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This article presents code which allows you to take a DIB and write it to a JPEG file. Other bitmap and palette example code at this web site can show you how to turn a bitmap into a DIB (DDBToDIB, etc.).

The code has been improved to handle all bitmap formats. The earlier version did not handle 32-bit or 16-bit DIB's.

Acknowledgements: This code depends on the jpeg.lib code written by the Independent JPEG Group (Thomas G. Lane and company).

The code assumes that you have installed their jpeg.lib library project.

To get jpeg.lib:

1) Go to Ulrich von Zadow's  "Class library for image file decoding" article, and click on the "Links" link.

2) Click on the "LIBJPEG ver 6b" link to download the "vanilla" libjpeg.

3) Do not use Mr. von Zadow's "Paintlib" version of libjpeg, because he seems to have made some internal changes for his own use.

The jpeg.lib project (LibJpeg) is set up to compile under numerous operating systems. The download includes a file called install.doc. Read the sections relevant to VC++/Windows. You have to rename a couple of files, etc.

The first time I installed and compiled libjpeg, it took a long time and was very aggravating. I then re-read the instructions, and it took about fifteen minutes. I'm sure there's a lesson here...

Testing Notes:

To test the code, I used MS Paint to create bitmap files in the four supported formats.

I then set up a VC++ project which read the bitmap files, and used the functions below to turn  convert them into JPEG files (.jpg).

I then used Julian Smart's CImage demo application to display them.

I also used Internet Explorer as well as MS Word to display them.

I also created a jpeg file by loading an internal resource (see the example below). The largest bitmap file I dealt with was about 390 KB.

Error trapping:

libjpeg includes a number of provisions for improving error detection, and reporting them in a good manner. I have not investigated these, and the code does not include any of them. Right now, an internal error will simply cause the code to stop cold.

To get at jpeg.lib properly in your project:

1. Under tools->options, directories tab:
   1. Set the Include Files to include the path to your jpeg.lib project
   2. Set the Library Files to include the path to your jpeg.lib release directory
2. Under Project->Settings, Link tab:
   1. Add jpeg.lib to the Object/library modules list

Note on Quality Settings:

For the IDB_TEST resource, I tried two quality settings: 10 and 100. At "10", the image had a lot of black spottiness, and took up 1 KB. At "100" the image looked very good, and took up 4 KB.

/////////////////////////////////////////////////////////////
//Example of use:
/////////////////////////////////////////////////////////////
void MakeJpeg()
{
    CBitmap  cBitmap;
    BITMAP   bm;
    CString  csMsg = "";

    cBitmap.LoadBitmap(IDB_TEST);
    cBitmap.GetBitmap(&bm);

    HANDLE  hDib = DDBToDIB((HBITMAP)cBitmap,
                             BI_RGB,
                             NULL);  //Use default palette

    //Turn DIB into JPEG file
    if (hDib != NULL)
    {
        if (!JpegFromDib(hDib,
                         100,   //Quality setting
                         "test.jpg",
                         &csMsg))
        {
            AfxMessageBox(csMsg);
        }

        else
            AfxMessageBox("test.jpg created");

        ::GlobalFree(hDib);
    }

    else
        AfxMessageBox("Failed to load IDB_TEST");
}

////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////
//Header file code
////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////


//In the relevant header file (up near the top):

#include "jpeglib.h"


BOOL JpegFromDib(HANDLE     hDib,     //Handle to DIB
                 int        nQuality, //JPEG quality (0-100)
                 CString    csJpeg,   //Pathname to target jpeg file
                 CString*   pcsMsg);  //Error msg to return

BOOL BuildSamps(HANDLE                      hDib,
                int                         nSampsPerRow,
                struct jpeg_compress_struct cinfo,
                JSAMPARRAY                  jsmpArray,
                CString*                    pcsMsg);

RGBQUAD QuadFromWord(WORD b16);

