (x,y)
plane) by a density plot:
f := proc(x, y) local c, z, n; begin c := x + I*y: z := 0.0: for n from 0 to 100 do z := z^2 + c: if abs(z) > 2 then break; end_if; end_for: if n < 70 then n mod 5; else n  70; end_if; end_proc:
Depending on your computer, the following computations may take
some time. On a very fast machine, you can increase the following
values of xmesh
, ymesh
. This
will use up more computing time but will lead to better graphical
results:
xmesh := 100: ymesh := 100:
The following region in the xy plane is to be considered:
xmin[1] := 2.0: xmax[1] := 0.5: ymin[1] := 1.2: ymax[1] := 1.2:
The region xmin_{1} ≤ x ≤ xmax_{1}, ymin_{1} ≤ y ≤ ymax_{1} is
divided into xmesh×ymesh rectangles.
Each rectangle is colored by a density plot according to the "escape
times" computed by the procedure f
. This
procedure can be passed directly to plot::Density
:
p1 := plot::Density(f, x = xmin[1].. xmax[1], y = ymin[1] .. ymax[1], Mesh = [xmesh, ymesh], FillColor = RGB::Black, FillColor2 = RGB::Red):
In addition, a rectangle is produced that indicates a region that is to be magnified in the following:
xmin[2] := 0.24: xmax[2] := 0.01: ymin[2] := 0.63: ymax[2] := 0.92: r1 := plot::Rectangle(xmin[2] .. xmax[2], ymin[2] .. ymax[2], LineColor = RGB::White):
plot(p1, r1):
The density values of the blowup are not computed directly
by plot::Density
. They are computed separately
and stored in an array A:
dx := (xmax[2]  xmin[2])/xmesh: dy := (ymax[2]  ymin[2])/ymesh: A := array(1..ymesh, 1..xmesh, [[f(xmin[2]+ (j  1/2)*dx, ymin[2] + (i  1/2)*dy) $ j = 1..xmesh] $ i = 1..ymesh]): p2 := plot::Density(A, x = xmin[2] .. xmax[2], y = ymin[2] .. ymax[2], FillColor = RGB::Black, FillColor2 = RGB::Red):
In addition, a further rectangle is produced to indicate a region of interest to be blown up lateron:
xmin[3] := 0.045: xmax[3] := 0.015: ymin[3] := 0.773: ymax[3] := 0.815: r2 := plot::Rectangle(xmin[3] .. xmax[3], ymin[3] .. ymax[3], LineColor = RGB::White):
plot(p2, r2):
The density values of the next blowup are again computed separately and stored in a nested list L:
dx := (xmax[3]  xmin[3])/xmesh: dy := (ymax[3]  ymin[3])/ymesh: L := [[f(xmin[3] + (j  1/2)*dx, ymin[3] + (i  1/2)*dy) $ j= 1..xmesh] $ i = 1..ymesh]: p3 := plot::Density(L, x = xmin[3] .. xmax[3], y = ymin[3] .. ymax[3], FillColor = RGB::Black, FillColor2 = RGB::Red):
plot(p3):
The density objects are to be placed in a single graphics. It
consists of the Mandelbrot set p1
as computed above
and of modifications of the density plots p2
and p3
.
Redefining the attributes XRange
, YRange
,
we move p2
, p3
to places in
the xy plane
where they are not overlapped by p1
. Note that
this does not change the graphical content of p2
, p3
,
because it is given by the data A and L,
respectively, which remain unchanged. (If the ranges were changed
in p1
, another plot
call of p1
would
call the procedure f
at different points of the
plane resulting in a different graphics.)
p2::XRange := 0.60 .. 1.60: p2::YRange := 0.05 .. 1.15: p3::XRange := 0.60 .. 1.60: p3::YRange := 1.15 .. 0.05:
The Mandelbrot set and the two blowups are placed in one scene.
In addition, some arrows are added to indicate the origin of the blowups.
Note that it is quite important here that the arrows are passed to
the plot
command
after the density plots. Otherwise, they would be hidden by the density
plots: graphical objects are painted in the ordering in which they
are passed to plot
:
plot(p1, p2, p3, plot::Arrow2d([(xmin[2] + xmax[2])/2, (ymin[2] + ymax[2])/2], [(p2::XMin + p2::XMax)/2, (p2::YMin + p2::YMax)/2], LineColor = RGB::Blue), plot::Arrow2d([1.50, 0.65], [(p3::XMin + p3::XMax)/2, (p3::YMin + p3::YMax)/2], LineColor = RGB::Blue) ):
delete f, xmesh, ymesh, xmin, xmax, ymin, ymax, dx, dy, p1, p2, p3, r1, r2, A, L:

The density values: an arithmetical expression in 2 variables x, y and the animation parameter a. Alternatively, a procedure that accepts 2 input parameters x, y or 3 input parameters x, y, a and returns a real density value.


Name of the horizontal variable: an identifier or an indexed identifier.


The range of the horizontal variable:


Name of the vertical variable: an identifier or an indexed identifier.


The range of the vertical variable:


An array of domain type


A list of lists of numerical density values or expressions of
the animation parameter a.
Each sublist of L represents
a row of the graphical array. The number of sublists in L yields
the value of the attribute


Animation parameter, specified as 