// Checks whether the image is completely black
public static unsafe bool IsBlack(this WriteableBitmap image)
{
// Check the arguments for null
ThrowIf.IsNullArgument(image, nameof(image));
// The RGB offsets from the beginning of the pixel (i.e., 0, 1 or 2)
WriteableBitmapExtensions.GetColorOffsets(image, out int blueOffset, out int greenOffset, out int redOffset);
// examine only a subset of pixels as otherwise this is an expensive operation
// check pixels from last to first as most cameras put a non-black status bar or at least non-black text at the bottom of the frame,
// so reverse order may be a little faster on average in cases of nighttime images with black skies
// TODO Calculate pixelStride as a function of image size so future high res images will still be processed quickly.
byte *currentPixel = (byte *)image.BackBuffer.ToPointer(); // the imageIndex will point to a particular byte in the pixel array
int pixelStride = Constant.ImageValues.DarkPixelSampleStrideDefault;
int totalPixels = image.PixelHeight * image.PixelWidth; // total number of pixels in the image
for (int pixelIndex = totalPixels - 1; pixelIndex > 0; pixelIndex -= pixelStride)
{
// get next pixel of interest
byte b = *(currentPixel + blueOffset);
byte g = *(currentPixel + greenOffset);
byte r = *(currentPixel + redOffset);
if (r != 0 || b != 0 || g != 0)
{
return(false);
}
}
return(true);
}
// Given three images, return an image that highlights the differences in common betwen the main image and the first image
// and the main image and a second image.
public static unsafe WriteableBitmap CombinedDifference(this WriteableBitmap unaltered, WriteableBitmap previous, WriteableBitmap next, byte threshold)
{
// Check the arguments for null
ThrowIf.IsNullArgument(unaltered, nameof(unaltered));
ThrowIf.IsNullArgument(previous, nameof(previous));
ThrowIf.IsNullArgument(next, nameof(next));
if (WriteableBitmapExtensions.BitmapsMismatched(unaltered, previous) ||
WriteableBitmapExtensions.BitmapsMismatched(unaltered, next))
{
return(null);
}
WriteableBitmapExtensions.GetColorOffsets(unaltered, out int blueOffset, out int greenOffset, out int redOffset);
int totalPixels = unaltered.PixelWidth * unaltered.PixelHeight;
int pixelSizeInBytes = unaltered.Format.BitsPerPixel / 8;
byte *unalteredIndex = (byte *)unaltered.BackBuffer.ToPointer();
byte *previousIndex = (byte *)previous.BackBuffer.ToPointer();
byte *nextIndex = (byte *)next.BackBuffer.ToPointer();
byte[] differencePixels = new byte[totalPixels * pixelSizeInBytes];
int differenceIndex = 0;
for (int pixel = 0; pixel < totalPixels; ++pixel)
{
byte b1 = (byte)Math.Abs(*(unalteredIndex + blueOffset) - *(previousIndex + blueOffset));
byte g1 = (byte)Math.Abs(*(unalteredIndex + greenOffset) - *(previousIndex + greenOffset));
byte r1 = (byte)Math.Abs(*(unalteredIndex + redOffset) - *(previousIndex + redOffset));
byte b2 = (byte)Math.Abs(*(unalteredIndex + blueOffset) - *(nextIndex + blueOffset));
byte g2 = (byte)Math.Abs(*(unalteredIndex + greenOffset) - *(nextIndex + greenOffset));
byte r2 = (byte)Math.Abs(*(unalteredIndex + redOffset) - *(nextIndex + redOffset));
byte b = WriteableBitmapExtensions.DifferenceIfAboveThreshold(threshold, b1, b2);
byte g = WriteableBitmapExtensions.DifferenceIfAboveThreshold(threshold, g1, g2);
byte r = WriteableBitmapExtensions.DifferenceIfAboveThreshold(threshold, r1, r2);
byte averageDifference = (byte)((b + g + r) / 3);
differencePixels[differenceIndex + blueOffset] = averageDifference;
differencePixels[differenceIndex + greenOffset] = averageDifference;
differencePixels[differenceIndex + redOffset] = averageDifference;
unalteredIndex += pixelSizeInBytes;
previousIndex += pixelSizeInBytes;
nextIndex += pixelSizeInBytes;
differenceIndex += pixelSizeInBytes;
}
WriteableBitmap difference = new WriteableBitmap(BitmapSource.Create(unaltered.PixelWidth, unaltered.PixelHeight, unaltered.DpiX, unaltered.DpiY, unaltered.Format, unaltered.Palette, differencePixels, unaltered.BackBufferStride));
return(difference);
}
// Given two images, return an image containing the visual difference between them
public static unsafe WriteableBitmap Subtract(this WriteableBitmap image1, WriteableBitmap image2)
{
// Check the arguments for null
ThrowIf.IsNullArgument(image1, nameof(image1));
ThrowIf.IsNullArgument(image2, nameof(image2));
if (WriteableBitmapExtensions.BitmapsMismatched(image1, image2))
{
return(null);
}
WriteableBitmapExtensions.GetColorOffsets(image1, out int blueOffset, out int greenOffset, out int redOffset);
int totalPixels = image1.PixelWidth * image1.PixelHeight;
int pixelSizeInBytes = image1.Format.BitsPerPixel / 8;
byte *image1Index = (byte *)image1.BackBuffer.ToPointer();
byte *image2Index = (byte *)image2.BackBuffer.ToPointer();
byte[] differencePixels = new byte[totalPixels * pixelSizeInBytes];
int differenceIndex = 0;
for (int pixel = 0; pixel < totalPixels; ++pixel)
{
byte b = (byte)Math.Abs(*(image1Index + blueOffset) - *(image2Index + blueOffset));
byte g = (byte)Math.Abs(*(image1Index + greenOffset) - *(image2Index + greenOffset));
byte r = (byte)Math.Abs(*(image1Index + redOffset) - *(image2Index + redOffset));
