Convert bitmap to PNG in-memory in C++ (win32)

Question

Can I convert a bitmap to PNG in memory (ie without writing to a file) using only the Platform SDK? (ie no libpng, etc.).

I also want to be able to define a transparent color (not alpha channel) for this image.

The GdiPlus solution seems to be limited to images of width divisible by 4 . Anything else fails during the call to Save(). Does anyone know the reason for this limitation and how/whether I can work around it?

Update: Bounty

I'm starting a bounty (I really want this to work). I implemented the GDI+ solution, but as I said, it's limited to images with quad width. The bounty will go to anyone who can solve this width issue (without changing the image dimensions), or can offer an alternative non-GDI+ solution that works.

Answer 1

LodePNG （ GitHub ）是一个无库的PNG编码器/解码器。

Answer 2

I read and write PNGs using libpng and it seems to deal with everthing I throw at it (I've used it in unit-tests with things like 257x255 images and they cause no trouble). I believe the API is flexible enough to not be tied to file I/O (or at least you can override its default behaviour eg see png_set_write_fn in section on customization )

In practice I always use it via the much cleaner boost::gil PNG IO extension , but unfortunately that takes char* filenames and if you dig into it the png_writer and file_mgr classes in its implementation it seem pretty tied to FILE* (although if you were on Linux a version using fmemopen and in-memory buffers could probably be cooked up quite easily).

Answer 3

On this site the code shows how convert a bitmap to PNG writing it to a file: http://dotnet-snippets.de/dns/gdi-speichern-eines-png-SID814.aspx . Instead of writing to a file, the Save method of Bitmap also supports writing to a IStream ( http://msdn.microsoft.com/en-us/library/ms535406%28VS.85%29.aspx ). You can create a Stream backed up by memory using the CreateStreamOnHGlobal API function. ( http://msdn.microsoft.com/en-us/library/aa378980%28VS.85%29.aspx ). The used library, GDI+, is included in Windows up from WindowsXP, and works in Windows up from Windows98. I've never done something with it, just googled around. Looks like you can use that, though.

Answer 4

The CImage class (ATL/MFC) supports saving into PNG format. Like the GDI+ solution, it also supports saving to a stream. Here's some code I use to save it to a CByteArray:

CByteArray baPicture;
IStream *pStream = NULL;
if (CreateStreamOnHGlobal(NULL, TRUE, &pStream) == S_OK)
{
    if (image.Save(pStream, Gdiplus::ImageFormatPNG) == S_OK)
    {
    ULARGE_INTEGER ulnSize;
        LARGE_INTEGER lnOffset;
        lnOffset.QuadPart = 0;
        if (pStream->Seek(lnOffset, STREAM_SEEK_END, &ulnSize) == S_OK)
        {
            if (pStream->Seek(lnOffset, STREAM_SEEK_SET, NULL) == S_OK)
            {                       
                baPicture.SetSize(ulnSize.QuadPart);
                ULONG ulBytesRead;
                pStream->Read(baPicture.GetData(), ulnSize.QuadPart, &ulBytesRead);
            }
        }
    }
}
pStream->Release();

I don't know if you'd want to use ATL or MFC, though.

Answer 5

I've used GDI+ for saving a bitmap as a PNG to a file. You should probably check out the MSDN info about GDI+ here and in particular this function GdipSaveImageToStream .

This tutorial here will probably provide some help as well.

Answer 6

GDI's (old school, non-plus) has a GetDIBits method that can be asked to output bits using PNG compression ( BITMAPINFOHEADER::biCompression == BI_PNG ). I wonder if this could be used to create a PNG file? Using GetDIBits to write standard bitmap files is complicated enough - so i suspect this would be even more difficult.

Answer 7

如果您只想使用Windows API， WIC是实现此目的的方法，它同时支持位图和PNG。

Answer 8

It would probably be better to use a library instead of reinventing the wheel yourself.

Look into freeImage