Ich glaube, das Problem kann gelöst werden, indem Sie überprüfen, dass resized
Bitmap von PixelFormat.Format1bppIndexed
ist. Wenn dies nicht der Fall ist, sollten Sie es in 1bpp Bitmap umwandeln und danach können Sie es ohne Probleme als Schwarz-Weiß-Png speichern.
Mit anderen Worten, sollten Sie folgenden Code verwenden, statt resized.Save(outputFilename, System.Drawing.Imaging.ImageFormat.Jpeg);
if (resized.PixelFormat != PixelFormat.Format1bppIndexed)
{
using (Bitmap bmp = convertToBitonal(resized))
bmp.Save(outputFilename, System.Drawing.Imaging.ImageFormat.Png);
}
else
{
resized.Save(outputFilename, System.Drawing.Imaging.ImageFormat.Png);
}
Ich verwende folgenden Code für convertToBitonal
:
private static Bitmap convertToBitonal(Bitmap original)
{
int sourceStride;
byte[] sourceBuffer = extractBytes(original, out sourceStride);
// Create destination bitmap
Bitmap destination = new Bitmap(original.Width, original.Height,
PixelFormat.Format1bppIndexed);
destination.SetResolution(original.HorizontalResolution, original.VerticalResolution);
// Lock destination bitmap in memory
BitmapData destinationData = destination.LockBits(
new Rectangle(0, 0, destination.Width, destination.Height),
ImageLockMode.WriteOnly, PixelFormat.Format1bppIndexed);
// Create buffer for destination bitmap bits
int imageSize = destinationData.Stride * destinationData.Height;
byte[] destinationBuffer = new byte[imageSize];
int sourceIndex = 0;
int destinationIndex = 0;
int pixelTotal = 0;
byte destinationValue = 0;
int pixelValue = 128;
int height = destination.Height;
int width = destination.Width;
int threshold = 500;
for (int y = 0; y < height; y++)
{
sourceIndex = y * sourceStride;
destinationIndex = y * destinationData.Stride;
destinationValue = 0;
pixelValue = 128;
for (int x = 0; x < width; x++)
{
// Compute pixel brightness (i.e. total of Red, Green, and Blue values)
pixelTotal = sourceBuffer[sourceIndex + 1] + sourceBuffer[sourceIndex + 2] +
sourceBuffer[sourceIndex + 3];
if (pixelTotal > threshold)
destinationValue += (byte)pixelValue;
if (pixelValue == 1)
{
destinationBuffer[destinationIndex] = destinationValue;
destinationIndex++;
destinationValue = 0;
pixelValue = 128;
}
else
{
pixelValue >>= 1;
}
sourceIndex += 4;
}
if (pixelValue != 128)
destinationBuffer[destinationIndex] = destinationValue;
}
Marshal.Copy(destinationBuffer, 0, destinationData.Scan0, imageSize);
destination.UnlockBits(destinationData);
return destination;
}
private static byte[] extractBytes(Bitmap original, out int stride)
{
Bitmap source = null;
try
{
// If original bitmap is not already in 32 BPP, ARGB format, then convert
if (original.PixelFormat != PixelFormat.Format32bppArgb)
{
source = new Bitmap(original.Width, original.Height, PixelFormat.Format32bppArgb);
source.SetResolution(original.HorizontalResolution, original.VerticalResolution);
using (Graphics g = Graphics.FromImage(source))
{
g.DrawImageUnscaled(original, 0, 0);
}
}
else
{
source = original;
}
// Lock source bitmap in memory
BitmapData sourceData = source.LockBits(
new Rectangle(0, 0, source.Width, source.Height),
ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
// Copy image data to binary array
int imageSize = sourceData.Stride * sourceData.Height;
byte[] sourceBuffer = new byte[imageSize];
Marshal.Copy(sourceData.Scan0, sourceBuffer, 0, imageSize);
// Unlock source bitmap
source.UnlockBits(sourceData);
stride = sourceData.Stride;
return sourceBuffer;
}
finally
{
if (source != original)
source.Dispose();
}
}
das ist brilliant. Vielen Dank. –