I already posted before about the how to use transformations with Kivy. There was still some gaps in the explanation. After a while of being working with Kivy I have a much better understanding of what it is happening so I came with a list of 10 things that you should know about the Kivy canvas.

  1. A Kivy canvas is not the space in which you paint. 

    The major problems I had at the beginning with the canvas were due to its name. Particularly considering all the buzz about HTML5 canvas. I initially visualize the canvas as the cloth that painters paint. Since all the widgets has one canvas, I started to get confused and asked why did the Widget seem to share the same canvas?

  2. The answer needs, first, to clarify what is the canvas? (2nd bullet point) and where is it that the widgets share? (3rd bullet point)

  3. A Kivy canvas is a set of instructions! 

    A canvas is basically a container of instructions. More like a recipe that tells you how to draw, rather than a place to draw. In other words the bitmap is not saved (like .bmp), but the vectors (like .gif). The canvas of the Widget basically keep the instructions to draw that performs the drawing.

  4. The widgets share the same coordinate space, not the same canvas.

    The final answer to my previous question is that all the widgets have a different canvas but all the canvas draw in exactly the same drawing space, i.e. the coordinate space. Moreover, the drawing space is not limited to the position and size of the widget. The 0,0 of the drawing space is always (except for the RelativeLayout and the ScatterLayout ) the bottom-left corner. If we want to draw a Rectangle that covers the whole area of the Widget we should do something like this:

    ...
    Widget:
        canvas:
            Rectangle:
                pos: self.pos
                size: self.size

    Notice that we need to specify explicitly that the size and pos of the Rectangle is the same of the Widget. There lies…

  5. … the little secret of RelativeLayout.

    The previous code doesn’t work with RelativeLayout. Instead we actually use the coordinate (0,0) to achieve the same result:

    ...
    Widget:
        canvas:
            Rectangle:
                pos: 0,0
                size: self.size

    The RelativeLayout includes a Translate instruction:

    Translate:
        xy: self.pos
    

    That instruction is going to translate the canvas to the self.pos position. In other words, the (0,0) is now self.pos.

  6. PushMatrix and PopMatrix to control the effects of sharing the same drawing space.

    Sharing the same drawing space has its consequences. The problem is that the instructions of one Widget canvas may affect the next one. If I rotate the drawing space, the drawing space will be rotated until some other instructions rotate it back again. It is up to us to guarantee that the context of the drawing space remains the same when the canvas stop drawings. Kivy provides two instructions PushMatrix and PopMatrix. RelativeLayout uses a PushMatrix before the instructions of the canvas and a PopMatrix after it.

    PushMatrix:
    ...
    Translate:
        xy: self.pos
    ...
    PopMatrix:
    

    The PushMatrix saves the current context state (rotations, translations, zoomings) and the PopMatrix recovers it and let thing as they were before.

  7. The color is an exception.

    That said, PushMatrix and PopMatrix doesn’t have an effect over the Color instruction.

    PushMatrix:
    Color:
        rgb: 1,0,0
    Rectangle:
        ...
    PopMatrix:
    Line:
        ... 
    

    It doesn’t matter which was the previous Color before PushMatrix, the Rectangle and and the Line would be red (rgb: 1,0,0).

  8. The order in which canvas instructions are executed.

    The execution order follow a traversal order. There is recursivity involved but, let’s stick with what you have to consider. When you are using a Widget canvas, the instructions of all the canvas of the parent and grandparents were executed. Also the instructions of the canvas of the first added siblings. The instructions of the canvas children hasn’t been executed yet. Neither the ones of the siblings added after. When you are coding you actually perceive a normal top-down execution and thinking that way is ok for most of the cases. When you start using dynamic interfaces, adding and removing children, strange things seem to happen and you will need to remember the order of execution. Perhaps, more important is to know how to do to manipulate this order.

  9. Drawing after the children with canvas.after

    What happens if we want to execute instructions after the children were added. In that case we have another canvas (called canvas.after) that is called after traverse all the children.

    Widget:
        canvas:
            ...
        canvas.after:
            ...
        Button:
            canvas.before:
               ...
            canvas:
               ...
            canvas.after:
               ...
    

