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Save Image as PNG - Grayscale 8bit

Posted on 2010-11-08
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Last Modified: 2012-05-10
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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Question by:Ola3
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Expert Comment

by:x77
ID: 34086327
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:
http://efreedom.com/Question/1-2517477/Converting-24BPP-4BPP-GDI-VBNET
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Erick37 earned 500 total points
ID: 34086893
Give this a shot.
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

		pngBitmapEncoder.Frames.Add(BitmapFrame.Create(fcb))

		Dim fileStream As Stream = File.Open("c:\output.png", FileMode.Create)

		pngBitmapEncoder.Save(fileStream)

		fileStream.Close()



	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

		attributes.SetColorMatrix(colorMatrix)



		'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

		g.Dispose()

		Return newBitmap



	End Function



End Class

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Expert Comment

by:Erick37
ID: 34088011
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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