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Comparing Two Bitmaps for Equality

By , 19 Dec 2006
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Introduction

Presented here is a simple method to compare two HBITMAP handles to see if they contain the same bitmap. The code itself is nothing fancy, so I will just include it in the article text. No separate download or demo application is needed.

How to Compare Two Bitmaps for Equality

The first thing to do is compare the HBITMAP handles themselves. If they are the same handle, then the two bitmaps are obviously equal. If one of the handles is NULL, then they are obviously not equal. If the HBITMAP handles are different and not NULL, then use GetDIBits to fill two BITMAPINFO structures -- one for each bitmap -- and compare the BITMAPINFOHEADERs. If the BITMAPINFOHEADERs contain different data, then the bitmaps are different. If they contain the same data, then the next step is to compare the actual bitmap data bits.

The Code

/////////////////////////////////////////////////////////////////////////////
//
//  CompareBitmaps
//    Compares two HBITMAPs to see if they contain the same image
//
//  Parameters :
//    HBitmapLeft  [in] : The HBITMAP handles that are to be compared
//    HBitmapRight [in] :
//
//  Returns :
//    true if the bitmaps are the same
//    false if they are different
//
/////////////////////////////////////////////////////////////////////////////


bool CompareBitmaps(HBITMAP HBitmapLeft, HBITMAP HBitmapRight)
{
    if (HBitmapLeft == HBitmapRight)
    {
        return true;
    }

    if (NULL == HBitmapLeft || NULL == HBitmapRight)
    {
        return false;
    }

    bool bSame = false;

    HDC hdc = GetDC(NULL);
    BITMAPINFO BitmapInfoLeft = {0};
    BITMAPINFO BitmapInfoRight = {0};

    BitmapInfoLeft.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    BitmapInfoRight.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);

    if (0 != GetDIBits(hdc, HBitmapLeft, 0, 0, NULL, &BitmapInfoLeft, DIB_RGB_COLORS) &&
        0 != GetDIBits(hdc, HBitmapRight, 0, 0, NULL, &BitmapInfoRight, DIB_RGB_COLORS))
    {
        // Compare the BITMAPINFOHEADERs of the two bitmaps

        if (0 == memcmp(&BitmapInfoLeft.bmiHeader, &BitmapInfoRight.bmiHeader, 
            sizeof(BITMAPINFOHEADER)))
        {
            // The BITMAPINFOHEADERs are the same so now compare the actual bitmap bits

            BYTE *pLeftBits = new BYTE<BitmapInfoLeft.bmiHeader.biSizeImage>;
            BYTE *pRightBits = new BYTE<BitmapInfoRight.bmiHeader.biSizeImage>;
            BYTE *pByteLeft = NULL;
            BYTE *pByteRight = NULL;

            PBITMAPINFO pBitmapInfoLeft = &BitmapInfoLeft;
            PBITMAPINFO pBitmapInfoRight = &BitmapInfoRight;

            // calculate the size in BYTEs of the additional

            // memory needed for the bmiColor table

            int AdditionalMemory = 0;
            switch (BitmapInfoLeft.bmiHeader.biBitCount)
            {
            case 1:
                AdditionalMemory = 1 * sizeof(RGBQUAD);
                break;
            case 4:
                AdditionalMemory = 15 * sizeof(RGBQUAD);
                break;
            case 8:
                AdditionalMemory = 255 * sizeof(RGBQUAD);
                break;
            case 16:
            case 32:
                AdditionalMemory = 2 * sizeof(RGBQUAD);
            }

            if (AdditionalMemory)
            {
                // we have to allocate room for the bmiColor table that will be

                // attached to our BITMAPINFO variables

                pByteLeft = new BYTE[sizeof(BITMAPINFO) + AdditionalMemory];
                if (pByteLeft)
                {
                    memset(pByteLeft, 0, sizeof(BITMAPINFO) + AdditionalMemory);
                    memcpy(pByteLeft, pBitmapInfoLeft, sizeof(BITMAPINFO));
                    pBitmapInfoLeft = (PBITMAPINFO)pByteLeft;
                }

                pByteRight = new BYTE[sizeof(BITMAPINFO) + AdditionalMemory];
                if (pByteRight)
                {
                    memset(pByteRight, 0, sizeof(BITMAPINFO) + AdditionalMemory);
                    memcpy(pByteRight, pBitmapInfoRight, sizeof(BITMAPINFO));
                    pBitmapInfoRight = (PBITMAPINFO)pByteRight;
                }
            }

            if (pLeftBits && pRightBits && pBitmapInfoLeft && pBitmapInfoRight)
            {
                // zero out the bitmap bit buffers

                memset(pLeftBits, 0, BitmapInfoLeft.bmiHeader.biSizeImage);
                memset(pRightBits, 0, BitmapInfoRight.bmiHeader.biSizeImage);

                // fill the bit buffers with the actual bitmap bits

                if (0 != GetDIBits(hdc, HBitmapLeft, 0, 
                    pBitmapInfoLeft->bmiHeader.biHeight, pLeftBits, pBitmapInfoLeft, 
                    DIB_RGB_COLORS) && 0 != GetDIBits(hdc, HBitmapRight, 0, 
                    pBitmapInfoRight->bmiHeader.biHeight, pRightBits, pBitmapInfoRight, 
                    DIB_RGB_COLORS))
                {
                    // compare the actual bitmap bits of the two bitmaps

                    bSame = 0 == memcmp(pLeftBits, pRightBits, 
                        pBitmapInfoLeft->bmiHeader.biSizeImage);
                }
            }

            // clean up

            delete[] pLeftBits;
            delete[] pRightBits;
            delete[] pByteLeft;
            delete[] pByteRight;
        }
    }

    ReleaseDC(NULL, hdc);

    return bSame;
}

History

  • 19 December, 2006 -- Original version posted
  • 12 March, 2008 -- Article edited and moved to The Code Project main article base

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

PJ Arends
President
Canada Canada
No Biography provided

Comments and Discussions

 
GeneralSmall Addition - detect minor changes PinmemberAlexander Shilonosov10-Jun-10 0:16 
Newsported to C [modified] PinmemberMikke828-Jan-10 13:49 
Questionam I doing somthing wrong? It crashes. PinmemberMember 137233911-Mar-08 4:09 
GeneralRe: am I doing somthing wrong? It crashes. PinmemberPJ Arends12-Mar-08 4:43 
GeneralExtending this Idea Pinmemberceejeeb27-Jan-07 0:36 
GeneralRe: Extending this Idea PinmemberPJ Arends27-Jan-07 9:52 
GeneralRe: Extending this Idea Pinmemberceejeeb28-Jan-07 2:43 
GeneralRe: Extending this Idea PinmemberPJ Arends28-Jan-07 9:17 
GeneralRe: Extending this Idea Pinmemberceejeeb5-Feb-07 0:35 
GeneralRe: Extending this Idea PinmemberPJ Arends6-Feb-07 9:58 
GeneralRe: Extending this Idea Pinmemberceejeeb7-Feb-07 3:55 
GeneralRe: Extending this Idea PinmemberPJ Arends7-Feb-07 7:43 
GeneralRe: Extending this Idea PinmemberRafal Struzyk11-Jul-07 7:09 
GeneralRe: Extending this Idea PinmemberPJ Arends23-Mar-07 18:56 
GeneralGreat but far too simple PinmemberIntVestor21-Dec-06 6:53 
GeneralRe: Great but far too simple PinmemberPJ Arends21-Dec-06 7:04 
GeneralRe: Great but far too simple PinmemberLampros G10-Jan-07 5:41 

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