Python图形艺术指南：2D与3D图像创作秘籍

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11 plt.grid(True)

13 plt.show()

Listing 1-1 produces the plotting area shown in Figure1-3. It has a horizontal width

of 20 and a vertical height of 20. I could have made these numbers 200 and 200, and

the area would appear in an output pane as the same physical size but with different

numerical values on the axes. Line 13 contains the command plt.show(). The purpose

of this is to display the program’s results in the output pane. With modern versions of

Python it isn’t required since the plots are automatically displayed when the program is

run. With older versions it may or may not be displayed. plt.show() can also be placed

within a program in order to show plots created during execution. Even though it may

not be necessary, it’s a good idea to include this command at the end of your script since

it can serve as a convenient marker for the end of your program. Lines 1, 2, 10, and 11in

Listing 1-1 will be explained in the following sections. These commands, or variations of

them, will appear in all of our programs.

Figure 1-3. Plotting area produced by Listing 1-1 with (0,0) located in the center

of the area

Chapter 1 essential python Commands andFunCtions

1.4 Importing Plotting Commands

While Python has many built-in commands and functions available, some math and

graphics commands must be imported. Lines 1 and 2in Listing 1-1 do this. The import

numpy as np statement in line 1 imports math functions such as sin(ϕ), e

, and so on.

The np in this statement is an abbreviation that may be used when referring to a numpy

function. When used in a program, these functions must be identified as coming from

numpy. For example, if you were to code v=e

, the program statement would be v=np.

exp(α) where α would have been previously defined. You don’t have to write out the full

length numpy.exp(α) since you defined the shorthand np for numpy in line 1. Graphics

commands are handled similarly. The statement import matplotlib.pyplot as plt

imports the library pyplot, which contains graphics commands. plt is an abbreviation

for pyplot. For example, if you want to plot a dot at x,y you would write plt.scatter(x,y).

I will talk more about plt.scatter() shortly.

Functions may also be imported directly from numpy. The statement from numpy

import sin, cos, radians imports the sin(), cos(), and radians() functions. When

imported in this manner they may be used without the np prefix. There is also a math

library that operates in a similar way. For example, from math import sin, cos, radians

is equivalent to importing from numpy. You will be using all these variations in the

coming programs.

There is also a graphics library called glib that contains graphics commands. glib

uses a different syntax than pyplot. Since pyplot is used more widely, you will use it in

your work here.

Line 8in Listing 1-1, plt.axis([x1,x2,y1,y2]), is the standard form of the command

that sets up the plotting area. This is from the pyplot library and is preceded by the

plt. prefix. There are attributes to this command and there are other ways of defining

a plotting area, notably the linspace() command, but the form in line 8 is sufficient

for most purposes and is the one you will use. x1 and x2 define the values of the left

and right sides, respectively, of the plotting area; y1 and y2 define the bottom and top,

respectively. With the numeric values in lines 8-11 you get the plotting area shown in

Figure1-3. x1,x2,y1, and y2 always have the locations shown in Figure1-3. That is, x1 and

y1 always refer to the lower left corner, y2 to other end of the y axis, and x2 to the other

end of the x axis. Their values can change, but they always refer to these locations. They

may be negative, as shown in Figure1-4.

Chapter 1 essential python Commands andFunCtions

1.5 Displaying thePlotting Area

In line 10 of Listing 1-1 the statement plt.axis('on') displays the plotting area with its

frame and numerical values. If you omit this command, the frame will still be displayed

with numerical values. So why include this command? Because, when creating a plot it

is sometimes desirable to turn the frame off. To do that, replace plt.axis('on') with plt.

axis('off'). Having the command there ahead of time makes it easy to type 'off' over

'on' and vice versa to switch between the frame showing and not showing. Also, after

you have finished with a plot, you may wish to use it in a document, in which case you

may not want the frame. Note that 'on' and 'off' must appear in quotes, either single or

double.

1.6 The Plotting Grid

Line 11 of Listing 1-1, plt.grid(True), turns on the dotted grid lines, which can be an aid

when constructing a plot, especially when it comes time to position textual information.

If you do not include this command, the grid lines will not be shown. To turn off the

grid lines, change the True to False. Note the first letter in True and False is capitalized.

True and False do not appear in quotations marks. As with plt.axis(), having the plt.

grid(True) and plt.grid(False) commands there makes it easy to switch back and forth.

Again, note that both True and False must have the first letter capitalized and do not

appear in quotes.

1.7 Saving aPlot

The easiest way to save a plot that appears in the output pane is to put your cursor over it

and right-click. A window will appear allowing you to give it a name and specify where it

is to be saved. It will be saved the .png format. If you are planning to use it in a program

such as Photoshop, the .png format works. Some word processing and document

programs may require the .eps (encapsulated Postscript) format. If so, save it in the .png

format, open it in Photoshop, and resave it in the .eps format. It’s a bit cumbersome but

it works.

Chapter 1 essential python Commands andFunCtions

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