The simplest code is
Here bounding box is drawn. You can see that symbol '#' start comment (any character after it is ignored). Also you may note that arguments of commands is delimited one from another by space (or by tabulation).
The script is executed line-by-line and any settings influence only on later commands. This is the matter why I put command rotate 40 60 first (before drawing command) and this is main difference from some other plotting languages.
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Let me show command of axes transformation. MathGL has the following commands: subplot, inplot, aspect and rotate. The order of its calling is strictly determined. At first one changes the position of axes in image area (commands subplot and inplot). After it one may rotate plot (command rotate). Finally one may change aspects of axes (command aspect). The following code illustrates it:
Here I used command text for printing text in arbitrary position of picture. Text coordinates and size is connected with axes. However, text coordinates may be everywhere, including outside the bounding box. I shall show its features later. String arguments is denoted by symbol ' at start and at the end of the string.
MathGL library can draw not only the bounding box but also the axes, grids, labels and so on. The limits of axes and its origin (the point of intersection) are determined by command axis with numerical arguments. Ticks on axis are specified by commands xtick, ytick, ztick. If its first argument is positive then it gives the step between ticks, if it is negative then it gives the number of ticks on the axis, if it is zero then logarithmic ticks is drawn. The second argument give numbers of subticks between ticks.
Command axis with one string or without arguments draws axes. Its textual string (if present) shows in which directions the axis or axes will be drawn (by default 'xyz', command draws axes in all directions). command grid draws grid perpendicularly to specified directions. Example of axes and grid drawing is:
This example shows the importance of correct choosing of the number of ticks on axis. If tick step is too small then its text may overlap and become unreadable. This code has the example of [xyz]label command. It draws label for axis in specified direction. The text position on axis is specified by second argument of [xyz]label command. If it is positive then text is drawn near maximum of axis, if negative then near minimum of axis, if zero then at center of axis.
Next point is the using of curvilinear coordinates. In difference from other systems of plot creation, MathGL uses textual formulas for connection of old (data) and new (output) coordinates. This allows one to plot in arbitrary coordinates. The following code plots the line y=0, z=0 in Cartesian, polar, parabolic and spiral coordinates:
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MathGL prints text by vector Hershey font. There are commands for manual specifying of text position (like text) and for its automatic selection (like [xyz]label, legend and so on). MathGL prints text always in specified position even if it lies outside the bounding box. The default size of font is specified by second argument of font command. However, the actual size of output string depends on position of axes (depends on commands subplot, inplot). Switching of font style (italic, bold, gothic, script and so on) can be done for the whole string (by command parameter) or inside the string. By default MathGL parses TeX-like commands for symbols and indexes. Example of MathGL font drawing is:
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Variable name is arbitrary combination of symbols (except spaces and ') started from a letter and with length less than 64. It is possible to use subarrays (like in subdata command) as command argument. For example, a(1) or a(1,:) or a(1,:,:) is second row, a(:,2) or a(:,2,:) is third column, a(:,:,0) is first slice and so on. Special variables nan=#QNAN, pi=3.1415926..., on=1, off=0, :=-1 are treated as number if they were not redefined by user. Before the first using all variables must be defined with the help of commands new, var, list, copy or read. Note, that if a variable with the same name exists then it will be overwritten during definition.
One can create a variable and fill it by several ways. Let me show it:
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MathGL has command for data processing: differentiating, integrating, smoothing and so on. Let us consider some examples. The simplest ones are integration and differentiation. The direction in which operation will be performed is specified by textual string, which may contain symbols 'x', 'y' or 'z'. For example, call diff a 'x' will differentiate data along x direction; call integrate a 'xy' will do double integration of data along x and y direction; call diff2 c 'xyz' will apply 3d Laplace operator to data and so on. Example of this operations on 2d array a = x*y is presented in code:
Data smoothing (command smooth) is more interesting and important. This command has 2 main arguments: type of smooting and its direction. Now 4 methods are supported: 0 – does nothing for delta=0 or approaches data to zero with the step delta, 1 – linear averaging by 3 points (default), 2 – linear averaging by 5 points, 3 – quadratic (parabolic) averaging by 5 points. Let me demonstrate it for 1d case:
Finally one can create new data arrays on base of existing one: extract slice, row or column of data (subdata), summarize along some of direction(s) (sum), find distribution of data elements (hist). Note that all these commands require the definedvariables.
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Let me now show how to plot data. MathGL generally has 2 types of plotting commands. Simple variant require singles data array for plotting, other data (coordinates) are considered uniformly distributed in interval specified by axis or [xyz]range commands. Second variant requires data arrays for all coordinates. It allows one to plot rather complex multivalent curves and surfaces (in case of parametric dependencies). Arguments setting to default values allows one to plot data in standard form. Manual arguments setting gives possibility for fine tuning of colors, positions and view of graphics. Note that the call of drawing command adds something to picture butdoes not clear the previous plots (as it does in Matlab). Another difference from Matlab is that all setup (like transparency, lightning, axis borders and so on) must be specified before plotting commands.
