[Python] In-memory save png

[DunkleMaterie]

Hello,

I don't know, whether such a feature is realizable without certain changes to the Gio, but let me just describe my intent:

In a plugin, I want to send an "copy" of current image to a service using urllib.request.Request. Currently, I save the png image as usual using Gimp.file_save(), reopen it to read the data as bytes and attach them to a boundarized body. Then I send this body using the content-type 'multipart/form-data; boundary=xxxx' to the web service. I get an answer, evaluate the response and add the retrieved image to a new created layer. This all is working well and stable.

But I just want to get rid of storing the image into a file and read it again to a byte buffer. Already tried Gio.File.new_for_uri() and point the uri to memory, but my current knowledge for this aspect of the Gio library is literally not existent 

Maybe there is a more elegant way to implement a in-memory file storage, as the export to PNG format is crucial.

Does someone has an approach in mind and could give me some pointers to study?

Thanks

[Ofnuts]

Probably unnecessary optimization. Your file is still cached in the RAM buffers so it won't be reread from disk.

Otherwise I would see if perhaps GEGL can do a PNG encoding to a ram buffer.

Kudos for using urlllib. I thought us humans were better off using requests?

[DunkleMaterie]

Hi ofnuts,

thanks for your very fast response, highly appreciated

And thank you for the pointer, I will try to find information about this GEGL topic. I've also a maybe-working way by implementing the Gio.File interface in plain python to utilize only memory storage and offer a method to return the bytes when appending it to request. But this could be some pain and may not fully working or instable, I think. What do you think about this approach?

Yes, urllib instead of request is used, because of the fact, that I implemented the plugin for Gimp 2.10 as well, which seems not to ship 'requests', but urllib, and I don't want users are forced to install additional libs if it is not necessary. Finally, I come from the Java world and like the feeling of Apache Http Client, where you can control every aspect of the connection by yourself.

[Ofnuts]

(07-20-2025, 06:20 AM)DunkleMaterie Wrote: And thank you for the pointer, I will try to find information about this GEGL topic. I've also a maybe-working way by implementing the Gio.File interface in plain python to utilize only memory storage and offer a method to return the bytes when appending it to request. But this could be some pain and may not fully working or instable, I think. What do you think about this approach?

Frankly? Waste of time. What problem are you trying to solve? How will a memory file be significantly better that a file mapped to a tmpfs (on Linux you can count on /run/user/{id}) or even a plain filesystem given the existence of file buffers all over the place?
Otherwise, Gio has MemoryInputStream and MemoryOutputStream.

[DunkleMaterie]

Yes, you are right. It would be total waste of time. I "solved" it now. "Solved" because, it is not beautiful code, but better, than storing a file on drive (Windows does not have such a feature as "tmpfs" as far as I know), which is deleted immediately after the sending, only needs to exist to retrieve the png bytes.

The solution makes use of Gegl.Node for a preprocessing to convert the Gimp.Layer into a GdkPixbuf in order to store it as png using save_to_bufferv().

I want to share it anyway, maybe somebody find it useful or as inspiration:

Code:

def export_layer_to_png_bytes(layer:Gimp.Layer) -> bytes:
   """
   Converts a given layer into PNG bytes after applying gamma correction.

   Parameters:
   layer (Gimp.Layer): The input layer to be converted.

   Returns:
   bytes: The PNG bytes representing the converted image.
   """
   Gegl.init(None)

   width = layer.get_width()
   height = layer.get_height()
   source_buffer = layer.get_buffer()

   # Gamma correction for GdkPixbuf
   node = Gegl.Node()
   # Input node
   load = node.create_child("gegl:buffer-source")
   # Unpremultiply node
   unpremul = node.create_child("gegl:unpremultiply")
   # Gamma correction node
   gamma = node.create_child("gegl:gamma")
   # Output node
   save = node.create_child("gegl:write-buffer")

   # Connect the source buffer from input layer to input node
   load.set_property("buffer", source_buffer)

   # Connect a result buffer to output layer node
   result_buffer = Gegl.Buffer.new("RGBA u8", 0, 0, width, height)
   save.set_property("buffer", result_buffer)

   # Set gamma correction to non-linear sRGB for GdkPixbuf
   gamma.set_property("value", 1.0 / 2.2)

   # Now connect the graph nodes to each other [load] -> [unpremul] -> [gamma] -> [save]
   load.connect_to("output", unpremul, "input")
   unpremul.connect_to("output", gamma, "input")
   gamma.connect_to("output", save, "input")

   # Baam!
   save.process()

   # Help to retrieve the bytes from result buffer after gamma correction for GdkPixbuf conversion
   rect = Gegl.Rectangle.new(0, 0, width, height)
   pixel_bytes = result_buffer.get(rect, 1.0, "RGBA u8", Gegl.AbyssPolicy.CLAMP)

   rowstride = width * 4
   gbytes = GLib.Bytes.new(pixel_bytes)
   pixbuf = GdkPixbuf.Pixbuf.new_from_bytes(gbytes, GdkPixbuf.Colorspace.RGB, True, 8, width, height, rowstride)

   success, png_bytes = pixbuf.save_to_bufferv("png", [], [])
   if not success:
       Gimp.message("Pixbuf export to png bytes failed")
       png_bytes = bytes()

   return png_bytes