arrays - Converting wx bitmap to numpy using BitmapBufferFormat_RGBA (python)

Question

I'm trying to capture a window with wxPython and process the result with cv2. This seems fairly straight forward as wx has a built in function to convert a bitmap object to a simple RGB array.

The problem is that I can't figure out the syntax. The documentation is sparse and the few examples I can find are either deprecated or incomplete.

Here's basically what I want

app = wx.App(False)
img = some_RGBA_array #e.g. cv2.imread('some.jpg')
s = wx.ScreenDC()
w, h = s.Size.Get()
b = wx.EmptyBitmap(w, h)
m = wx.MemoryDCFromDC(s)
m.SelectObject(b)
m.Blit(0, 0, w, h, s, 0, 0)
m.SelectObject(wx.NullBitmap)
b.CopyFromBuffer(m, format=wx.BitmapBufferFormat_RGBA, stride=-1)#<----problem is here
img.matchSizeAndChannels(b)#<----placeholder psuedo
img[:,:,0] = np.where(img[:,:,0] >= 0, b[:,:,0], img[:,:,0])#<---copy a channel

For simplicity this doesn't specify a window and only processes one channel but it should give an idea of what I'm attempting to do.

Whenever I try to run it like that using CopyFromBuffer it tells me that the bitmap stored in "b" isn't a readable buffer object yet if I pass it to SaveFile it writes out the image as expected.

Not sure what I'm doing wrong here.

EDIT: Turns out what I'm doing wrong is trying to use BitmapBufferFormat_RGBA to convert wxBitmaps to cv2 rgb's. As per the answer below I should use the following (where "b" is the bitmap):

wxB = wx.ImageFromBitmap(b)#input bitmap 
buf = wxB.GetDataBuffer() 
arr = np.frombuffer(buf, dtype='uint8',count=-1, offset=0)
img2 = np.reshape(arr, (h,w,3))#convert to classic rgb
img2 = cv2.cvtColor(img2, cv2.COLOR_RGB2BGR)#match colors to original image

Answer 1

Haven't done this for some while: But an OpenCV bitmap is essentially a numpy array. And for creating a wx.Bitmap from a generic array you'll have to take the wx.Image route. See the entry from the wxPython wiki (somewhere in the middle) regarding converting numpy arrays:

array = ... # the OpenCV image
image = ... # wx.Image
image.SetData(array.tostring())
wxBitmap = image.ConvertToBitmap()       # OR:  wx.BitmapFromImage(image)

EDIT: And to go the other way round:

import numpy
img = wx.ImageFromBitmap(wxBitmap)
buf = img.GetDataBuffer() # use img.GetAlphaBuffer() for alpha data
arr = numpy.frombuffer(buf, dtype='uint8')

# example numpy transformation
arr[0::3] = 0 # turn off red
arr[1::3] = 255 # turn on green