Possibility to disable (or enhance) logging from png and jpeg extensions (maybe others)

[misos1]

Is your feature request related to a problem? Please describe.

In case of error both libjpeg and libpng will write message to stderr in contrast with libtiff where can be logging turned off.

Describe the solution you'd like

Possibility to turn off writing error messages to stderr.

Describe alternatives you've considered

Maybe incorporate error messages from used libraries into exception error string. So instead of something like this:

stderr: libpng error: [00][00][00][00]: invalid chunk type C++ exception what(): "png is invalid: iostream error"

It could be:

C++ exception what(): "png is invalid: [00][00][00][00]: invalid chunk type"

This would be nice for all extensions including tiff.

[simmplecoder]

Could you please provide an example where a warning would be generated? I believe I can set a custom handler for libpng warnings and just throw a warning exception (not the best idea?). I believe it is allowed to return to the caller if a warning is encountered, thus a user provided function might be reasonable idea as well. I'll re-read and quote the official docs later on the last sentence.

[misos1]

In #401: "libpng error: Not enough image data"

Error mentioned in this issue should be generated for example when using png data:

		0x89, 'P', 'N', 'G', 0x0D, 0x0A, 0x1A, 0x0A,
		0x00, 0x00, 0x00, 0x00,
		0x00, 0x00, 0x00, 0x00,

[simmplecoder]

At the moment I am forwarding whatever error message is passed to the custom error handler, please have a look at https://github.com/boostorg/gil/pull/428 . I was thinking if warnings should be propagated upwards too, or just swallowed.

[misos1]

Regarding warnings I found in sources for example this:

https://github.com/glennrp/libpng/blob/301f7a14295a3bdfaf406dbb5004d0784dc137ea/png.c#L2546-L2559

But seems libpng must be compiled with PNG_WARNINGS_SUPPORTED. So maybe I would try with IHDR like this:

		0x00, 0x00, 0x00, 0x0D,
		'I', 'H', 'D', 'R',
		0x00, 0x00, 0x00, 0x00,
		0x00, 0x00, 0x05, 0xA9,
		0x08, 0x02, 0x00, 0x00, 0x00,
		// crc32 checksum of this IHDR

[misos1]

I think warning function should return to libpng and do not trigger exceptions. If are warnings in libpng disabled by default maybe would be best to not set warning handler. Or provide user function as you mentioned.

[simmplecoder]

Perhaps I could create a header file that would define the no warnings by default, and be empty (except include guards), if an option is passed through CMake/build2?

I don’t know how to do that, but with some guidance I am eager to learn.

/cc @mloskot , @stefanseefeld, what do you think?