Certain members in the BITMAPHANDLE structure mentioned below are intended for internal use, including
These members are used when window-leveling. More information can be found in the LTKRN.H header file, which describes the entire structure.
12-/16-bit bitmaps can use a lookup table (LUT) for displaying images or performing image processing. However, using LUT is optional (it can be NULL). If the LUT is not NULL, it can be used for 'displaying' or for 'displaying and performing image processing'. The LUT data is created by window-leveling (L_ApplyLinearVOILUT, L_WindowLevel) or by loading a DICOM or TIFF file with window-leveling information.
There are 3 cases:
The LUT data is stored in the BITMAPHANDLE using the following fields:
Pointer to an array of
L_RGBQUAD values containing the corresponding color for each LUT entry. Usually, the
L_RGBQUAD entries contain grayscale colors.
But sometimes, the
L_RGBQUAD entries contain non-grayscale colors. The number of entries is determined by
PaintHighBit entries in the BITMAPHANDLE structure.
pLUT is NULL and
pLUT16 is NULL, then there is no LUT (case '1' above).
pLUT, except that the array contains
L_RGBQUAD16 values and each color value is 16-bit. 16-bit values provide extra precision compared to 8-bit values.
If TRUE, the LUT data is used for displaying and performing image processing (case '3' above). In this case, the number of LUT entries is determined by
LUT Length = 1 << (HighBit – LowBit + 1)
If FALSE, the LUT is used only for display. In this case, the number of LUT entries is determined by
LUT Length = 1 << (PaintHighBit – PaintLowBit + 1)
pLUT16 are synchronized (that is, they contain the same information). If
pLUT == NULL and
pLUT16 != NULL, then there is a LUT. Conversely, if
pLUT != NULL and
pLUT16 == NULL, there is a LUT.
pLUT != NULL and
pLUT16 != NULL, there is a LUT and both arrays will point to the same colors and have the same length.
PaintLowBit/PaintHighBit can be different from
LowBit/HighBit, but usually they are the same.
Here are the 3 cases again:
pLUT == NULL
pLUT != NULLand
Flags.UseLUT == FALSE(or 0)
pLUT != NULLand
Flags.UseLUT == TRUE(or 1)
In cases '1' and '2', image processing will be performed on image data.
In case '3', image processing is performed and there is a LUT. The image can look completely different after image processing, so it is best to regenerate the LUT data to make sure you see all the details in the new bitmap. Regenerate the LUT by doing something like this:
L_GetMinMaxVal(pBitmap, MinVal, MaxVal);
L_ApplyLinearVOILUT(pBitmap, (MinVal + MaxVal / 2), (MaxVal – MinVal) / 2, 0);
In case '3', the image processing would typically take the LUT colors into account.
There are several types of image processing functions and the output will vary, producing one of these results:
Image processing does not change the size, and the output of each pixel depends only on the color of the original pixel (for example, when inverting the colors or changing the brightness). In this case, image processing will be performed on the LUT colors only and the data will be unchanged.
Image processing changes the size of the image. In this case, image processing is performed on the image data and the LUT entries are taken into account, if appropriate.
Flags.UseLUTto FALSE before the image processing operation, then they may restore it after. Users who are interested in the image processing function and applying it on an image as they see it in the viewer with LUT, they need to convert their 12- or 16-bit grayscale image to an 8-bit grayscale or a 24-bit RGB bitmap using
L_WindowLevelBitmap, then perform image processing on the converted image.
In some instances image processing performs well when using LUT, while in other cases it is better if the LUT is ignored. You can choose whichever mode is more appropriate for your application. Here is how you can get to each mode:
L_WindowLevel(pBitmap, pBitmap->LowBit, pBitmap->HighBit, NULL, 0, WINDOWLEVEL_PAINT)
(pBitmap, lowbit, highbit, pLUT, LUTLength, WINDOWLEVEL_PAINT)
(…, WINDOWLEVEL_PAINT_AND_PROCESSING)or L_ApplyLinearVOILUT.
You can also switch between '2' and '3' by switching the
pBitmap->Flags.UseLUT flag between 0 and 1. But when doing so, make sure
PaintLowBit == LowBit and
PaintHighBit == HighBit.
The above L_WindowLevel calls can also be replaced with L_WindowLevelExt function calls in order to use 16-bit entries for the LUT.
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