Porting system.drawing to imagesharp

[raiserle]

Hi,

I would like to replace System.Drawing with ImageSharp.
The following code works great with System.Drawing, but with ImageSharp the image comes out horizontally shifted and in gray.

	private static byte[] CreatePreviewBytesFromImageData(int width, int height, byte[] imageData)
	{
		int Format = 8 | (24 << 8);
		int bitsPerPixel = ((int)Format & 0xff00) >> 8;
		int bytesPerPixel = (bitsPerPixel + 7) / 8;
		int stride = 4 * ((width * bytesPerPixel + 3) / 4);
		
		Bitmap bitmap = new Bitmap(width, height, PixelFormat.Format24bppRgb);
		BitmapData bitmapData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.WriteOnly, PixelFormat.Format24bppRgb);
		IntPtr scan0 = bitmapData.Scan0;
		int strideWidth = Math.Abs(stride);
		int numSkipBytes = strideWidth - width * 3;
		byte[] bitmapPixelData = new byte[strideWidth * height];
		int writtenBytes = 0;
		int readBytes = 0;

		for (int h = 0; h < height; h++)
		{
			for (int w = 0; w < width; w++)
			{
				// GDI+ bitmap raw pixel data is in BGR format, red & blue values need to be flipped around for each pixel.
				bitmapPixelData[writtenBytes] = imageData[readBytes + 2];
				bitmapPixelData[writtenBytes + 1] = imageData[readBytes + 1];
				bitmapPixelData[writtenBytes + 2] = imageData[readBytes];
				
				writtenBytes += 3;
				readBytes += 3;
			}

			// GDI+ bitmap stride / scan width has to be a multiple of 4, so the end of each stride / scanline can contain extra bytes
			// in the bitmap raw pixel data that are not present in the image data and should be skipped when copying.
			writtenBytes += numSkipBytes;
		}

		SixLabors.ImageSharp.Image bitmap2 =
			SixLabors.ImageSharp.Image.LoadPixelData<SixLabors.ImageSharp.PixelFormats.Bgr24>(bitmapPixelData,
				width, height);
		bitmap2.Save("abc.bmp");
		
		Marshal.Copy(bitmapPixelData, 0, scan0, bitmapPixelData.Length);
		bitmap.UnlockBits(bitmapData);
		bitmap.Save("def.bmp");
		
		return bitmapPixelData;     
	}

abc.bmp => ImageSharp

def.bmp => System.Drawing

What did I do wrong with the ImageSharp-Code?

regards
raiserle

[JimBobSquarePants]

The problem lies in how ImageSharp handles stride and padding compared to System.Drawing. Your code assumes GDI+ (System.Drawing) behavior, which enforces that each row of pixel data is aligned to a 4-byte boundary. However, ImageSharp does not require such padding for rows, and the pixel data should be tightly packed with no extra padding bytes between rows.

[raiserle]

Thank you so much.
Yes the 4Byte alignment was the error 👍

private static byte[] CreatePreviewBytesFromImageData(int width, int height, byte[] imageData)
{
		byte[] bitmapPixelData = new byte[imageData.Length];
		int writtenBytes = 0;
		int readBytes = 0;

		for (int h = 0; h < height; h++)
		{
			for (int w = 0; w < width; w++)
			{
				// GDI+ bitmap raw pixel data is in BGR format, red & blue values need to be flipped around for each pixel.
				bitmapPixelData[writtenBytes] = imageData[readBytes + 2];
				bitmapPixelData[writtenBytes + 1] = imageData[readBytes + 1];
				bitmapPixelData[writtenBytes + 2] = imageData[readBytes];
				
				writtenBytes += 3;
				readBytes += 3;
			}
		}
		
		SixLabors.ImageSharp.Image bitmap2 =
			SixLabors.ImageSharp.Image.LoadPixelData<SixLabors.ImageSharp.PixelFormats.Bgr24>(bitmapPixelData,
				width, height);
		bitmap2.Save("abc.bmp");
		
		return bitmapPixelData;
}

This code is now working perfect.

regards
raiserle

[tocsoft]

I believe you shouldn't need to be doing any translation with the image data array, if I'm right you shouldn't need to do any of this in ImageSharp.. your source data seems to be in RGB format so you can just decode directly from that.

private static void SaveImageFromImageData(int width, int height, byte[] imageData)
{
    using SixLabors.ImageSharp.Image bitmap2 =
	SixLabors.ImageSharp.Image.LoadPixelData<SixLabors.ImageSharp.PixelFormats.Rgb24>(imageData,
		width, height); //use Rgb24 directly
    bitmap2.Save("abc.bmp");
}