////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////
// Source file code
////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////

extern "C"
{
#include "jpeglib.h"
}
         
#include 


struct ima_error_mgr
{
  struct  jpeg_error_mgr pub; //"public" fields
  jmp_buf setjmp_buffer;	  //for return to caller
};

////////////////////////////////////////////////////////////////////////////
//This function takes the contents of a DIB
//and turns it into a JPEG file.
//
//The DIB may be monochrome, 16-color, 256-color, or 24-bit color.
//
//Any functions or data items beginning with "jpeg_" belong to jpeg.lib,
//and are not included here.
//
//The function assumes 3 color components per pixel.
/////////////////////////////////////////////////////////////////////////////
BOOL JpegFromDib(HANDLE     hDib,     //Handle to DIB
                 int        nQuality, //JPEG quality (0-100)
                 CString    csJpeg,   //Pathname to jpeg file
                 CString*   pcsMsg)   //Error msg to return
{
    //Basic sanity checks...
    if (nQuality <0 || nQuality> 100 ||
        hDib   == NULL ||
        pcsMsg == NULL ||
        csJpeg == "")
    {
        if (pcsMsg != NULL)
            *pcsMsg = "Invalid input data";

        return FALSE;
    }

    *pcsMsg = "";

    LPBITMAPINFOHEADER lpbi = (LPBITMAPINFOHEADER)hDib;

    byte *buf2 = 0;

    //Use libjpeg functions to write scanlines to disk in JPEG format
    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr       jerr;

    FILE*      pOutFile;     //Target file 
    int        nSampsPerRow; //Physical row width in image buffer 
    JSAMPARRAY jsmpArray;    //Pixel RGB buffer for JPEG file

    cinfo.err = jpeg_std_error(&jerr); //Use default error handling (ugly!)

    jpeg_create_compress(&cinfo);

    if ((pOutFile = fopen(csJpeg, "wb")) == NULL)
    {
        *pcsMsg = "Cannot open ";
		*pcsMsg += csJpeg;
        jpeg_destroy_compress(&cinfo);
        return FALSE;
    }

    jpeg_stdio_dest(&cinfo, pOutFile);

    cinfo.image_width      = lpbi->biWidth;  //Image width and height, in pixels 
    cinfo.image_height     = lpbi->biHeight;
    cinfo.input_components = 3;              //Color components per pixel
                                             //(RGB_PIXELSIZE - see jmorecfg.h)
    cinfo.in_color_space   = JCS_RGB; 	     //Colorspace of input image

    jpeg_set_defaults(&cinfo);

    jpeg_set_quality(&cinfo,
                     nQuality, //Quality: 0-100 scale
                     TRUE);    //Limit to baseline-JPEG values

    jpeg_start_compress(&cinfo, TRUE);

    //JSAMPLEs per row in output buffer
    nSampsPerRow = cinfo.image_width * cinfo.input_components; 

    //Allocate array of pixel RGB values
    jsmpArray = (*cinfo.mem->alloc_sarray)
                ((j_common_ptr) &cinfo,
                 JPOOL_IMAGE,
                 nSampsPerRow,
                 cinfo.image_height);

    if (DibToSamps(hDib,
                   nSampsPerRow,
                   cinfo,
                   jsmpArray,
                   pcsMsg))
    {
        //Write the array of scan lines to the JPEG file
        (void)jpeg_write_scanlines(&cinfo,
                                   jsmpArray,
                                   cinfo.image_height);
    }

    jpeg_finish_compress(&cinfo); //Always finish

    fclose(pOutFile);

    jpeg_destroy_compress(&cinfo); //Free resources

    if (*pcsMsg != "")
        return FALSE;

