L_GetBitmapHistogram

#include "l_bitmap.h"

L_INT EXT_FUNCTION L_GetBitmapHistogram(pBitmap, pHisto, uHistoLen, uFlags)

Creates an array that charts how many times each intensity level occurs in a bitmap. This function can chart red, green, and blue separately or together. It is used for all resolutions, including 12 and 16-bit grayscale.

Returns

Comments

For 1, 2, 3, 4, 5, 6, 7, 8, 16, 24, and 32 bit color bitmaps, intensity levels range from 0 to 255. Therefore, the resulting array has 256 items, indexed by intensity value. The value of each item is the number of occurrences of the intensity level.

This function supports 12 and 16-bit grayscale and 48 and 64-bit color images. Support for 12 and 16-bit grayscale and 48 and 64-bit color images is available only in the Document/Medical toolkits.

For 48 and 64-bit images, the intensity levels range from 0 to 65535. Therefore, the resulting array has 65536 items, indexed by intensity value. The value of each item is the number of occurrences of the intensity level.

This functions also works on 12 and 16-bit grayscale images. Intensity values in these bitmaps can range from 0 to (2^16 – 1) for 16-bit grayscale or from 0 to (2^12 – 1) for 12- bit grayscale. The LowBit and HighBit identify which bits in a 12-bit or 16-bit entry are used. The LowBit and HighBit can be obtained by using L_GetMinMaxBits or by looking at the corresponding fields in the BITMAPHANDLE itself.

Specifically, the table must hold at least 2^(HighBit – LowBit + 1) entries if HIST_LOWHIGH_BITS is set in uFlags. If HIST_ALL_BITS is set in uFlags, the table must hold at least 4096 for 12-bit bitmaps and 65536 for 16-bit bitmaps.

For example, suppose HIST_LOWHIGH_BITS is set in uFlags and you have a 16-bit grayscale image with LowBit = 2 and HighBit = 7. There are 2^(7-2+1) = 64 possible grayscale intensities. The table must therefore hold at least 64 entries. The number of grayscale values with bits 2 – 7 set to 0 can be found in pHisto [0]. The number of grayscale values with bit 2 set to 1 and bits 3 – 7 set to 0 can be found in pHisto [1], and so on up to pHisto [63]. Since the low bit is 2 and the high bit is 7, bits 0, 1, and bits 8 – 15 must all be 0. Therefore the values set in bits 2 – 7 determine the intensities present in the image. In the table below, the gray columns represent those bits that are always 0 for the image. The columns bit 7 through bit 2 represent possible settings for those bits. The last column gives the location within the pHisto array of the number of grayscale values having the corresponding settings. For example, pHisto [0] contains the number of grayscale values of intensity 0 (all bits set to 0). pHisto [1] contains all the grayscale values with intensity 4 (the 2 bit position set to 1). pHisto [2] contains all the grayscale values with intensity 8 (the 3 bit set to 1 and the remaining bits set to 0) and so on.

As another example, suppose you have a 16-bit grayscale image with LowBit = 0 and HighBit = 15. The table must hold 2^16 = 65,536 L_UINT32 values.

To update a status bar or detect a user interrupt during execution of this function, refer to L_SetStatusCallback.

This function supports signed data images.

Required DLLs and Libraries

Platforms

Windows 95 / 98 / Me, Windows 2000 / XP.

See Also

Example

/* This example loads a bitmap at 24 bit per pixel and creates a red-channel histogram, finds the brightest
   and darkest intensities in the histogram, and displays the values */
L_TCHAR szMessage[80];               /* Buffer for the MessageBox string */
BITMAPHANDLE LeadBitmap;    /* Bitmap handle to hold the loaded image */
L_UINT32 Histogram[256];           /* Array for the histogram */
L_INT nIndex;                               /* Array index */
L_INT nBrightest;                         /* Brightest value */
L_INT nDarkest;                           /* Darkest value */
/* Load the bitmap at 24  bits per pixel */
L_LoadBitmap (TEXT("IMAGE3.CMP"), &LeadBitmap, sizeof(BITMAPHANDLE), 24, ORDER_BGR, NULL, NULL);
/* Create the red-channel histogram */
L_GetBitmapHistogram(&LeadBitmap, Histogram, 256, CHANNEL_RED | HIST_ALL_BITS);
/* Initialize the darkest and brightest index values */
nDarkest = 0;
nBrightest = 0;
/* Find the brightest and darkest intensities in the histogram */
for(nIndex = 0; nIndex < 256; nIndex++)
{
    if (Histogram[nIndex] != 0)
    {
        nBrightest = nIndex;
        if (nDarkest == 0)
           nDarkest = nIndex;
    }
}
/* Display the values in a message box */
wsprintf (szMessage, TEXT("Darkest red = %d;  Brightest red = %d"), nDarkest, nBrightest);
MessageBox (NULL, szMessage, TEXT("Notice"), MB_OK);