Term "1D data" means that data depend on single index (parameter) like curve in parametric form {x(i),y(i),z(i)}, i=1...n. There are 5 generally different types of data representations: simple line plot (plot), line plot with filling under it (area), stairs plot (step), bar plot (bars) and vertical lines (stem).Each type of plotting has similar interface. There are 3D version and two 2D versions. One of last requires single array. The parameters of line and marks are specified by the string argument. If the string parameter is '' then solid line with color from palette is used.
Below I shall show the features of 1D plotting on base of plot command. Let us start from sinus plot:
Style of line is not specified in plot command. So MathGL uses the solid line with first color of palette (this is blue). Next subplot shows array y1 with 2 rows:
As previously I did not specify the style of lines. As a result, MathGL again uses solid line with next colors in palette (there are green and red). Now let us plot a circle on the same subplot. The circle is parametric curve x=cos(π t), y=sin(π t). I will set the color of the circle (dark yellow, 'Y') and put marks ('+') at point position:
Note that solid line is used because I did not specify the type of line.
Drawing in 3D space is mostly the same. Let us draw spiral with default line style. Now its color is 4-th color from palette (this is cyan):
Note that line style is empty ('') here. Usage of other 1D plotting commands looks similar:
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Surfaces surf and other 2D plots (mesh, dens, cont, boxs, axial, tile, fall, belt, contf) are drown the same simpler as 1D one. The difference is that the string parameter specifies not line style but the color scheme of the plot. Here I draw attention on 4 most interesting color schemes. There is gray scheme where color is changed from black to white (string 'kw') or from white to black (string 'wk'). Another scheme is useful for accentuation of negative (by blue color) and positive (by red color) regions on plot (string 'BbwrR'). Last one is the popular "jet" scheme (string 'BbcyrR').
Now I shall show the example of a surface drawing. At first let us switch lightning on
and draw the surface, considering coordinates x,y to be uniformly distributed in bounding box
Color scheme was not specified. So previous color scheme is used. In this case it is default color scheme ("jet") for the first plot. Next example is a sphere. The sphere is parametrically specified surface:
I set color scheme to 'BbwrR' that corresponds to red top and blue bottom of the sphere.
Surfaces will be plotted for each of slice of the data if nz>1. Next example draws surfaces for data arrays with nz=3:
Note that it may entail a confusion. However, if one will use density plot then the picture will look better:
Note that the previous color scheme is used in last plots because there are no direct specification of the one.
Drawing of other 2D plots is analogous. The only peculiarity is the usage of flag '#' in string argument. By default this flag switches on the drawing of a grid on plot (grid or mesh for plots in plain or in volume). However, for isosurfaces (including surfaces of rotation axial) this flag switches the face drawing off. Figure becomes wired. The following code gives example of flag '#' using (compare with normal command drawing as in its description):
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Drawing procedures for 3D plot looks analogously ones for 1D and 2D plots described above. There are 3 general types of 3D plots: (i) plots on slices or on projections (dens3, cont3), (ii) isosurfaces (surf3), (iii) cloud-like plots (cloud). Plots on slice are clear enough – one specifies a slice (as its index or as value of coordinate) and MathGL draws contour lines or density plot on slice plane. Isosurface gives more information. Isosurface is 3D analog of contour line cont. It shows the region where values of data array exceed specified isosurface level. Plot becomes more informative if one adds transparency, lightning or set color scheme depending on coordinates. Generalization of isosurface is cloud-like plot. This plot draws by darker color and less transparent regions with high values of data. Contrary the regions with low values are transparent. For plotting of the phase of fields (or beams or pulses) one can use isosurface which transparency depends on other data array (see command surf3a). As example of 3D data plots let us draw Gaussian beam diffraction in space. Beam propagates along x axis:
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MathGL library has advanced features for setting and handling the surface transparency. The simplest way to add transparency is the using of command alpha. As result all further surfaces (and isosurfaces, density plots and so on) become transparent. However, its look can be additionally improved.
First, the selected surface will be non-transparent if one sets transparento ff before the surface drawing and sets it on after the drawing.
Second, the value of transparency can be different from surface to surface. To do it just change the value of alphadef before the drawing of selected surface. If its value is close to 0 then the surface becomes more and more transparent. Contrary, if its value is close to 1 then the surface becomes practically non-transparent. This is some analog of transparent off. One may use option alpha here to change alphadef only for one graphics:
Third feature is the changing of the way how the light goes through overlapped surfaces. The command transptype defines it. By default the usual transparency is used (transptype 0) – surfaces below is less visible than upper ones. A "glass-like" transparency (transptype 1) has a different look when the surface just decreases the background light (the surfaces are commutable in this case). A "neon-like" transparency (transptype 2) has more interesting look. In this case a surface is the light source (like lamp on dark background) and just adds some intensity to color. At this the library automatically sets black color for background and changes default line color to white.
