libpng error invalid chunk type when loading png from memory - c++

Question

I'm trying to load a PNG from a memory buffer so I can access the ImageData without having to save it as a file first.
The memory buffer contains a valid png-file, when using fwrite to save it as a file on disk I get the following image: https://dl.dropboxusercontent.com/u/13077624/test.png
This represents a depth Image received by a Kinect sensor, for those of you wondering.
This is the code that gives errors:
struct mem_encode
{
char *buffer;
png_uint_32 size;
png_uint_32 current_pos;
};
void handle_data(const boost::system::error_code& error,
size_t bytes_transferred)
{
if (!error)
{
cout<<"Saving as file: "<<determinePathExtension(PNGFrame,"png");
FILE* fp=fopen("test.png","wb");
fwrite(data_,bytes_transferred,1,fp);
fclose(fp);
//get PNG file info struct (memory is allocated by libpng)
png_structp png_ptr = NULL;
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (!png_ptr) {
std::cerr << "ERROR: Couldn't initialize png read struct" << std::endl;
cin.get();
return; //Do your own error recovery/handling here
}
// get PNG image data info struct (memory is allocated by libpng)
png_infop info_ptr = NULL;
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr) {
std::cerr << "ERROR: Couldn't initialize png info struct" << std::endl;
cin.get();
png_destroy_read_struct(&png_ptr, (png_infopp)0, (png_infopp)0);
return; //Do your own error recovery/handling here
}
struct mem_encode pngdata;
pngdata.buffer=data_;
pngdata.size=(png_uint_32)bytes_transferred;
pngdata.current_pos=0;
png_set_read_fn(png_ptr,&pngdata, ReadData);
//Start reading the png header
png_set_sig_bytes(png_ptr, 8);
png_read_info(png_ptr,info_ptr);
//... Program crashes here
}
else
{
cout<<error.message()<<" Bytes received: "<<bytes_transferred<<endl;
delete this;
}
}
static void ReadData(png_structp png_ptr, png_bytep outBytes,
png_size_t byteCountToRead){
struct mem_encode* p=(struct mem_encode*)png_get_io_ptr(png_ptr);
size_t nsize=p->size + byteCountToRead;
if(byteCountToRead>(p->size-p->current_pos)) png_error(png_ptr,"read error in read_data_memory (loadpng)");
/* copy new bytes */
memcpy(outBytes,p->buffer + p->size,byteCountToRead);
p->current_pos+=byteCountToRead;
}
Calling the method results in the program crashing with the following error:
libpng error: [00][00][00][00]: invalid chunk type
data_ represents the the databuffer storing the PNG-image and is a char *.
Any help would be appreciated.
Sources I used:
http://www.libpng.org/pub/png/libpng-1.0.3-manual.html
http://blog.hammerian.net/2009/reading-png-images-from-memory/
http://santosdev.blogspot.be/2012/08/loading-png-image-with-libpng-1512-or.html
http://www.piko3d.net/tutorials/libpng-tutorial-loading-png-files-from-streams/
Could this be caused by network bytes being translated badly?

Answer 1

I think you forgot to read the PNG signature bytes. Use
if (png_sig_cmp(data, 0, 8)
png_error(png_ptr, "it's not a PNG file");
Then your
png_set_sig_bytes(png_ptr,8);
lets libpng know you have already read the signature.
Or, you could use png_set_sig_bytes(png_ptr, 0); and
let libpng do the checking for you.

Answer 2

Are you sure your ReadData function is correct? Why do you start memcpy from the address p->buffer + p->size - isn't it the end of the buffer? And what does nsize do?