L_AutoColorLevelBitmap

Summary

Applies one of several types of automatic color leveling to an image. This function is also useful for pre-processing images to improve barcode recognition results.

Syntax

L_LTIMGCLR_API L_INT L_AutoColorLevelBitmap(pBitmap, pLvlClr, uBlackClip, uWhiteClip, uFlags)

Parameters

pBITMAPHANDLE pBitmap

Pointer to the bitmap handle.

pLVLCLR pLvlClr

Pointer to a LVLCLR structure that specifies the color leveling information used by the function. If AUTO_NOPROCESS is not set in uFlags, you can pass NULL if you are not interested in using these values. If AUTO_NOPROCESS is set in uFlags, then this parameter cannot be NULL.

L_UINT uBlackClip

An unsigned integer that represents the percentage of black pixels to clip from the bitmap, in hundredths of a percent. Valid values range from 0 to 10000 (which represents 100%). Best results are found using 0.3 to 0.9 %. The default is DEFAULT_BLACK_CLIP, which indicates to clip 0.5% of the black pixels.

L_UINT uWhiteClip

An unsigned integer that represents the percentage of pixels to clip from the bitmap, in hundredths of a percent. Valid values range from 0 to 10000 (which represents 100%). The default is DEFAULT_WHITE_CLIP, which indicates to clip 0.5% of the white pixels.

L_UINT uFlags

A flag that indicates the type of leveling to perform. Possible values are:

Returns

Comments

This function makes dark values darker and light values lighter, automatically enhancing shadows and brightening the image. It also maximizes the tonal range of the image. It enhances contrast and intensity using the image's histogram and a color distribution-based algorithm.

Before calling L_AutoColorLevelBitmap, initialize the size of the LVLCLR structure. The other fields of the structure will be updated with the uMinInput, uMaxInput, uMinOutput, uMaxOutput and uGamma values used by the function for the appropriate channel(s).

If you set the AUTO_NOPROCESS flag in uFlags, the pLvlClr parameter will be updated with the color leveling information used by the L_AutoColorLevelBitmap function. If you then pass the updated pLvlClr structure to the L_ColorLevelBitmap function, you will obtain the same result that you would have obtained if you had called the L_AutoColorLevelBitmap function with the leveling flag by itself.

If you set AUTO_NOPROCESS in uFlags and pass NULL to pLvlClr the function will return an error code.

If you pass only AUTO_NOPROCESS to uFlags then the function will return an error code.

If the image is a grayscale image, then AUTO_LEVEL, AUTO_CONTRAST and AUTO_INTENSITY give identical results. Also the member variables master, red, green and blue of the structure pLvlClr will have the same values.

If the bitmap has a region, the effect will be applied only on the region.

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 in theDocument and Medical Imaging toolkits.

This function does not support signed data images. It returns the error code ERROR_SIGNED_DATA_NOT_SUPPORTED if a signed data image is passed to this function.

This function does not support 32-bit grayscale images. It returns the error code ERROR_GRAY32_UNSUPPORTED if a 32-bit grayscale image is passed to this function.

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

For an example, click here.

Calculating Master Channel Values

In order to speed up widely used image processing filters in LEADTOOLS, the grayscale value (master channel) of a colored image is calculated using the following formulas:

#define CalcGrayValue(r, g, b) ((L_UCHAR)(((L_UCHAR) (((2 * (L_UINT) (r)) + (5 * (L_UINT) (g)) + (L_UINT) (b) + 4) / 8)))) 
#define CalcGrayValue16(r, g, b) ((L_UINT16) (((2 * (L_UINT32) (r)) + (5 * (L_UINT32) (g)) + (L_UINT32) (b) + 4) / 8)) 
#define CalcGrayValue32(r, g, b) ((L_UINT32) (((2 * (L_UINT32) (r)) + (5 * (L_UINT32) (g)) + (L_UINT32) (b) + 4) / 8)) 

View additional platform support for this Auto Color Level function.

Required DLLs and Libraries

• LTIMGCLR
• For a listing of the exact DLLs and Libraries needed, based on the toolkit version, refer to Files To Be Included With Your Application.

Platforms

Win32, x64, Linux.

See Also

Functions

• L_SharpenBitmap
• L_PosterizeBitmap
• L_MosaicBitmap
• L_EmbossBitmap
• L_MedianFilterBitmap
• L_AddBitmapNoise
• L_IntensityDetectBitmap
• L_SpatialFilterBitmap
• L_BinaryFilterBitmap
• L_MaxFilterBitmap
• L_MinFilterBitmap
• L_OilifyBitmap
• L_SolarizeBitmap
• L_DlgColor
• L_WindowLevel
• L_ColorLevelBitmap
• L_SelectiveColorBitmap
• L_GammaCorrectBitmapExt
• L_AdjustBitmapTint
• L_ColorHalfToneBitmap

Topics

• Raster Image Functions: Modifying Intensity Values
• Changing Brightness and Contrast
• Raster Image Functions: Changing Brightness and Contrast
• Correcting Colors
• Color Halftone and Halftone Images
• Raster Image Functions: Correcting Colors

Example

This example loads a bitmap and applies auto image leveling.

L_INT AutoColorLevelBitmapExample(L_VOID) 
{ 
   L_INT nRet; 
   BITMAPHANDLE LeadBitmap;   /* Bitmap handle to hold the loaded image. */ 
   LVLCLR lc;        /* Structure to hold the level values, used for display only */ 
   L_TCHAR buffer[100]; 
 
 
   /* Load the bitmap, keeping the bits per pixel of the file */ 
   nRet = L_LoadBitmap(MAKE_IMAGE_PATH(TEXT("Image1.jpg")), &LeadBitmap, sizeof(BITMAPHANDLE), 0, ORDER_BGR, NULL, NULL); 
   if (nRet != SUCCESS) 
      return nRet; 
 
   /* Apply "Auto Leveling" to the bitmap */ 
   lc.uStructSize = sizeof(LVLCLR); 
   nRet = L_AutoColorLevelBitmap(&LeadBitmap, &lc, DEFAULT_BLACK_CLIP, DEFAULT_WHITE_CLIP, AUTO_LEVEL); 
   if (nRet != SUCCESS) 
      return nRet; 
 
   wsprintf(buffer, TEXT("Min and Max input values for the blue channel using auto level are %d and %d"), lc.blue.nMinInput, lc.blue.nMaxInput); 
   if (LeadBitmap.Flags.Allocated) 
      L_FreeBitmap(&LeadBitmap); 
 
   return SUCCESS; 
}