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Asked: 2026-06-06T17:42:44+00:00

I am currently trying to implement a PNG encoder in C++ based on libpng

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I am currently trying to implement a PNG encoder in C++ based on libpng that uses OpenMP to speed up the compression process.
The tool is already able to generate PNG files from various image formats.
I uploaded the complete source code to pastebin.com so you can see what I have done so far: http://pastebin.com/8wiFzcgV

So far, so good! Now, my problem is to find a way how to parallelize the generation of the IDAT chunks containing the compressed image data. Usually, the libpng function png_write_row gets called in a for-loop with a pointer to the struct that contains all the information about the PNG file and a row pointer with the pixel data of a single image row.

(Line 114-117 in the Pastebin file)

//Loop through image
for (i = 0, rp = info_ptr->row_pointers; i < png_ptr->height; i++, rp++) {
    png_write_row(png_ptr, *rp);
}

Libpng then compresses one row after another and fills an internal buffer with the compressed data. As soon as the buffer is full, the compressed data gets flushed in a IDAT chunk to the image file.

My approach was to split the image into multiple parts and let one thread compress row 1 to 10 and another thread 11 to 20 and so on. But as libpng is using an internal buffer it is not as easy as I thought first 🙂 I somehow have to make libpng write the compressed data to a separate buffer for every thread. Afterwards I need a way to concatenate the buffers in the right order so I can write them all together to the output image file.

So, does someone have an idea how I can do this with OpenMP and some tweaking to libpng? Thank you very much!

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1 Answer

  1. Editorial Team

    I finally got it to parallelize the compression process.
    As mentioned by Cameron in the comment to his answer I had to strip the zlib header from the zstreams to combine them. Stripping the footer was not required as zlib offers an option called Z_SYNC_FLUSH which can be used for all chunks (except the last one which has to be written with Z_FINISH) to write to a byte boundary. So you can simply concatenate the stream outputs afterwards. Eventually, the adler32 checksum has to be calculated over all threads and copied to the end of the combined zstreams.

    If you are interested in the result you can find the complete proof of concept at https://github.com/anvio/png-parallel

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