Is there a max image size (pixel width and height) within wx where png images lose there transparency?

Question

Initially, I loaded in 5 .png's with transparent backgrounds using wx.Image() and every single one kept its transparent background and looked the way I wanted it to on the canvas (it kept the background of the canvas). These png images were about (200,200) in size. I proceeded to load a png image with a transparent background that was about (900,500) in size onto the canvas and it made the transparency a black box around the image. Next, I opened the image up with gimp and exported the transparent image as a smaller size. Then when I loaded the image into python the image kept its transparency. Is there a max image size (pixel width and height) within wx where png images lose there transparency? Any info would help. Keep in mind that I can't resize the picture before it is loaded into wxpython. If I do that, it will have already lost its transparency.

import wx
import os

def opj(path):
    return apply(os.path.join, tuple(path.split('/')))

def saveSnapShot(dcSource):
    size = dcSource.Size

    bmp= wx.EmptyBitmap(size.width, size.height)
    memDC = wx.MemoryDC()
    memDC.SelectObject(bmp)

    memDC.Blit(0, 0, size.width, size.height, dcSource, 0,0)
    memDC.SelectObject(wx.NullBitmap)

    img = bmp.ConvertToImage()
    img.SaveFile('path to new image created', wx.BITMAP_TYPE_JPEG)

def main():
    app = wx.App(None)
    testImage = wx.Image(opj('path to original image'), wx.BITMAP_TYPE_PNG).ConvertToBitmap()

    draw_bmp = wx.EmptyBitmap(1500, 1500)
    canvas_dc = wx.MemoryDC(draw_bmp)

    background = wx.Colour(208, 11, 11)
    canvas_dc.SetBackground(wx.Brush(background))
    canvas_dc.Clear()

    canvas_dc.DrawBitmap(testImage,0, 0)
    saveSnapShot(canvas_dc)
if __name__ == '__main__':
    main()

that sounds odd. can you post a minimum runnable example code that reproduces the problem? — otterb
I posted code, but it wouldn't let me put the transparent images in here. What I did was used a transparent png from google images and I was able to reproduce the problem. — user3170662
Is it just because it was saved as jpg? Have you tried EmptyBitmapRGBA instead? — otterb

otterb otterb · Accepted Answer · 2014-05-28T12:54:47

I don't know if I got this right. But if I convert your example from MemoryDC to PaintDC, then I could fix the transparency issue. The key was to pass True to useMask in DrawBitmap method. If I omit useMask parameter, it will default to False and no transparency will be used.

The documentation is here: http://www.wxpython.org/docs/api/wx.DC-class.html#DrawBitmap

I hope this what you wanted to do...

import wx

class myFrame(wx.Frame):
    def __init__(self, testImage):
        wx.Frame.__init__(self, None, size=testImage.Size)
        self.Bind(wx.EVT_PAINT, self.OnPaint)
        self.testImage = testImage
        self.Show()

    def OnPaint(self, event):
        dc = wx.PaintDC(self)
        background = wx.Colour(255, 0, 0)
        dc.SetBackground(wx.Brush(background))
        dc.Clear()
        #dc.DrawBitmap(self.testImage, 0,  0)  # black background
        dc.DrawBitmap(self.testImage, 0,  0, True)  # transparency on, now red


def main():
    app = wx.App(None)
    testImage = wx.Image(r"path_to_image.png", wx.BITMAP_TYPE_PNG).ConvertToBitmap()
    Frame = myFrame(testImage)
    app.MainLoop()


if __name__ == '__main__':
    main()

(Edit) Ok. I think your original example can be fixed in a similar way

memDC.Blit(0, 0, size.width, size.height, dcSource, 0,0,  useMask=True)

canvas_dc.DrawBitmap(testImage,0, 0, useMask=True)

Just making sure that useMask is True was enough to fix the transparency issue in your example, too.

Is there a max image size (pixel width and height) within wx where png images lose there transparency?

1 Answers