CODEWITHSUNDEEP
c++winapipng
Assaf Lavie
Assaf Lavie

Reputation: 75913

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

Can I convert a bitmap to PNG in memory (i.e. without writing to a file) using only the Platform SDK? (i.e. 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

I implemented the GDI+ solution, but as I said, it's limited to images with quad width. I am seeking to solve this width issue without changing the image dimensions, or an alternative non-GDI+ solution that works.

Upvotes: 20

Views: 39117

Answers (8)

Johan Kotlinski
Johan Kotlinski

Reputation: 25731

LodePNG (GitHub) is a lib-less PNG encoder/decoder.

Upvotes: 21

Ana Betts
Ana Betts

Reputation: 74654

If you want to only use Windows APIs, WIC is the way to accomplish this, and it supports both Bitmaps and PNGs.

Upvotes: 4

Aardvark
Aardvark

Reputation: 8561

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.

Upvotes: 6

djeidot
djeidot

Reputation: 4642

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.

Upvotes: 8

timday
timday

Reputation: 24902

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 e.g 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).

Upvotes: 14

Twotymz
Twotymz

Reputation: 412

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.

Upvotes: 6

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506837

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.

Upvotes: 12

DShook
DShook

Reputation: 15654

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

Look into freeImage

Upvotes: 2

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