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wx.MemoryDC¶

A memory device context provides a means to draw graphics onto a bitmap.

When drawing in to a mono-bitmap, using WHITE , WHITE_PEN and WHITE_BRUSH will draw the background colour (i.e. 0) whereas all other colours will draw the foreground colour (i.e. 1).

A bitmap must be selected into the new memory DC before it may be used for anything. Typical usage is as follows:

# Create a memory DC
temp_dc = wx.MemoryDC()
temp_dc.SelectObject(test_bitmap)

# We can now draw into the memory DC...
# Copy from this DC to another DC.
old_dc.Blit(250, 50, BITMAP_WIDTH, BITMAP_HEIGHT, temp_dc, 0, 0)

Note that the memory DC must be deleted (or the bitmap selected out of it) before a bitmap can be reselected into another memory DC.

And, before performing any other operations on the bitmap data, the bitmap must be selected out of the memory DC:

temp_dc.SelectObject(wx.NullBitmap)

This happens automatically when wx.MemoryDC object goes out of scope.

Note that the scaling factor of the bitmap determines the scaling factor used by this device context, so when using a memory device context as a back buffer for a window, you should create the bitmap using the same scale factor as the one used by the window itself, e.g.

class MyWindow(wx.Window):
    ...
    def OnPaint(self, event):
    bmp = wx.Bitmap();
    bmp.CreateWithDIPSize(self.GetClientSize(), self.GetDPIScaleFactor())

    memdc = wx.MemoryDC(bmp)
    ... use memdc to draw on the bitmap ...

    dc = wx.PaintDC(self)
    dc.DrawBitmap(bmp, wx.Point(0, 0))

Class Hierarchy¶

Inheritance diagram for class MemoryDC:

Known Subclasses¶

wx.BufferedDC

Methods Summary¶

`__init__`	Constructs a new memory device context.
`GetSelectedBitmap`
`SelectObject`	Allow using this device context object to modify the given bitmap contents.
`SelectObjectAsSource`	Selects the given bitmap into the device context, to use as the memory bitmap.

Properties Summary¶

SelectedBitmap

See GetSelectedBitmap

Class API¶

class wx.MemoryDC(DC)¶

Possible constructors:

MemoryDC() -> None

MemoryDC(dc) -> None

MemoryDC(bitmap) -> None

A memory device context provides a means to draw graphics onto a bitmap.

Methods¶

__init__(self, *args, **kw)¶

Overloaded Implementations:

__init__ (self)

Constructs a new memory device context.

Use the wx.DC.IsOk member to test whether the constructor was successful in creating a usable device context. Don’t forget to select a bitmap into the DC before drawing on it.

Return type:: None

__init__ (self, dc)

Constructs a new memory device context having the same characteristics as the given existing device context.

This constructor creates a memory device context compatible with dc in wxMSW, the argument is ignored in the other ports. If dc is nullptr, a device context compatible with the screen is created, just as with the default constructor.

Note that the DPI scaling factor is not inherited from dc, but is determined by the scaling factor of the bitmap selected into this device context later.

Parameters:: dc (wx.DC)
Return type:: None

__init__ (self, bitmap)

Constructs a new memory device context and calls SelectObject with the given bitmap.

Use the wx.DC.IsOk member to test whether the constructor was successful in creating a usable device context.

Parameters:: bitmap (wx.Bitmap)
Return type:: None

GetSelectedBitmap(self)¶

Return type:: wx.Bitmap

SelectObject(self, bitmap)¶

Allow using this device context object to modify the given bitmap contents.

Note that if you need to only use the existing bitmap contents instead of modifying it, you should use SelectObjectAsSource instead.

Before using the updated bitmap data, make sure to select it out of context first either by selecting wx.NullBitmap into the device context or destroying the device context entirely.

If the bitmap is already selected in this device context, nothing is done. If it is selected in another context, the function asserts and drawing on the bitmap won’t work correctly.

Note that this function changes the scale factor of this device context (as returned by wx.DC.GetContentScaleFactor ) to be the same as the bitmap scale factor (as returned by wx.Bitmap.GetScaleFactor ).

Parameters:: bitmap (wx.Bitmap)
Return type:: None

SelectObjectAsSource(self, bitmap)¶

Selects the given bitmap into the device context, to use as the memory bitmap.

Selecting the bitmap as source into a memory DC allows you to copy its contents to another device context using wx.DC.Blit . Note that using wx.DC.DrawBitmap or wx.DC.DrawIcon is a simpler way to do the same thing.

If the argument is wx.NullBitmap (or some other uninitialised wx.Bitmap) the current bitmap is selected out of the device context, allowing the current bitmap to be destroyed safely.

Parameters:: bitmap (wx.Bitmap)
Return type:: None

Note

Modifying a bitmap selected only as a source may not work correctly and can notably modify the other bitmaps sharing the same data due to the use of reference counting (see Reference Counting).

Properties¶

SelectedBitmap¶: See GetSelectedBitmap

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