Thinking about large files.

[Boudewijn Rempt]

One think Krita can not handle are really big files. A 125MB tif file is 
loading into memory in one go, making Krita take about 289MB of memory -- 
with no image loaded, Krita takes a more reasonable 41MB. And during loading, 
Krita needs 800MB, and that's a bit much.

Since my employer has paid to upgrade my laptop to 1GB of memory, that's not a 
problem for me :-). But it's the principle of the thing. Besides, loading the 
file does take a few minutes to convert. And you don't want to do a quick 
sharpen on a file of this size.

Now it has never been a design goal to handle really enormous images, vips 
does that a lot better than any general purpose paint app can ever aspire to, 
but we should give some thought to our way of handling big images.

The problem is layered: it's not enough to have lazy loading of krita files
(although we should store, when an image gets above a certain size, an .png 
file of the rendered image that we can quickly load and display, and then we 
should only load the tiles we actually need for editing/rerendering from the 
file), but we should also be able to do the same with tiled tiffs and exr 
images that support random access loading.

This must be done somewhere in the datamanager code. If an image reader 
(whether it's a filter or Krita's own file format reader) indicates that it 
supports random access, then a file handle and a the reader object is passed 
to the tile manager. 

The tile manager either does nothing until it gets a request for a certain 
chunk of data (through one of the read functions or the creation of an 
read/write iterator), and only then it starts reading and decoding image 
data. Or, the tile manager starts a background thread and begins converting 
the alien image data to Krita tiles, carefully caching them in a temp file.

Maybe we could even devise an algorithm to delete unused tiles from memory and 
cache them on disk; same with undo information.

Anyway, I just wanted to start thinking about this, not working; I'll add a 
note in the doc directory.

--
Boudewijn Rempt 
http://www.valdyas.org/fading/index.cgi
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