byte averageDifference = (byte)((b + g + r) / 3);
differencePixels[differenceIndex + blueOffset] = averageDifference;
differencePixels[differenceIndex + greenOffset] = averageDifference;
differencePixels[differenceIndex + redOffset] = averageDifference;
image1Index += pixelSizeInBytes;
image2Index += pixelSizeInBytes;
differenceIndex += pixelSizeInBytes;
}
WriteableBitmap difference = new WriteableBitmap(BitmapSource.Create(image1.PixelWidth, image1.PixelHeight, image1.DpiX, image1.DpiY, image1.Format, image1.Palette, differencePixels, image1.BackBufferStride));
return(difference);
}
[HandleProcessCorruptedStateExceptions] // Necessary for unsafe code to implement try/catch
public static unsafe FileSelectionEnum IsDark(this WriteableBitmap image, int darkPixelThreshold, double darkPixelRatio, out double darkPixelFraction, out bool isColor)
{
// Check the arguments for null
ThrowIf.IsNullArgument(image, nameof(image));
// The RGB offsets from the beginning of the pixel (i.e., 0, 1 or 2)
WriteableBitmapExtensions.GetColorOffsets(image, out int blueOffset, out int greenOffset, out int redOffset);
// various counters that we will use in calculation of image darkness
int darkPixels = 0;
int uncoloredPixels = 0;
int countedPixels = 0;
// There was a nasty bug that occurred when checking for dark pixels on loading, which happened only (I think) on older machines and operating systems.
// I suspect its due to a threading issue, but it proved impossible to debug as only release (vs. debug) versions would crash
// The try / catch mitigates this, although there is a chance that the bug may overwrite some vital memory.
try
{
// Examine only a subset of pixels as otherwise this is a very expensive operation
// TODO: Calculate pixelStride as a function of image resolution so future high res images will still be processed quickly.
byte *currentPixel = (byte *)image.BackBuffer.ToPointer(); // the imageIndex will point to a particular byte in the pixel array
int pixelSizeInBytes = image.Format.BitsPerPixel / 8;
int pixelStride = Constant.ImageValues.DarkPixelSampleStrideDefault;
int totalPixels = image.PixelHeight * image.PixelWidth; // total number of pixels in the image
for (int pixelIndex = 0; pixelIndex < totalPixels; pixelIndex += pixelStride)
{
// get next pixel of interest
byte b = *(currentPixel + blueOffset);
byte g = *(currentPixel + greenOffset);
byte r = *(currentPixel + redOffset);
// The numbers below convert a particular color to its greyscale equivalent, and then checked against the darkPixelThreshold
// Colors are not weighted equally. Since pure green is lighter than pure red and pure blue, it has a higher weight.
// Pure blue is the darkest of the three, so it receives the least weight.
int humanPercievedLuminosity = (int)Math.Round(0.299 * r + 0.5876 * g + 0.114 * b);
if (humanPercievedLuminosity <= darkPixelThreshold)
{
++darkPixels;
}
// Check if the pixel is a grey scale vs. color pixel, using a heuristic.
// In precise grey scales, r = g = b. However, we allow a bit of slop as some cameras actually
// have a bit of color in their dark shots (don't ask me why, it just happens).
// i.e. if the total delta is less than the color slop, then we consider it a grey level.
// Given a pixel's rgb values, calculate the delta between all those values. This is used to determine if its a grey scale pixel.
// In practice a grey scale pixel's rgb are all equal (i.e., delta = 0) but we need the value as we want to see how 'close' the pixel
// actually is to 0, i.e., to allow some slop in determining grey versus color pixels.
int rgbDelta = Math.Abs(r - g) + Math.Abs(g - b) + Math.Abs(b - r);
if (rgbDelta <= Constant.ImageValues.GreyscalePixelThreshold)
{
++uncoloredPixels;
}
// update other loop variables
++countedPixels;
currentPixel += pixelSizeInBytes * pixelStride; // Advance the pointer to the beginning of the next pixel of interest
}
// Check if its a grey scale image, i.e., at least 90% of the pixels in this image (given this slop) are grey scale.
// If not, its a color image so judge it as not dark
double uncoloredPixelFraction = 1d * uncoloredPixels / countedPixels;
if (uncoloredPixelFraction < Constant.ImageValues.GreyscaleImageThreshold)
{
darkPixelFraction = 1 - uncoloredPixelFraction;
isColor = true;
return(FileSelectionEnum.Ok);
}
// It is a grey scale image. If the fraction of dark pixels are higher than the threshold, it is dark.
darkPixelFraction = 1d * darkPixels / countedPixels;
isColor = false;
if (darkPixelFraction >= darkPixelRatio)
{
return(FileSelectionEnum.Dark);
}
return(FileSelectionEnum.Ok);
}
//#pragma warning disable CA2153 // Do Not Catch Corrupted State Exceptions
catch
//#pragma warning restore CA2153 // Do Not Catch Corrupted State Exceptions
{
// Without this catch, Timelapse will crash on some machines and OS during loads (see notes in method comments)
// Note that we have to set isColor and darkPixelFraction to some value, although these are nonsensical as
// the method has not completed.
// Returning a Corrupted state informs the caller that the check for dark has not completed successfully.
isColor = true;
darkPixelFraction = 0;
return(FileSelectionEnum.Corrupted);
}
}
