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Canvas

The Canvas is the root object used for drawing by a Widget. Check the class documentation for more information about the usage of Canvas.

class kivy.graphics.instructions.Instruction

Bases: kivy.event.ObjectWithUid

Represents the smallest instruction available. This class is for internal usage only, don’t use it directly.

proxy_ref

Return a proxy reference to the Instruction i.e. without creating a reference of the widget. See weakref.proxy for more information.

New in version 1.7.2.

class kivy.graphics.instructions.InstructionGroup

Bases: kivy.graphics.instructions.Instruction

Group of Instructions. Allows for the adding and removing of graphics instruction.

add()

Add a new Instruction to our list.

clear()

Remove all the Instructions.

get_group()

Return an iterable for all the Instructions with a specific group name.

insert()

Insert a new Instruction into our list at index.

remove()

Remove an existing Instruction from our list.

remove_group()

Remove all Instructions with a specific group name.

class kivy.graphics.instructions.ContextInstruction

Bases: kivy.graphics.instructions.Instruction

The ContextInstruction class is the base for the creation of instructions that don’t have a direct visual representation, but instead modify the current Canvas’ state, e.g. texture binding, setting color parameters, matrix manipulation and so on.

class kivy.graphics.instructions.VertexInstruction

Bases: kivy.graphics.instructions.Instruction

The VertexInstruction class is the base for all graphics instructions that have a direct visual representation on the canvas, such as Rectangles, Triangles, Lines, Ellipse and so on.

source

This property represents the filename to load the texture from. If you want to use an image as source, do it like this:

with self.canvas:
    Rectangle(source='mylogo.png', pos=self.pos, size=self.size)

Here’s the equivalent in Kivy language:

<MyWidget>:
    canvas:
        Rectangle:
            source: 'mylogo.png'
            pos: self.pos
            size: self.size

Note

The filename will be searched for using the kivy.resources.resource_find() function.

tex_coords

This property represents the texture coordinates used for drawing the vertex instruction. The value must be a list of 8 values.

A texture coordinate has a position (u, v), and a size (w, h). The size can be negative, and would represent the ‘flipped’ texture. By default, the tex_coords are:

[u, v, u + w, v, u + w, y + h, u, y + h]

You can pass your own texture coordinates if you want to achieve fancy effects.

Warning

The default values just mentioned can be negative. Depending on the image and label providers, the coordinates are flipped vertically because of the order in which the image is internally stored. Instead of flipping the image data, we are just flipping the texture coordinates to be faster.

texture

Property that represents the texture used for drawing this Instruction. You can set a new texture like this:

from kivy.core.image import Image

texture = Image('logo.png').texture
with self.canvas:
    Rectangle(texture=texture, pos=self.pos, size=self.size)

Usually, you will use the source attribute instead of the texture.

class kivy.graphics.instructions.Canvas

Bases: kivy.graphics.instructions.CanvasBase

The important Canvas class. Use this class to add graphics or context instructions that you want to be used for drawing.

Note

The Canvas supports Python’s with statement and its enter & exit semantics.

Usage of a canvas without the with statement:

self.canvas.add(Color(1., 1., 0))
self.canvas.add(Rectangle(size=(50, 50)))

Usage of a canvas with Python’s with statement:

with self.canvas:
    Color(1., 1., 0)
    Rectangle(size=(50, 50))
after

Property for getting the ‘after’ group.

ask_update()

Inform the canvas that we’d like it to update on the next frame. This is useful when you need to trigger a redraw due to some value having changed for example.

before

Property for getting the ‘before’ group.

clear()

Clears every Instruction in the canvas, leaving it clean.

draw()

Apply the instruction to our window.

has_after

Property to see if the after group has already been created.

New in version 1.7.0.

has_before

Property to see if the before group has already been created.

New in version 1.7.0.

opacity

Property to get/set the opacity value of the canvas.

New in version 1.4.1.

The opacity attribute controls the opacity of the canvas and its children. Be careful, it’s a cumulative attribute: the value is multiplied to the current global opacity and the result is applied to the current context color.

For example: if your parent has an opacity of 0.5 and a child has an opacity of 0.2, the real opacity of the child will be 0.5 * 0.2 = 0.1.

Then, the opacity is applied on the shader as:

frag_color = color * vec4(1.0, 1.0, 1.0, opacity);
class kivy.graphics.instructions.CanvasBase

Bases: kivy.graphics.instructions.InstructionGroup

CanvasBase provides the context manager methods for the Canvas.

class kivy.graphics.instructions.RenderContext

Bases: kivy.graphics.instructions.Canvas

The render context stores all the necessary information for drawing, i.e.:

  • The vertex shader
  • The fragment shader
  • The default texture
  • The state stack (color, texture, matrix...)
shader

Return the shader attached to the render context.

use_parent_modelview

If True, the parent modelview matrix will be used.

New in version 1.7.0.

Before:

rc['modelview_mat'] = Window.render_context['modelview_mat']

Now:

rc = RenderContext(use_parent_modelview=True)
use_parent_projection

If True, the parent projection matrix will be used.

New in version 1.7.0.

Before:

rc['projection_mat'] = Window.render_context['projection_mat']

Now:

rc = RenderContext(use_parent_projection=True)
class kivy.graphics.instructions.Callback

Bases: kivy.graphics.instructions.Instruction

New in version 1.0.4.

A Callback is an instruction that will be called when the drawing operation is performed. When adding instructions to a canvas, you can do this:

with self.canvas:
    Color(1, 1, 1)
    Rectangle(pos=self.pos, size=self.size)
    Callback(self.my_callback)

The definition of the callback must be:

def my_callback(self, instr):
    print('I have been called!')

Warning

Note that if you perform many and/or costly calls to callbacks, you might potentially slow down the rendering performance significantly.

The updating of your canvas does not occur until something new happens. From your callback, you can ask for an update:

with self.canvas:
    self.cb = Callback(self.my_callback)
# then later in the code
self.cb.ask_update()

If you use the Callback class to call rendering methods of another toolkit, you will have issues with the OpenGL context. The OpenGL state may have been manipulated by the other toolkit, and as soon as program flow returns to Kivy, it will just break. You can have glitches, crashes, black holes might occur, etc. To avoid that, you can activate the reset_context option. It will reset the OpenGL context state to make Kivy’s rendering correct after the call to your callback.

Warning

The reset_context is not a full OpenGL reset. If you have issues regarding that, please contact us.

ask_update()

Inform the parent canvas that we’d like it to update on the next frame. This is useful when you need to trigger a redraw due to some value having changed for example.

New in version 1.0.4.

reset_context

Set this to True if you want to reset the OpenGL context for Kivy after the callback has been called.