Cache QImage

krzysieklfc

I have an application which lists 32bit .exr image files and creates a thumbnail once each item from the list widget is selected. It would be useful to cache each thumbnail in memory in case an item is selected more than once. I have read that the memory managment beetween python and qt is tricky [1][2].

I tried collecting the QImages in a dictionary to simply keep the references but it is not working properly (most times I get image artifacts, sometimes the thumbnail is looking fine but it's from another file). I'm sure there's better approach than this

class Image(object):
	WIDTH = 260
	HEIGHT = 130
	thumbnails = dict()

	@staticmethod
	def createThumbnail(name, buffer):
		image = QtGui.QImage(buffer, Image.WIDTH, Image.HEIGHT, QtGui.QImage.Format_RGB888)
		Image.collectThumbnail(name, image)
		return image

	@staticmethod
	def collectThumbnail(name, image):
		if name not in Image.thumbnails:
			Image.thumbnails[name] = image

#[...]

if selected_file in Image.thumbnails:
	image = Image.thumbnails[selected_file]
    self.setThumbnail(image)

I noticed QCache is missing in PySide2 bindings.

I'm using Pyside 2 (Qt 5.6.3)

Kent-Dorfman

The only thing that comes to mind is whether your buffer is in the format that you think it is. Also, I don't generally use the dict() constructor, but prefer dict = {}. finally, I usually rely upon the KeyError exception to see if an item already exists in a dictionary.

Nothing really jumps out as being obviously wrong with the above, so my suspicion is the format of buffer.

krzysieklfc

@Kent-Dorfman I totally missed the concept that QImage references the buffer, not copies it. I included it in my dictionary and now it works. Thank you.

Kent-Dorfman

@krzysieklfc said in Cache QImage:

@Kent-Dorfman I totally missed the concept that QImage references the buffer, not copies it. I included it in my dictionary and now it works. Thank you.

Well yeah, reuse of the buffer ist verboten. Gotta make a copy. Glad you figured it out