    else
        return TRUE;
}

////////////////////////////////////////////////////////////////
//This function fills a jsmpArray with the RGB values
//for the CBitmap.
//
//It has been improved to handle all legal bitmap formats.
//
//A jsmpArray is a big array of RGB values, 3 bytes per value.
//
//Note that rows of pixels are processed bottom to top:
//The data in the jsamp array must be arranged top to bottom.
////////////////////////////////////////////////////////////////
BOOL DibToSamps(HANDLE                      hDib,
                int                         nSampsPerRow,
                struct jpeg_compress_struct cinfo,
                JSAMPARRAY                  jsmpPixels,
                CString*                    pcsMsg)
{
   //Sanity...
   if (hDib == NULL    ||
     nSampsPerRow <= 0 || pcsMsg="=" NULL) { if (pcsMsg !="NULL)" *pcsMsg="Invalid input data" ; return FALSE; } int r="0," p="0," q="0," b="0," n="0," nUnused="0," nBytesWide="0," nUsed="0," nLastBits="0," nLastNibs="0," nCTEntries="0," nRow="0," nByte="0," nPixel="0;" BYTE bytCTEnt="0;" LPBITMAPINFOHEADER pbBmHdr="(LPBITMAPINFOHEADER)hDib;" //The bit count tells you the format of the bitmap: //Decide how many entries will be in the color table (if any) switch (pbBmHdr->biBitCount)
   {
      case 1:
         nCTEntries = 2;   //Monochrome
         break;

      case 4:
         nCTEntries = 16;  //16-color
         break;

      case 8:
         nCTEntries = 256; //256-color
         break;

      case 16:
      case 24:
      case 32:
         nCTEntries = 0;   //No color table needed
         break;

      default:
         *pcsMsg = "Invalid bitmap bit count";
         return FALSE; //Unsupported format
   }

   //Point to the color table and pixels
   DWORD     dwCTab = (DWORD)pbBmHdr + pbBmHdr->biSize;
   LPRGBQUAD pCTab  = (LPRGBQUAD)(dwCTab);
   LPSTR     lpBits = (LPSTR)pbBmHdr +
                      (WORD)pbBmHdr->biSize +
                      (WORD)(nCTEntries * sizeof(RGBQUAD));

   //Different formats for the image bits
   LPBYTE   lpPixels = (LPBYTE)  lpBits;
   RGBQUAD* pRgbQs   = (RGBQUAD*)lpBits;
   WORD*    wPixels  = (WORD*)   lpBits;

   //Set up the jsamps according to the bitmap's format.
   //Note that rows are processed bottom to top, because
   //that's how bitmaps are created.
   switch (pbBmHdr->biBitCount)
   {
      case 1:
         nUsed      = (pbBmHdr->biWidth + 7) / 8;
         nUnused    = (((nUsed + 3) / 4) * 4) - nUsed;
         nBytesWide = nUsed + nUnused;
         nLastBits  = 8 - ((nUsed * 8) - pbBmHdr->biWidth);

         for (r=0; r 
biHeight; r++)
         {
            for (p=0,q=0; p 
biHeight-r-1) * nBytesWide;
               nByte =  nRow + p;

               int nBUsed = (p <(nUsed 1)) ? 8 : nLastBits; for(b="0;" b < nBUsed;b++) { bytCTEnt="lpPixels[nByte]" << b; bytCTEnt="bytCTEnt">> 7;

                  jsmpPixels[r][q+0] = pCTab[bytCTEnt].rgbRed;
                  jsmpPixels[r][q+1] = pCTab[bytCTEnt].rgbGreen;
                  jsmpPixels[r][q+2] = pCTab[bytCTEnt].rgbBlue;

                  q += 3;
               }
            }
         }
         break;

      case 4:
         nUsed      = (pbBmHdr->biWidth + 1) / 2;
         nUnused    = (((nUsed + 3) / 4) * 4) - nUsed;
         nBytesWide = nUsed + nUnused;
         nLastNibs  = 2 - ((nUsed * 2) - pbBmHdr->biWidth);

         for (r=0; r 
biHeight;r++)
         {
            for (p=0,q=0; p 
biHeight-r-1) * nBytesWide;
               nByte = nRow + p;

               int nNibbles = (p 
 > (4-(n*4));

