GDI+ how to query a pixel

Question

I am using the GDI+ library for Delphi 6 by progdigy.com .

I loaded an image using TGPImage.Create(AFilename) .

The question is, how can I query a single pixel (RGBA values)? The object TGPImage does not seem to have something like that. The documentation in the library only links to the Microsoft documentation of GDI+, which is a broken link by now.

PS: I can draw the image by using

TGPGraphics.Create(PaintBox1.Canvas.Handle).DrawImage(Image, GPRect(ALeft, ATop, AWidth, AHeight));

but I don't know if this helps since I need to query only the pixels without needing to draw the whole picture.

Answer 1

You can borrow the GetPixel method from the TGPBitmap class, which is used just for getting pixel colors. To implement this method, you will need to subclass the original TGPImage class to get access to the protected NativeImage field (holding the pointer to the Image object).

Here is an example using interposer class:

uses
  GDIPAPI, GDIPOBJ;

type
  // we need to subclass the TGPImage to access the protected NativeImage field
  TGPImage = class(GDIPOBJ.TGPImage)
  public
    function GetPixel(X, Y: Integer; out Color: TGPColor): TStatus;
  end;

implementation

{ TGPImage }

function TGPImage.GetPixel(X, Y: Integer; out Color: TGPColor): TStatus;
begin
  Result := SetStatus(GdipBitmapGetPixel(GPBITMAP(NativeImage), X, Y, Color));
end;