    The canvas execution order is as follows:
    (1) The Widget canvas
    (2) All the Button canvas (canvas.before, canvas, canvas.after)
    (3) The Widget canvas.after
    Notice that all the canvas of the Button are executed in between canvas and canvas.after.

  10. What about canvas.before?

    You probably notice that there is a third canvas (canvas.before). It executes just before the canvas. I found the canvas.before particularly useful when
    (1) I need to manipulate the Color of an instance and, (2) I need to manipulate inheritance. Let’s say I have MyWidget that draws a Rectangle inside.

    <MyWidget@Widget>:
        canvas:
            Rectangle:
            ...
    

    So I can modify the Color of a particular instance doing something like this:

        ...
        mywidget = MyWidget
        mywidget.canvas.before:
            Color(rgba=(1,0,0,0))
    

    If I use canvas instead of canvas.before in RedWidget, the instruction is going to be executed after the Rectangle, so it won’t apply to the Rectangle.
    (2) Similarly, when I need to manipulate inheritance. Let’s say I have MyWidget that draws a Rectangle inside. I can do the following to change the color of the base class Rectangle:

    <RedWidget@MyWidget>:
        canvas.before:
            Color:
                rgb: 1,0,0
    

    The RedWidget inherits from MyWidget and I would have the same problem if I just use the canvas. Of course, similar applications goes for the canvas.after.

  11. Drawing on top of a Button Widget with canvas.after.

    This works for any other Widget and not just for the Label but it is probably the most common case. If you want to draw over the Button background you can use the canvas.after. Otherwise the background will cover your drawings:

    Button:
        text:
        canvas.after:
            Rectangle:
                pos: self.pos
                size: self.size
    

    The rectangle of the previous code will actually cover its text.

This post turned out to be longer than I was expected. I hope I didn’t make any mistake in my explanation. If so, please let me know.

8 thoughts on “10 things you should know about the Kivy canvas

  1. Hi Roberto,

    Thanks for these explanations about Canvas in Kivy. I would say that there is not so much documentation about all that… so it’s complex to do something out of the standard (and too much simple) way.

    Do you have an idea on how to change the source string of the Reclangle when you have this (when I click on a button, by example) :
    ListItemButton:
    id: iconListItemButton_id
    canvas.before:
    Color:
    rgba: 1,1,1, 1
    Rectangle:
    source: “images/ic_action_star.png”
    pos: self.pos
    size: self.size

    I’ve tried this in Python:
    theRect = iconButton.canvas.before.get_group(“? ? ?”)
    to change the source string, but no name group match on the Reclangle.

    Is it a way to get the Reclagle Instruction (and play with it in Python language) ?

    Julien

    Reply
  2. Helpful thanks. I have a question about screen transitions. I’m using canvas to set screen backgrounds but the problem is that unless you use canvas.after, you get black in between screens and not a nice smooth transition. I don’t know why it’s only smooth with canvas.after, I can’t find any documentation.

    Problem described here with gifs:
    http://stackoverflow.com/questions/39724267/kivy-black-screen-between-transitions/

    The problem with filling the screen with canvas.after of course is that it will cover up every other widget. I can’t seem to find a way to have smooth transitions with canvas.before.

    Is there a way to use canvas.after but somehow add widgets on top of that? Or do you know another way to solve this problem?

    Thanks.

    Reply
    • Thanks for your question. I added an answer to the original question (http://stackoverflow.com/a/40093938/1060349). I tested, and I have no problems using `canvas` or `canvas.before`. I also reply your comment in another answer I gave regarding the canvas (http://stackoverflow.com/a/17007957/1060349). I tried to expand on the use of the canvas. There is a couple of sections in my book that deal with the canvas that you might find useful, or you can ask me directly (preferable with a stackoverflow link that includes code sample) when you get stuck in anything.

      I hope I had helped, and if not don’t hesitate in asking again. Cheers!

      Reply
  3. Pingback: Kivy For The Non Computer Scientist – Part 11: Using KV Language To Set Widget Text Size, Text Position And Text And Button Colours | ....brad's blog....

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