Unknown PNG chunks

[mattl...@live.com]

Hi folks, I am trying to have a look at supporting PNG chunks when decoding images. As far as I can tell, I need to compile with the "PNG_READ_UNKNOWN_CHUNKS_SUPPORTED" flag set, and then this code is enabled:

https://github.com/google/skia/blob/de7a810734f3323b0830f8bd8ed76c35603b85f5/src/codec/SkPngCodec.cpp#L819-L827

        png_set_keep_unknown_chunks(png_ptr, PNG_HANDLE_CHUNK_ALWAYS, (png_byte*)"", 0);
        png_set_read_user_chunk_fn(png_ptr, (png_voidp) chunkReader, sk_read_user_chunk);

This seems to work from a logic perspective, but at runtime, the "png_ptr->num_chunk_list" value is 0, so it just skips over. When I step through, I see it reach the "png_handle_as_unknown" in "png.c", and then it just return with a value of "0".

Are chunks supported? Should I even be going down this path?

Thanks!

[Leon Scroggins]

I'm not sure what's going on with the png_ptr->num_chunk_list value, but you should be able to handle these with SkPngChunkReader. You can look at an example implementation here.

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 •  Leon Scroggins

 •  scr...@google.com

[李華斯]

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[mattl...@live.com]

I am using the reader - and that is where I noticed it did not pick up the iTXt or pHYs chunks. 

Does the reader only work with some types?

[Leon Scroggins]

It's been a while since I've looked at this very deeply. I think this only supports unknown chunks - iTXt and pHYs are standard chunks, which are handled directly by libpng. (There might be a way to tell libpng to let you handle known chunks, but I'm not sure.)

There are standard libpng methods for retrieving the data from pHYs:

png_get_pixels_per_meter

png_get_x_pixels_per_meter

png_get_y_pixels_per_meter

I don't remember how to get iTXt.

The Skia API doesn't give you a way to get those chunks (or their data), though, and I'm not sure it should.

To view this discussion on the web visit https://groups.google.com/d/msgid/skia-discuss/076ad618-cf2a-4e68-b7e7-c10c04e657a6n%40googlegroups.com.

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 •  Leon Scroggins

 •  scr...@google.com

[mattl...@live.com]

Thanks! This makes a bunch of sense.