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 silly 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.

[DNSB]

Setting QT_IMAGEIO_MAXALLOC to 320 worked here as well. I had also used an image editor to resize the image to 500x1630 which reduced the size to 117KB which is still large for a fleuron but Sigil had no issues with it. I wasn't expecting any fixes since the image size is ridiculous. After the resize, the ePub file dropped from 2384KB to to 684KB.

Allowing for the differences between Windows and MacOS, the error message I received was identical.

[KevinH]

I will add thE env var, QT_IMAGEIO_MAXALLOC to the next update for the Sigil User Guide to document its use.