                  jsmpPixels[r][q+0] = pCTab[bytCTEnt].rgbRed;
                  jsmpPixels[r][q+1] = pCTab[bytCTEnt].rgbGreen;
                  jsmpPixels[r][q+2] = pCTab[bytCTEnt].rgbBlue;

                  q += 3;
               }
            }
         }
         break;

      default:
      case 8: //Each byte is a pointer to a pixel color
         nUnused = (((pbBmHdr->biWidth + 3) / 4) * 4) -
                   pbBmHdr->biWidth;

         for (r=0;r 
biHeight; r++)
         {
            for (p=0,q=0; p 
biWidth; p++,q+=3)
            {
               nRow   = (pbBmHdr->biHeight-r-1) * (pbBmHdr->biWidth + nUnused);
               nPixel =  nRow + p;

               jsmpPixels[r][q+0] = pCTab[lpPixels[nPixel]].rgbRed;
               jsmpPixels[r][q+1] = pCTab[lpPixels[nPixel]].rgbGreen;
               jsmpPixels[r][q+2] = pCTab[lpPixels[nPixel]].rgbBlue;
            }
         }
         break;

      case 16: //Hi-color (16 bits per pixel)
         for (r=0;r 
biHeight; r++)
         {
            for (p=0,q=0; p 
biWidth; p++,q+=3)
            {
               nRow    = (pbBmHdr->biHeight-r-1) * pbBmHdr->biWidth;
               nPixel  = nRow + p;

               RGBQUAD quad = QuadFromWord(wPixels[nPixel]);

               jsmpPixels[r][q+0] = quad.rgbRed;
               jsmpPixels[r][q+1] = quad.rgbGreen;
               jsmpPixels[r][q+2] = quad.rgbBlue;
            }
         }
         break;

      case 24:
         nBytesWide =  (pbBmHdr->biWidth*3);
         nUnused    =  (((nBytesWide + 3) / 4) * 4) -
                       nBytesWide;
         nBytesWide += nUnused;

         for (r=0;r 
biHeight;r++)
         {
            for (p=0,q=0;p <(nBytesWide-nUnused); p+="3,q+=3)" { nRow="(pbBmHdr-">biHeight-r-1) * nBytesWide;
               nPixel  = nRow + p;

               jsmpPixels[r][q+0] = lpPixels[nPixel+2]; //Red
               jsmpPixels[r][q+1] = lpPixels[nPixel+1]; //Green
               jsmpPixels[r][q+2] = lpPixels[nPixel+0]; //Blue
            }
         }
         break;

      case 32:
         for (r=0; r 
biHeight; r++)
         {
            for (p=0,q=0; p 
biWidth; p++,q+=3)
            {
               nRow    = (pbBmHdr->biHeight-r-1) *
                          pbBmHdr->biWidth;
               nPixel  = nRow + p;

               jsmpPixels[r][q+0] = pRgbQs[nPixel].rgbRed;
               jsmpPixels[r][q+1] = pRgbQs[nPixel].rgbGreen;
               jsmpPixels[r][q+2] = pRgbQs[nPixel].rgbBlue;
            }
         }
         break;
   }   //end switch

return TRUE;
}
 
////////////////////////////////////////
//This function turns a 16-bit pixel
//into an RGBQUAD value.
////////////////////////////////////////
RGBQUAD QuadFromWord(WORD b16)
{
   BYTE bytVals[] =
   {
     0,  16, 24, 32,  40, 48, 56, 64,
     72, 80, 88, 96, 104,112,120,128,
     136,144,152,160,168,176,184,192,
     200,208,216,224,232,240,248,255
   };

   WORD wR = b16;
   WORD wG = b16;
   WORD wB = b16;

   wR <<= 1; wR >>= 11;
   wG <<= 6; wG >>= 11;
   wB <<= 11; wB >>= 11;

   RGBQUAD rgb;

   rgb.rgbReserved = 0;
   rgb.rgbBlue     = bytVals[wB];
   rgb.rgbGreen    = bytVals[wG];
   rgb.rgbRed      = bytVals[wR];

   return rgb;
}