Use GetDIBits to load the image's pixel values into a device-independent bitmap (DIB) structure. Modify the pixel values and use SetDIBits to update the image.

Note that this program introduces a small amount of color degredation if you are running with less than 24-bit color depth because GetDIBits and SetDIBits store color values in 3 bytes (essentially 24-bit color) but the system uses fewer bits to store the values.

' Prepare the bitmap description.
wid = Picture1.ScaleWidth
hgt = Picture1.ScaleHeight
With bitmap_info.bmiHeader
    .biSize = 40
    .biWidth = wid
    ' Use negative height to scan top-down.
    .biHeight = -hgt
    .biPlanes = 1
    .biBitCount = 32
    .biCompression = BI_RGB
    bytes_per_scanLine = ((((.biWidth * .biBitCount) + 31) _
        \ 32) * 4)
    pad_per_scanLine = bytes_per_scanLine - (((.biWidth * _
        .biBitCount) + 7) \ 8)
    .biSizeImage = bytes_per_scanLine * Abs(.biHeight)
End With

' Load the bitmap's data.
ReDim pixels(1 To 4, 1 To wid, 1 To hgt)
GetDIBits Picture1.hdc, Picture1.Image, _
    0, hgt, pixels(1, 1, 1), _
    bitmap_info, DIB_RGB_COLORS

' Modify the pixels.
For Y = 1 To hgt
    For X = 1 To wid
        ' Make the pixel black.
        pixels(pixR, X, Y) = 0
        pixels(pixG, X, Y) = 0
        pixels(pixB, X, Y) = 0

        If ((X \ 20) Mod 2) <> ((Y \ 20) Mod 2) Then
            ' Set the pixel color.
            pixels(clr, X, Y) = 255
        End If
    Next X
Next Y

' Display the result.
SetDIBits Picture1.hdc, Picture1.Image, _
    0, hgt, pixels(1, 1, 1), _
    bitmap_info, DIB_RGB_COLORS
Picture1.Picture = Picture1.Image

• Fill a large area one pixel at a time using PSet (slowest)
• Fill a large area one pixel at a time using a Bitmap object in VB.NET (faster)
• Fill a large area one pixel at a time using SetDIBits (even faster)
• Fill a large area one pixel at a time using SetBitmapBits (fastest)