Creating image file from pointer to HGLOBAL

[alleyes 0]

In an app that is used to capture images, the information used to store the image data is referenced by a pointer to HGLOBAL. That is then used to re-create the image so it can be written to disk with a call to GlobalFree. I have two problems: I need to attach a file header so I can write the image to a file. The image data is of native type DWORD so I cant use the .NET classes without massaging the image data. What I have done thus far: BITMAPINFOHEADER* imgHeader = (BITMAPINFOHEADER*)GlobalLock(gPtrImage); array<BYTE>^ imgArray = gcnew array<BYTE>(imgHeader->biSizeImage); if (!imgHeader) { GlobalFree(gPtrImage); return; } BITMAPFILEHEADER* bfh = new BITMAPFILEHEADER(); bfh->bfType = 0x4d42; //BM bfh->bfSize = (DWORD) (sizeof(BITMAPFILEHEADER) + imgHeader->biSize + imgHeader->biSizeImage); bfh->bfReserved1 = 0; bfh->bfReserved2 = 0; bfh->bfOffBits = (DWORD) (bfh->bfSize - imgHeader->biSizeImage); LPDWORD Pixels = (LPDWORD)((LPBYTE)imgHeader + sizeof(BITMAPINFOHEADER)); Marshal::Copy(static_cast<IntPtr>(bfh), imgArray, 0, sizeof(BITMAPFILEHEADER)); Marshal::Copy(static_cast<IntPtr>(Pixels), imgArray, (sizeof(BITMAPFILEHEADER) + 1), imgHeader->biSizeImage); GlobalUnlock(gPtrImage); System::Environment::SpecialFolder saveFolder = System::Environment::SpecialFolder::MyDocuments; SaveFileDialog^ sfd = gcnew SaveFileDialog(); sfd->InitialDirectory = System::Environment::GetFolderPath(saveFolder); sfd->Filter = "Device Independent Bitmaps |*.bmp|All files|*.*"; if (sfd->ShowDialog() == System::Windows::Forms::DialogResult::OK) { System::IO::Stream^ imgFileStream = sfd->OpenFile(); if (imgFileStream != nullptr) { imgFileStream->Write(imgArray, 0, imgArray->Length); imgFileStream->Close(); } } GlobalFree(gPtrImage); gPtrImage = NULL; The file size is a lot larger than I would expect for a 640*480 32 bit BMP and it can't be displayed. The image is fine when written to a picturebox, the data is then unlocked but NOT freed. As a test, I save the image of the picturebox i.e PictureBox->Image and that is fine. The data needs to to be handed off to another application that requires native code so I need to re-create the image from the pointer to HGLOBAL and that's where it falls down What might be wrong with this? Thanks for any help

[Mark Salsbery]

BITMAPINFOHEADER.biSizeImage is allowed to be 0... I would personally feel better about checking for that unless you know for sure it's never going to be 0...maybe something like

if (imgHeader->biSizeImage == 0)
imgHeader->biSizeImage = (((imgHeader->biWidth * (long)imgHeader->biBitCount + 31L) & (~31L)) / 8L) * abs(imgHeader->biHeight);

Al_S wrote:

array<BYTE>^ imgArray = gcnew array<BYTE>(imgHeader->biSizeImage);

Your array isn't big enough. What about the size of the BITMAPFILEHEADER and the BITMAPINFOHEADER?

Al_S wrote:

Marshal::Copy(static_cast(Pixels), imgArray, (sizeof(BITMAPFILEHEADER) + 1), imgHeader->biSizeImage);

Array indexes are 0-based... That should be Marshal::Copy(static_cast(Pixels), imgArray, sizeof(BITMAPFILEHEADER), imgHeader->biSizeImage); Also I'm not sure about those static_casts... why not just use IntPtr(bfh)/IntPtr(Pixels)

Al_S wrote:

The file size is a lot larger than I would expect for a 640*480 32 bit BMP

Should be a little over 1MB - just how big is it? You're the one calculating the size - what do you see in the debugger?

Mark Salsbery Microsoft MVP - Visual C++ :java:

[alleyes 0]

Hi Mark. Since the original image is from a global pointer to HGLOBAL, which was previously created and displayed, I am aware of all the fields of the BITMAPINFOHEADER. They are all set from grabbing an image in other code. I am trying to re-create the image after handing off the pointer to HGLOBAL elsewhere With that: biSizeImage = 0x12c000 (about 1.2M) biBitCount = 0x20 ... ... I saw that the array size was goofy so I fixed that - I think. array^ imgArray = gcnew array(imgHeader->biSizeImage + sizeof(BITMAPFILEHEADER)); I also fixed copying to the array: Marshal::Copy(static_cast<IntPtr>(bfh), imgArray, 0, sizeof(BITMAPFILEHEADER)); Marshal::Copy(static_cast<IntPtr>(Pixels), imgArray, sizeof(BITMAPFILEHEADER), imgHeader->biSizeImage); All this washes out to a correct file size. When viewed in a hex editor, the BITMAPFILEHEADER appears where it belongs correctly. The rest of the data is different though. The resultant image can not be viewed X| As far as the use of the static_cast, I don't have any problems with that and I think I've used it properly. Thanks for responding.

[Mark Salsbery]

alleyes wrote:

I saw that the array size was goofy so I fixed that - I think. array^ imgArray = gcnew array<BYTE>(imgHeader->biSizeImage + sizeof(BITMAPFILEHEADER));

Still not big enough :) array<BYTE>^ imgArray = gcnew array<BYTE>(imgHeader->biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER));

alleyes wrote:

Marshal::Copy(static_cast<IntPtr>(Pixels), imgArray, sizeof(BITMAPFILEHEADER), imgHeader->biSizeImage);

You're not writing the BITMAPINFOHEADER to the output array... I missed that one the first time, sorry :) Marshal::Copy(static_cast<IntPtr>(imgHeader), imgArray, sizeof(BITMAPFILEHEADER), sizeof(BITMAPINFOHEADER) + imgHeader->biSizeImage);

Mark Salsbery Microsoft MVP - Visual C++ :java:

modified on Tuesday, January 12, 2010 4:47 PM

[alleyes 0]

array size change plus the proper copy nails it. Thanks very much! :-D

[alleyes 0]

Hi Mark, Is there a way to do the reverse of this? That is, one HGLOBAL to store different images, where it normally is one HGLOBAL relating to one image. Thanks for the valuable help!

[Mouzam Basheer]

Hi Developers. i am going to wrap lib(static library ) file in c++. one function return HGLOBAL DIB. how can i access Byte Array or Bitmap Image from this and pass this Byte Array or bitmap to My C# Project. please help me. thanks in advance. Here is function that return HGLOBAL DIB Type.

HGLOBAL m_hDib;
int retval = XRaySensorIsImageAvailable();

    if ( 0 < retval )
    {
        if (m\_hDib ) ::GlobalFree(m\_hDib);
        m\_hDib = XRaySensorGetDIB();

	}