How do I read text chunks quicker with libpng? - c++

Question

With libpng, I’m trying to extract text chunks in a 44-megabyte PNG image (and preferably validate that the PNG data is not malformed (e. g. lacking IEND, etc.)). I could do that with png_read_png and png_get_text, but it took way too long for me, 0.47 seconds, which I’m pretty sure is because of the massive amount of the IDAT chunks the image has. How do I do this in a quicker manner?
I didn’t need the pixels, so I tried to make libpng ignore the IDAT chunks.
To have libpng ignore IDAT chunks, I tried:
png_read_info(p_png, p_png_information); png_read_image(p_png, nullptr); png_read_end(p_png, p_png_information); to skip IDAT chunks; crashed and failed.
png_set_keep_unknown_chunks to make libpng unknow about IDAT, and png_set_read_user_chunk_fn(p_png, nullptr, discard_an_unknown_chunk) (discard_an_unknown_chunk is a function that does return 1;) to discard unknown chunks; a weird CRC error occurred on the first IDAT chunk and failed.
And failed to do that.
Edit
Running as a Node.js C++ addon, mostly written in C++, on Windows 10, with i9-9900K CPU # 3.6 GHz and gigabytes of memory.
Read the image file on an SSD with fs.readFileSync, a Node.js method returning a Buffer, and tossed it to the libpng to process.
Yes, at first, I blamed libpng for the prolonged computation. Now I see there might be other reasons causing the delay. (If that’s the case, this question would be a bad one with an XY problem.) Thank you for your comments. I’ll check my code out again more thoroughly.
Edit 2
With every step for feeding the PNG data input to the C++ addon kept the same, I ended up manually picking and decoding text chunks only, with my C pointer magic and some C++ magic. And, the performance was impressive (0.0020829 seconds on processing), being almost immediate. Don’t know why and how though.
B:\__A2MSUB\image-processing-utility>npm run test
> image-processing-utility#1.0.0 test B:\__A2MSUB\image-processing-utility
> node tests/test.js
----- “read_png_text_chunks (manual decoding, not using libpng.)” -----
[
{
type: 'tEXt',
keyword: 'date:create',
language_tag: null,
translated_keyword: null,
content: '2020-12-13T22:01:22+09:00',
the_content_is_compressed: false
},
{
type: 'tEXt',
keyword: 'date:modify',
language_tag: null,
translated_keyword: null,
content: '2020-12-13T21:53:58+09:00',
the_content_is_compressed: false
}
]
----- “read_png_text_chunks (manual decoding, not using libpng.)” took 0.013713 seconds.
B:\__A2MSUB\image-processing-utility>

Answer 1

I had to do something similar, but where I wanted libpng to do all of the metadata chunk parsing (e.g. eXIf, gAMA, pHYs, zEXt, cHRM, etc. chunks). Some of these chunks can appear after the IDAT, which means the metadata can't be read with just png_read_info. (The only way to get to them would be to do a full decode of the image, which is expensive, and then call png_read_end.)
My solution was to create a synthetic PNG byte stream that is fed to libpng via the read callback set using png_set_read_fn. In that callback, I skip all IDAT chunks in the source PNG file, and when I get to an IEND chunk, I instead emit a zero-length IDAT chunk.
Now I call png_read_info: it parses all of the metadata in all of the chunks it sees, stopping at the first IDAT, which in my synthetic PNG stream is really the end of the source PNG image. Now I have all of the metadata and can query libpng for it via the png_get_xxx functions.
The read callback that creates the synthetic PNG stream is a little complicated due to it being called by libpng multiple times, each for small sections of the stream. I solved that using a simple state machine that processes the source PNG progressively, producing the synthetic PNG stream on-the-fly. You could avoid those complexities if you produce the synthetic PNG stream up-front in memory before calling png_read_info: without any real IDATs, your full synthetic PNG stream is bound to be small...
While I don't have benchmarks to share here, the final solution is fast because IDATs are skipped entirely and not decoded. (I use a file seek to skip each IDAT in the source PNG after reading the 32-bit chunk length.)

Answer 2

You can check that all the correct PNG chunks are in a file, in the correct order, and not repeated and with correct checksums using pngcheck. It is open source so you could look at how it works.
If you add the parameter -7, you can not only check the structure but also extract the text:
pngcheck -7 a.png
Output
File: a.png (60041572 bytes)
date:create:
2020-12-24T13:22:41+00:00
date:modify:
2020-12-24T13:22:41+00:00
OK: a.png (10000x1000, 48-bit RGB, non-interlaced, -0.1%).
I generated a 60MB PNG and the above check takes 0.067s on my MacBook Pro.