/// <summary>
/// Gets (256) array of histogram values for image.
/// </summary>
/// <param name="imageBytes">Image bytes</param>
/// <param name="pixelDepth">Pixel depth of image</param>
/// <returns>256 array of histogram values</returns>
public static int[] GetHistogramValues(byte[] imageBytes, int pixelDepth)
{
bool isColor = BitmapDataExt.IsColorPixelDepth(pixelDepth);
// 0-255
int[] histogram = new int[byte.MaxValue + 1];
int limit = BitmapDataExt.GetSafeArrayLimitForImage(isColor, pixelDepth, imageBytes);
byte avg;
// Find number of pixels at each level.
for (int i = 0; i < limit; i += pixelDepth)
{
if (isColor)
{
avg = (byte)((imageBytes[i] + imageBytes[i + 1] + imageBytes[i + 2]) / 3);
}
else
{
avg = imageBytes[i];
}
// double-check within range.
if (avg >= 0 && avg < histogram.Length)
{
histogram[avg]++;
}
}
return(histogram);
}
/// <summary>
/// Histogram Equalization will enhance general contrast
/// by distributing grey levels wider and more evenly.
/// </summary>
/// <param name="imageBytes">Image bytes</param>
/// <param name="pixelDepth">Pixel depth of image</param>
/// <param name="imageWidth">Image width</param>
/// <param name="imageHeight">Image height</param>
public static void HistogramEqualizationDirect(byte[] imageBytes, int pixelDepth, int imageWidth, int imageHeight)
{
int[] histogram = ImageHistogram.GetHistogramValues(imageBytes, pixelDepth);
double numberOfPixels = imageWidth * imageHeight;
double numberOfLevels = histogram.Length;
int cumulativeFrequency = 0;
int equalizedValue;
// Calculate cumulative distribution for histogram
for (int i = 0; i < histogram.Length; i++)
{
cumulativeFrequency += histogram[i];
// Calculate equalized value
equalizedValue = (int)Math.Round((numberOfLevels * cumulativeFrequency) / numberOfPixels) - 1;
// Ensure +ve value
histogram[i] = Math.Max(0, equalizedValue);
}
bool isColor = BitmapDataExt.IsColorPixelDepth(pixelDepth);
int limit = BitmapDataExt.GetSafeArrayLimitForImage(isColor, pixelDepth, imageBytes);
// Apply distribution to image to equalize
if (isColor)
{
// Exclude alpha/transparency byte
int pixelDepthWithoutAlpha = Math.Min(pixelDepth, Constants.PixelDepthRGB);
for (int i = 0, j; i < limit; i += pixelDepth)
{
for (j = 0; j < pixelDepthWithoutAlpha; j++)
{
imageBytes[i + j] = (byte)histogram[imageBytes[i + j]];
}
}
}
else
{
for (int i = 0; i < limit; i++)
{
imageBytes[i] = (byte)histogram[imageBytes[i]];
}
}
}
