Processing an Image

Most image processing functions act on a bitmap in memory, which means that the changes become permanent when you save the image in a file. Some low-level functions act on a buffer that you manage, for example when you process data as it is loaded.

Changing the Data Format

You can change the color resolution (bits per pixel) of an image using a number of high-level and low-level functions. For a list of these functions, refer to Doing Color Expansion or Reduction.

Windows always uses the RGB color-space model, and when loading or saving a file, LEADTOOLS converts image data to or from RGB, as necessary. Nevertheless, LEADTOOLS provides functions for other color-space models. You can use the high-level L_ColorSeparateBitmap and L_ColorMergeBitmap functions to create and merge color separations using a number of color-space models, including RGB, CMYK, CMY, HSV, and HLS. You can also use the low-level L_ConvertColorSpace function to convert raw data in a buffer from one color-space model to another, including RGB, YUV, CMYK, CMY, YIQ, HSV, and HLS.

Changing the Data Type (Signed / Unsigned)

Image data can be changed from signed to unsigned and vice versa. Signed image data may contain some negative values. Converting data from signed to unsigned, and vice versa, is done by shifting the image data/intensity values by a specific value. The ability to shift image data back and forth between signed and unsigned is often useful in medical or analytical applications.

To change image data from signed to unsigned, which is often done before applying image processing or analysis functions, there are the following options:

$image\sqrblit.gif$ L_ConvertBitmapUnsignedToSigned shifts the image data based on internal aspects of the bitmap.

$image\sqrblit.gif$ L_ShiftMinimumToZero provides the user with the value by which the image data was shifted. This information can be used later, after image processing, to return the image to signed data.

$image\sqrblit.gif$ L_ClearNegativePixels sets all negative pixels to 0, therefore creating an unsigned image.

Once signed images have been converted to unsigned images image processing or analysis functions can be performed on the unsigned images. (Most image processing functions work only on unsigned data.) When the image processing or analysis is complete, convert the unsigned data back to signed data.

To change image data from unsigned to signed, which is often done after applying image processing or analysis functions, there are the following options:

$image\sqrblit.gif$ L_ConvertBitmapSignedToUnsigned shifts the image data based on internal aspects of the bitmap.

$image\sqrblit.gif$ L_ShiftZeroToNegative gives the user the ability to specify the amount by which to shift the image data, and dictates the minimum and maximum values that are output by the function.

Doing Geometric Transformations

Geometric transformations include resizing, trimming, rotating, shearing, flipping, or reversing a bitmap. Some geometric transformation functions are designed mainly for document imaging. For example, L_DeskewBitmap (Document/Medical only) and L_DeskewBitmapExt (Document/Medical only) let you automatically straighten scanned documents. For resizing any kind of bitmap, L_SizeBitmap provides the most up-to-date options, but you may also find the alternative and low-level functions useful.

LEADTOOLS also provides the L_TransformFile function for performing lossless flips, rotations and reversals. However, only certain file formats are supported at this time. This function provides better results than loading an image, transforming it using L_RotateBitmap, L_FlipBitmap or L_ReverseBitmap and then resaving it. It uses the TRANSFORMFILECALLBACK function for processing the file. The TRANSFORMFILECALLBACK function in turn uses the LEADMARKERCALLBACK function for writing any updated markers within the file. For more information, refer to L_TransformFile, TRANSFORMFILECALLBACK and LEADMARKERCALLBACK. The L_DisplaceMapBitmap function displaces bitmap pixels horizontally and vertically according to the values of the displacement map image. The L_PerspectiveBitmap function gives a bitmap a 3-D depth, as if it exists on a flat plane and has been skewed into a different shape.

For a list of functions in this group, refer to Raster Image Functions: Doing Geometric Transformations. For information about memory allocation when a bitmap gets bigger, refer to Resizing Considerations.

Lightening, Darkening, Filtering, and So Forth

A number of image processing functions let you change the values of pixels across a bitmap (or a region in the bitmap) using various algorithms and filters. You might do this to improve the appearance of the image, to analyze details in the image, or to apply artistic effects. The following topics provide details:

Changing Brightness and Contrast

Removing Noise

Detecting and Enhancing Edges and Lines

Applying Artistic Effects

Correcting Colors

Adding Another Image to a Bitmap

The L_CombineBitmap function lets you add all or part of another bitmap to the target bitmap. You can pass flags to control whether the new pixels simply replace the old ones or are combined with the old ones in a specified way. You can also define regions in either or both of the bitmaps, and if you do, the function affects only the intersection of the regions. The L_CombineBitmapExt extends the capabilities of the L_CombineBitmap function by letting the user combine bitmaps of different bits per pixel and specify which color planes to use when combining the bitmaps. For information about regions, refer to Defining and Using a Bitmap Region.

Source and destination bitmaps may be combined by warping all or part of a source bitmap into part of a destination bitmap. To combine images in this manner, use L_CombineBitmapWarp.

The L_UnderlayBitmap function also combines images, but it has a much more specific purpose. It combines two bitmaps so that one appears to be an underlying texture for the other.

LEADTOOLS provides several functions for picturizing an image. The L_PicturizeBitmap function replaces an image with a new image created by combining images present in a specific directory. L_PicturizeBitmapList replaces an image with a new image created by combining images present in a bitmap list. The L_PicturizeBitmapSingle function picturizes an image using various versions of a single image.

Two bitmaps can also be blended by combining the images with an opacity value. The L_AlphaBlendBitmap function combines two bitmaps with a fixed opacity value to create a new blended image. The L_FeatherAlphaBlendBitmap function combines two bitmaps with feathering by using a variable opacity that depends on a fade mask. A fade mask bitmap can be created using the L_CreateFadedMask function.

Borders and frames may be added to a bitmap using the L_AddBorder and L_AddFrame functions.

The L_BumpMapBitmap function lets you combine a bitmap with a bump image to create a three-dimensional texture pattern.

The L_CanvasBitmap function applies an image as an overlay for another image.

A number of other functions let you get and put pixel data. For example, you can fill a bitmap with a color, and you can get and put individual pixel values. For a list of functions, refer to Getting and Setting Pixel Values.

Treating the bitmap as a device context opens up many possibilities. The L_CreateLeadDC function lets you get the device context. You can then use Windows GDI functions to draw lines, text, or images on the bitmap. You can also use the LEADTOOLS special effects functions to add three-dimensional shapes or text, or to combine images in interesting ways. For details, refer to Implementing Special Effects.

L_TextureAlphaBlendBitmap combines image data from pBitmapMask and pBitmapUnderlay with an underlay effect. The result is used as a fade mask that will be used to combine pBitmapSrc and pBitmapDst with variable opacity. The result is combined again with pBitmapDst using a constant opacity (using nOpacity). The L_BricksTextureBitmap function creates a brick texture that makes the image look like it has been drawn on a brick wall.

The L_DigitalSubtractBitmap function does not combine images, but subtracts the live (destination) bitmap from the mask (source) bitmap to show the differences between the two bitmaps.

Comparing Images

Two bitmaps or parts of two bitmaps can be compared using the L_CorrelationBItmap function. This function compares all or part of one bitmap with all the areas of the same dimensions in another bitmap and finds those areas that match according to the measure of correlation. The L_CorrelationListBitmap function compares the images in hCorList with all the areas of the same dimensions in pBitmap and finds those portions that match according to the measure of correlation.

Extracting Bitmaps From A Series

The L_SliceBitmap function is designed to extract the individual slices from radiographic scanned film.