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Canvas Widgets
Introduction
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The Canvas widget supplies graphics facilities for Tkinter. Among these graphical objects are lines, circles, images, and evenother widgets. With this widget it's possible to draw graphs and plots, create graphics editors, and implement various kinds of custom widgets.We demonstrate in our first example, how to draw a line.
The method create_line(coords, options) is used to draw a straight line. The coordinates 'coords' aregiven as four integer numbers: x1, y1, x2, y2 This means that the line goes from the point (x
![Canvas Canvas](https://www.featurepics.com/StockImage/20120414/hand-drawing-flowers-stock-illustration-2198562.jpg)
We kept the first example intentionally very simple. We create a canvas and draw a straight horizontal line into this canvas. This line vertically cuts the canvas into two areas.
The casting to an integer value in the assignment 'y = int(canvas_height / 2)' is superfluous, becausecreate_line can work with float values as well. They are automatically turned into integer values. In the following you can see the code of our first simple script:
If we start this program, using Python 3, we get the following window:
For creating rectangles we have the method create_rectangle(coords, options). Coords isagain defined by two points, but this time the first one is the top left point and the bottomright point of the rectangle.
The window, you see above, is created by the following Python tkinter code:The following image with the coordinates will simplify the understanding of application of create_lines and create_rectangle in our previous example.
Text on Canvas
We demonstrate now how to print text on a canvas. We will extend and modify the previous examplefor this purpose. The method create_text() can be applied to a canvas object to write text on it.The first two parameters are the x and the y positions of the text object. By default, the text is centred on this position. You can override this with the anchor option. For example, if the coordinate should be the upper left corner, set the anchor to NW. With the keyword parameter text, we can define the actual text to be displayed on the canvas.Though the code of our example program is changed drastically, the graphical result looks still thesame except for the text 'Python':
You can understand the benefit of our code changes, if you change for example the height of the canvas to 190 and the width to 90 and modify the ratio for the first box to 0.3. Image doing this in the code of our first example. It would be a lot tougher. The result looks likethis:
Oval Objects
An oval (or an ovoid) is any curve resembling an egg (ovum means egg in Latin). It resembles an ellipse,but it is not an ellipse. The term 'oval' is not well-defined. Many different curves are called ovals, but they all have in common:Canvas Draw 6 0 2 X 2
- They are differentiable, simple (not self-intersecting), convex, closed, plane curves
- They are very similar in shape to ellipses
- There is at least one axis of symmetry
We can create an oval on a canvas c with the following method:
This method returns the object ID of the new oval object on the canvas C.
The following script draws a circle around the point (75,75) with the radius 25:
We can define a small function drawing circles by using the create_oval() method.
Painting Interactively into a Canvas
We want to write an application for painting or writing into a canvas. Unfortunately, there isno way to paint just one dot into a canvas. But we can overcome this problem by using a small oval:Canvas Draw 6 0 2 X 20 44 X
Drawing Polygons
If you want to draw a polygon, you have to provide at least three coordinate points:create_polygon(x0,y0, x1,y1, x2,y2, ..)
In the following example we draw a triangle using this method:
It looks like this:
When you read this, there may or not be Christmas soon, but we present a way to improve yournext Christmas with some stars, created by Python and Tkinter. The first star is straight forward withhardly any programming skills involved:
As we have mentioned, this approach is very unskilful. What if we have to change the size or the thickness of the star? We have to change all the points manually, which is of coursean error-prone and tedious task to do. So, we present a new version ofthe previous script which involves more 'programming' and programming skills. First, we putthe creation of the star in a function, and we use an origin point and two lengths p and t tocreate the star:
Our new improved program looks like this now:
The result looks even more like Xmas and we are sure that nobody doubts that it would be hell todefine the polygon points directly, as we did in our first star example:
Bitmaps
Canvas Draw 6 0 2 X 2 Answer
The method create_bitmap() can be be used to include a bitmap on a canvas.The following bitmaps are available on all platforms:'error', 'gray75', 'gray50', 'gray25', 'gray12', 'hourglass', 'info', 'questhead', 'question', 'warning'
The following script puts all of these bitmaps on a canvas:
The result looks like this:
The Canvas Image Item
The Canvas method create_image(x0,y0, options ..) is used to draw an image on a canvas. create_image doesn't accept an image directly. It uses an object which is created by the PhotoImage() method. The PhotoImage class can only read GIF and PGM/PPM images from filesThe window created by the previous Python script looks like this:
Exercise
Write a function, which draws a checkered pattern into a canvas. The function gets called withcheckered(canvas, line_distance). 'canvas' is the Canvas object, which will be drawn into. line_distanceis the distance between the vertical and horizontal lines.Solution
The result of the previous script looks like this: