Save Image as PNG - Grayscale 8bit

I need to save an Image as an 8bit GrayScale PNG file in VB.NET

To verify that the format is correct, the header information for the saved PNG file is:

byte 0x00: 0x89
byte 0x01: 0x50
byte 0x02: 0x4E

byte 0x18: 0x08      '8bit
byte 0x19: 0x00      'GrayScale

I can get 8 bit using PixelFormat.Format8bppIndexed, but this does not give me gray-scale.
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Option Explicit On
Option Strict On

Imports System.Drawing.Imaging
Imports System.Windows.Media.Imaging
Imports System.IO

Public Class Form1

	Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

		''Load original image
		Dim image1 As Bitmap = New Bitmap("c:\input.png")

		'PictureBox1.Image = image1

		'Convert to Grayscale
		Dim image2 As Bitmap = MakeGrayscale(image1)

		'PictureBox2.Image = image2

		'Save as PNG, 8bbp
		Dim fcb As FormatConvertedBitmap = New FormatConvertedBitmap(System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(image2.GetHbitmap(System.Drawing.Color.Transparent), IntPtr.Zero, System.Windows.Int32Rect.Empty, _
		  BitmapSizeOptions.FromWidthAndHeight(image2.Width, image2.Height)), System.Windows.Media.PixelFormats.Gray8, BitmapPalettes.Gray256, 0.5)
		Dim pngBitmapEncoder As PngBitmapEncoder = New PngBitmapEncoder()
		pngBitmapEncoder.Interlace = PngInterlaceOption.Off
		Dim fileStream As Stream = File.Open("c:\output.png", FileMode.Create)

	End Sub

	Public Function MakeGrayscale(ByVal original As Bitmap) As Bitmap

		''create a blank bitmap the same size as original
		Dim newBitmap As Bitmap = New Bitmap(original.Width, original.Height)

		''get a graphics object from the new image
		Dim g As Graphics = Graphics.FromImage(newBitmap)

		'create the grayscale ColorMatrix
		Dim colorMatrix As ColorMatrix = New ColorMatrix(New Single()() { _
		  New Single() {0.3F, 0.3F, 0.3F, 0, 0}, _
		  New Single() {0.59F, 0.59F, 0.59F, 0, 0}, _
		  New Single() {0.11F, 0.11F, 0.11F, 0, 0}, _
		  New Single() {0, 0, 0, 1, 0}, _
		  New Single() {0, 0, 0, 0, 1}})

		'create some image attributes
		Dim attributes As ImageAttributes = New ImageAttributes()

		'set the color matrix attribute

		'draw the original image on the new image using the grayscale color matrix
		g.DrawImage(original, New Rectangle(0, 0, original.Width, original.Height), 0, 0, original.Width, original.Height, GraphicsUnit.Pixel, attributes)

		'dispose the Graphics object
		Return newBitmap

	End Function

End Class

Open in new window

You can compute each bit for grayscale. Note that you need use a factor for each R G B component.

You can use GDI API Functions:

1. Create an 1 plane 8 bits/pixel device independent bitmap using CreateDIBSection.
2. Set its color table to greyscale colors.
3. Use BitBlt to transfer your color bitmap into this one. Windows will make the necessary conversions automatically and the bitmap will appear in greyscale.

You can find many samples on internet:
Just realized in the sample above, the call to MakeGrayscale() is not needed since the FormatConvertedBitmap will do the conversion.

''Load original image
Dim image2 As Bitmap = New Bitmap("c:\input.png")

'Save as PNG, 8bbp
Dim fcb As FormatConvertedBitmap = ...
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