How to use DrawEllipse in a LockBits array (How to generate a set of pixels that forms an ellipse)

Question

I'm using this class to fill pixels of a bitmap based on the LockBits function:

Imports System.Drawing.Imaging
Imports System.Runtime.InteropServices.Marshal

Public Class Fill

    Public Shared Function Process(ByVal b As Bitmap) As Bitmap

    Dim bmd As BitmapData = _
    b.LockBits(New Rectangle(0, 0, b.Width, b.Height), _
    System.Drawing.Imaging.ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb)

    Dim scan0 As IntPtr = bmd.Scan0
    Dim stride As Integer = bmd.Stride

    ' Here's the speedier method.
    ' define an array to store each pixels color as an int32 
    Dim pixels(b.Width * b.Height - 1) As Integer

    ' this is system.runtime.interopservices.marshall.copy
    Copy(scan0, pixels, 0, pixels.Length)

    ' loop through all pixels and fill

    For i As Integer = 0 To pixels.Length - 1
        pixels(i) = Color.Red.ToArgb
    Next

    ' Copy the data back from the array to the locked memory
    Copy(pixels, 0, scan0, pixels.Length)

    ' finally we unlock the bits. 
    b.UnlockBits(bmd)
    Return b
    End Function
End Class

Now, instead of filling all the pixels, I need to fill an ellipse (actually it's going to be many ellipses, that's why I use LockBits), so I googled for a way to draw an ellipse pixel by pixel using some kind of formula, but I didn't find much help, also I'm not good with this math stuff. So, my question is: how to create an array of pixels that forms a filled ellipse? thank you

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supplement (feel free to ignore):

I'll explain exactly what I'm trying to do, so it might help you understand my situation.. Actually, I'm working on a function that's supposed to generate filled ellipses with random width & height (in specific range) on a specific area of a bitmap, while the filled pixels must have a percentage of the total number of pixels in that area, that's why I need to draw the ellipse pixel by pixel (or using an array of pixels) to keep track of the number of filled pixels.

The Graphics class is very good at generating an ellipse quickly. Do not use LockBits, use Graphics.FromImage(). — Hans Passant, Jul 20 '15 at 17:08
@HansPassant The problem is that I need to keep track of the number of pixels that the function fills — 41686d6564 stands w. Palestine, Jul 20 '15 at 17:20
Very hard to imagine the point of doing that. You can count them afterwards. — Hans Passant, Jul 20 '15 at 17:31

score 2 · Accepted Answer · answered Jul 20 '15 at 18:32

The formula for all the points inside an ellipse is:

(x - h) * (x - h) / a * a + (y - k) * (y - k) / b * b <= 1

where
x,y are the coordinates of any point on the ellipse,
a, b are the radius on the x and y axes respectively and
h,k the coordinates of the center

So the code:

Dim h, k, a, b, x, y As Integer
Dim res As Double

'arbitrary values of ellipse
h = 200
k = 200
a = 80
b = 60

For x = h - a To h + a
    For y = k - b To k + b
        res = CDbl((x - h) * (x - h)) / CDbl(a * a) + CDbl((y - k) * (y - k)) / CDbl(b * b)

        If res <= 1.0 Then
            'the point (x, y) is inside
        End If

    Next
Next

How to use DrawEllipse in a LockBits array (How to generate a set of pixels that forms an ellipse)

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supplement (feel free to ignore):

1 Answers1