PNG repair tools?

[Graf Zahl]

I have an annoying problem on my hand.
One tool I used at work was creating incorrect PNGs, it stores the transparency information before the palette, although the spec wants it after the palette.
For most software we are using this doesn't matter, it can just read them as it is. That's why we never noticed that something was wrong.

But today I had to read them into an older wxWidgets based application and wxWidgets, shitty as it is, not only performs a validation but also opens a MESSAGE BOX(!!!) each time one of these files gets opened making the application unusable because it's impossible to get it out of the loading code, I'd have to click away that stupid box for each single image.

I've been looking for a tool to fix this issue but no luck so far. Most do not see anything wrong and those which do strip out all custom chunks, which in this case render the 'repaired' files unusable because the custom chunks contain critical info.

So my question: Does anyone here know if such a tool exists or do I have to write my own? It's several 1000 images that need to be fixed so this is not something to be done manually.

[Caligari87]

Not sure if you've tried something like this already, but just a thought... Imagemagick includes a tool called mogrify that can easily batch-convert images. Perhaps you could you batch-convert them to transparent TIFF then back to PNG?

EDIT: Nevermind, that'll probably strip the custom chunks.

EDIT: Imagemagick actually does have a special option just for preserving or excluding PNG chunks: http://www.imagemagick.org/script/comma ... php#define dunno if that's helpful or not. Perhaps batch-converting the PNGs to new filenames then back to the old filenames while using the preservation option will do the trick?

[Graf Zahl]

No. I said they contain custom chunks. Mogrify destroys all of them. It also recompresses the image.

[Rachael]

Have you tried pngout? It basically rewrites PNG files. You can use the /k option to keep custom chunks and /f to force it even if the resulting file will be bigger.

You can use /s3 to do a standard-deflate compression, which is faster than Ken's custom super-deflate.

I don't know if that will help you, but it's worth a shot.

Also you can batch it in cmd.exe using this command:

Code: Select all

for %I in (*.png) do @pngout /k /s3 /f %I

[Tapwave]

Probably a stupid answer, but. Does the tool you use allows saving in different formats? You could save it in something sufficiently loseless, then open it with another tool that saves png properly and convert it that way. But I understand you'd like to avoid conversion cycling as much as possible.

[Caligari87]

@Graf: Dunno if you saw my edit, but imagemagick does have an option to preserve chunks and fiddle with compression settings, which mogrify should recognize. http://www.imagemagick.org/script/comma ... php#define Does that help at all?

Just throwing ideas.

[Gez]

Probably a stupid answer, but. Does the tool you use allows saving in different formats? You could save it in something sufficiently loseless, then open it with another tool that saves png properly and convert it that way. But I understand you'd like to avoid conversion cycling as much as possible.

The problem is preserving the custom chunks, something exceedingly few programs do despite the specs asking them to.

[Graf Zahl]

Probably a stupid answer, but. Does the tool you use allows saving in different formats? You could save it in something sufficiently loseless, then open it with another tool that saves png properly and convert it that way. But I understand you'd like to avoid conversion cycling as much as possible.

The tool is fixed now. But all the images we created over the last 6 months need to be corrected, too. Going back to the sources would cost even more time.

[Graf Zahl]

@Graf: Dunno if you saw my edit, but imagemagick does have an option to preserve chunks and fiddle with compression settings, which mogrify should recognize. http://www.imagemagick.org/script/comma ... php#define Does that help at all?

Just throwing ideas.

Unfortunately, ImageMagick completely refuses to load the broken images.
Well, I think I have to write that thing myself. I already wasted several hours on this shit in which I could have done it myself. Grrrrr...

[dpJudas]

Code: Select all

#include <vector>
#include <algorithm>
#include <cstdio>
#include <cstdint>
#include <stdlib.h>

struct PNGMagic { uint8_t magic[8]; };
struct PNGChunkStart { uint32_t size, uint32_t name; };
struct PNGChunkEnd { uint32_t crc; };
struct PNGChunk { PNGChunkStart start, std::vector<uint8_t> data; PNGChunkEnd end; };

#ifndef __BIG_ENDIAN__
#define MAKE_ID(a,b,c,d)	((uint32_t)((a)|((b)<<8)|((c)<<16)|((d)<<24)))
#else
#define MAKE_ID(a,b,c,d)	((uint32_t)((d)|((c)<<8)|((b)<<16)|((a)<<24)))
#endif

int main(int, char *args)
{
    PNGMagic magic; std::vector<PNGChunk> chunks;

    FILE *f = fopen(args[0], "rb");
    fread(&magic, sizeof(PNGMagic), 1, f);
    PNGChunk cur;
    while (fread(&cur.start, sizeof(PNGChunkStart), 1, f) == 1)
    {
        uint32_t size = _byteswap_ulong(cur.start.size);
        cur.data.resize(size);
        fread(cur.data.data(), size, 1, f);
        fread(&cur.end, sizeof(PNGChunkEnd), 1, f);
        chunks.push_back(cur);
    }
    fclose(f);

    auto itTrans = std::find(chunks.begin(), chunks.end(), [](PNGChunk &c) -> bool { return MAKE_ID('t', 'R', 'N', 'S') == c.name; });
    auto itPal = std::find(chunks.begin(), chunks.end(), [](PNGChunk &c) -> bool { return MAKE_ID('P', 'L', 'T', 'E') == c.name; });
    if (itTrans != chunks.end() && itPal != chunks.end()) std::swap(*itTrans, *itPal);

    f = fopen(args[1], "wb");
    fwrite(&magic, sizeof(PNGMagic), 1, f);
    for (auto &chunk : chunks)
    {
        fwrite(&chunk.start, sizeof(PNGChunkStart), 1, f);
        fwrite(chunk.data.data(), _byteswap_ulong(cur.start.size), 1, f);
        fwrite(&chunk.end, sizeof(PNGChunkEnd), 1, f);
    }
    fclose(f);
}

Totally untested of course, and assumes MSVC compiling it. May burn down your PC and kill your cat!

[Rachael]

My cat has been killed many times thanks to your code. She still has some of her 9 lives left, though...

[dpJudas]

Sorry about that. Did your PC burn down too?

[Rachael]

No, just overheated a bit.

[Edward-san]

code snippet

no checks for fopen possibly returning nullptr? Cat exterminator!

[Graf Zahl]

I already solved this yesterday by writing my own tool, after giving up on the shit the internet had to offer. It's truly amazing what bottom-feeding scum is trying to make some money off desperate people who trashed some data - and 90% of this stuff doesn't even work!

And my solution was quite similar to what dpJudas just posted. Read all image chunks, compare index of palette and trans, and if wrong swap them around and save the image back. Took me two hours to run this on all the affected images, though...