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In this section I have included some small hints and advices for the improving of the quality of plots and for the demonstration of some non-trivial features of MathGL library. In contrast to previous examples here I showed mostly the idea but not the whole drawing command.
The usual way to change bounding box is the using of axis command. But if you don't know the range of data or you want to change only one dimension (for example x) then the [xyzc]range command is a solution. This command has a variant for the manual rage setting (for example, xrange 0 1) and has a variant for automatic data rage determining (for example, zrange a).
It is much more interesting to use these commands as options. In these options one should define the range of auto-filled variables. For example, command plot y; xrange 0 2 will plot data y with x-coordinate distributed in range [0, 2]. At this, the range may exceed the bounding box (so that only part of data will be plotted).
One more interesting feature is that auto-filled coordinate vary from the first argument of the option to the second one. This allows to make mirrored plots:
The speeding up of the drawing is important task when the script is used often (for example, in UDAV program). There are several things that can be done.
once on and once off. As a result this code will be executed only once and will be omitted during coordinate rotation and other tuning action.As I have noted above, MathGL commands (except the special one, like clf) do not erase the previous plotting but just add the new one. It allows one to draw easily "compound" plots. For example, popular Matlab command surfc can be emulated in MathGL by 2 calls:
Here a is 2-dimensional data for the plotting, -1 is the value of z-coordinate at which the contour should be plotted (at the bottom in this example). Analogously one can draw density plot instead of contour lines and so on.
Another nice plot is contour lines plotted directly on the surface:
The possible difficulties arise in black&white case, when the color of the surface can be close to the color of a contour line. In that case I may suggest the following code:
The idea is to divide the color range on 2 parts (dark and bright) and to select the contrasting color for contour lines for each of part.
Similarly one can plot flow thread over density plot of vector field amplitude (this is another amusing plot from Matlab) and so on. The list of compound graphics can be prolonged but I hope that the general idea is clear.
Developing the previous hint one can make a plot with 2 or more axes. The idea is that the change of settings does not influence on the already drawn graphics. So, for 2-axis plot let us set the first axis and draw everything concerning it. Then let us setup the second axis and draw things for the second axis. The corresponding code is:
Note, that the first and the second axes look better if being placed in different corners. In the code presented above the first axis is placed in the left-bottom corner, and the second one is placed in the right-top corner.
The printing of nice titles for the plot is not so trivial task in general case. The problem is that rotation and aspect change lead to different looks for titles of different subplots. So, the resulting look is not so good as it could be. The solution is simple – to print titles exactly after subplot call and before any rotation, aspect change and so on! Analogously, the title for the whole picture looks better if it is printed first (before any subplot calls).
By default (for the user comfort), the color range is set equal to z-range of the plot. However, there are different ranges. So, one can obtain an amusing plot by the change of color range manually. For example, there are plots with one-color bottom (or top) or practically bi-color picture and so on. For example, compare 2 surfaces:
Sometimes an experimental or numerical surface has outstanding points. Visualization of such surface will lead to the hole(s) in place of such points. The standard method of "fighting" – tochange data values – is not always good and is not so convenient. MathGL library has another method – to set cut off. As a consequence, all outstanding point will be projected on the bounding box.
Such method is good not only for outstanding points but also for the case when one need to plane the bottom or the top of the plot. Exactly such case is demonstrated in the code:
It is an interesting result, is not it? Note that the same result can be achieved by options cut:
Vector field visualization (especially in 3d case vect or vectc) may look tangly – there are too many overlapping lines. I may suggest 2 ways to solve this problem. The first one is to change meshnum the number of hachures for decreasing. The second way is to use the flow thread chart flow. Unfortunately, I don't know any other methods to visualize 3d vector field. If you know any, e-mail me and I shall add it to MatGL.
In contrast to the most of the other programs, MathGL supports several (up to 10) light sources. Moreover, the color each of them can be different: white (this is usual), yellow, red, cyan, green and so on. The use of several light sources may be interesting for the highlighting of some peculiarities of the plot or just to make an amusing picture. Note, each light source can be switched on/off individually.
MathGL library has a feature for cutting of points in some region cut. Such an excision can be used to improve the look of the graphics. Moreover, this cutting may help to show an internal structure of an object (like isocaps plot in Matlab). For example, let us use the standard data array and show its interior:
Sometime ago I worked with mapping and have a question about its visualization. Let me remind you that mapping is some transformation rule for one set of number to another one. The 1d mapping is just an ordinary command – it takes a number and transforms it to another one. The 2d mapping (which I have used) is a pair of commands which take 2 numbers and transform them to another 2 ones. Except general plots (like surfc, surfa) there is a special plot – Arnold diagram. It shows the area which is the result of mapping of someinitial area (usually square).
I tried to make such plot in map. It shows the set of points or set of faces, which final position is the result of mapping. At this, the color gives information about their initial position and the height describes Jacobian value of the transformation. Unfortunately, it looks good only for the simplest mapping but for the real multivalent quasi-chaotic mapping it produces a confusion. So, use it if you like :).
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