Quick way to create a large bitmap from a bitmap array?

I have this code,

copy/paste in a new winform app and this will write a file on your desktop if you run it: test123abcd.png

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    Dim SquareSize = 5
    Dim GridX = 2500
    Dim GridY = 2500
    Dim SquareCount = GridX * GridY - 1
    Dim sw As New Stopwatch

    Dim Rect(4) As Rectangle
    Rect(0) = New Rectangle(0, 3, 3, 1)
    Rect(1) = New Rectangle(3, 0, 1, 3)
    Rect(2) = New Rectangle(3, 3, 3, 1)
    Rect(3) = New Rectangle(0, 0, 1, 3)

    Dim fullsw = Stopwatch.StartNew
    Using board = New Bitmap(SquareSize * (GridX + 1), SquareSize * (GridY + 1), Imaging.PixelFormat.Format32bppPArgb)
        Using graph = Graphics.FromImage(board)
            Using _board = New Bitmap(SquareSize, SquareSize, Imaging.PixelFormat.Format32bppPArgb)
                Using g As Graphics = Graphics.FromImage(_board)
                    For i = 0 To SquareCount
                        g.Clear(If((i And 1) = 1, Color.Red, Color.Blue))
                        g.FillRectangles(Brushes.White, Rect)
                        sw.Start()
                        graph.DrawImageUnscaled(_board, ((i Mod GridX) * SquareSize), ((i \ GridY) * SquareSize))
                        sw.Stop()
                    Next
                End Using
            End Using
        End Using
        fullsw.Stop()
        board.Save(Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory) & "\test123abcd.png", Imaging.ImageFormat.Png)
    End Using

    MessageBox.Show("Full SW: " & fullsw.ElapsedMilliseconds & Environment.NewLine &
                    "DrawImageUnscaled SW: " & sw.ElapsedMilliseconds)
End Sub

about 40% to 45% of the time spent is on DrawImageUnscaled, about 23 seconds on my current computer while the whole thing take about 50 seconds

is there a way to speed up DrawImageUnscaled? (and maybe the whole thing?)

EDIT - question in vb.net, answer in c#


By assuming that the generation part (g.FillRectangles(Brushes.White, Rect), pretty time-consuming too) cannot be avoided, the best thing you can do is avoiding a second graph-generation process (also for board) and just copying the information from _board. Copying is much quicker than a new generation (as shown below), but you have the problem that the source information (_board) do not match the destination format (board by relying on .SetPixel) and thus you will have to create a function determining the current pixel (X/Y point) from the provided information (current rectangle).

Below you can see a simple code showing the time requirement differences between both approaches:

Dim SquareSize As Integer = 5
Dim _board As Bitmap = Bitmap.FromFile("in.png")
Dim board As Bitmap = New Bitmap(_board.Width * SquareSize, _board.Height * SquareSize)

For x As Integer = 0 To _board.Width - 1
    For y As Integer = 0 To _board.Height - 1
        board.SetPixel(x * SquareSize, y * SquareSize, _board.GetPixel(x, y))
    Next
Next

board.Save("out1.png", Imaging.ImageFormat.Png)

board = New Bitmap(_board.Width, _board.Height)

Using board
    Using graph = Graphics.FromImage(board)
        Using _board
            Using g As Graphics = Graphics.FromImage(_board)
                For x As Integer = 0 To _board.Width - 1
                    For y As Integer = 0 To _board.Height - 1
                        graph.DrawImageUnscaled(_board, x, y)
                    Next
                Next
            End Using
        End Using
    End Using
    board.Save("out2.png", Imaging.ImageFormat.Png)
End Using

Bear in mind that it is not a "properly-working code". Its whole point is showing how to copy pixels between bitmaps (by multiplying by a factor, just to get different outputs than inputs); and putting the DrawImageUnscaled method under equivalent conditions (although the output picture is, logically, different) to get a good feeling of the differences in time requirements between both methodologies.

As said via comment, this is all what I can do under the current conditions. I hope that will be enough to help you find the best solution.