Issue with large PNG image

[DNSB]

I edited an ePub for my spouse today and thee was a rather large .png image which Sigil's image viewer refused to open when I double-clicked on it. Using the View Image option allowed me to see the image with the size reported as 0x0 pixels. But doesn't everybody use a 4671x15231 pixel image for a chapter header?

[KevinH]

Thanks for the test case. I will try to pass that image through a png validator to make sure it is not corrupt. If it is valid, I will try to figure out why Qt QImage (what Sigil's Image Tab uses) can not read it but View Image (QtWebEngine) can?

[DNSB]

For what it may be worth, I checked the dagger.png file using PNGCheck 4.01 and it seemed happy.

D:\tester\>pngcheck -v dagger.png
File: dagger.png (1930112 bytes)
chunk IHDR at offset 0x0000c, length 13
4671 x 15231 image, 32-bit RGB+alpha, non-interlaced
chunk pHYs at offset 0x00025, length 9: 11811x11811 pixels/meter (300 dpi)
chunk iTXt at offset 0x0003a, length 1481, keyword: XML:com.adobe.xmp
uncompressed, no language tag
no translated keyword, 1460 bytes of UTF-8 text
chunk IDAT at offset 0x0060f, length 1048576
zlib: deflated, 32K window, default compression
chunk IDAT at offset 0x10061b, length 879953
chunk IEND at offset 0x1d7378, length 0
No errors detected in dagger.png (6 chunks, 99.3% compression).

[KevinH]

Actually, I get a nice error message from MacOS or Qt that says the following:

Cannot load /Users/kbhend/Library/ Application Support/sigil-ebook/sigil/ workspace/Sigil-kORsSx/OEBPS/Images/ daggerL.png.

You can see the nice error message in my screenshot attached below.

Do you get the same error message box?

So this image breaks some limit internal to Qt QImages.

I will see if there is any way to relax that limit.

[KevinH]

And the issue is QImage converts all images to 32 bpp to enable the highest quality editing. This image should have been at 8bpp but QImage makes it a 32 bpp on load. There is no way to change that until after loading so this is a catch-22.

So 1 byte vs 4 bytes per pixel change. So if this image is loaded in a QImage this image is at least 4 times bigger than the approximately 2 megabyte (or more once uncompressed) current file size!

And of course, there is also an allocation limit internal to QImage, but there is an environment variable to change it.

QT_IMAGEIO_MAXALLOC

This is supposed to be in "megabytes" but I have no idea. So you might want try setting this env variable to 320.

Does that allow this image to seen/edited in its Tab?


Update: I checked this and its value is in megabytes and in Qt master this is set to:

https://github.com/qt/qtbase/blob/ac...eader.cpp#L435


256 MB.

And 4671 x 15231 image, 32-bit (4 bytes per pixel) = 284576004 bytes which exceeds that limit.

Setting that environment var to 0 should disable the maximum limit according to the Qt source.

But that is just such a stupid size (4671 x 15231) for an epub image!

Not sure there is anything we on Sigil's end should do, here.

[KevinH]

Tested setting that env var to 320 and sure enough Sigil's Image Tab was able to load and edit it.