CODEWITHSUNDEEP
delphipngimaging
RaelB
RaelB

Reputation: 3481

PNG loses transparency when converting from a Bitmap

I load a png file into a TPNGImage, and then display it in a TImage. No problem. Then I create a TBitmap and assign it to the TPNGImage, and display it in a TImage. No problem.

Then I create a second TPNGImage and assign it to the TBitmap. In this case if I display it in a TImage, it has lost it's transparency. If I set the TBitmap.Transparent to True, before assigning to the PNGImage, the PNGImage is mostly transparent, but there is a small area where the transparency was lost, showing in black.

var
  Bmp: TBitmap;
  PngImage: TPNGImage;
  PngImage2: TPNGImage;
begin
  PngImage := TPNGImage.Create;
  try
    PngImage.LoadFromFile(FILE_NAME);
    Image1.Picture.Assign(PngImage);

    Bmp := TBitmap.Create;
    try
      Bmp.Assign(PngImage);
      Image2.Picture.Assign(Bmp);

      PngImage2 := TPNGImage.Create;
      try
        //Bmp.Transparent := True;
        PngImage2.Assign(Bmp);
        Image3.Picture.Assign(PngImage2);
      finally
        PngImage2.Free;
      end;
    finally
      Bmp.Free;
    end;
  finally
    PngImage.Free;
  end;
end;

Result without setting Bitmap.Transparent to True:

enter image description here

Result when I set Bitmap.Transparent to True: There is a small bit of black in the Image

enter image description here

How can I assign the PNGImage to the Bitmap without losing any transparency?

Upvotes: 0

Views: 799

Answers (2)

RaelB
RaelB

Reputation: 3481

I can confirm what @SertacAkyuz said in the comments. There is no problem with:

Bmp.Assign(PngImage);

The result is a 32bit image that preserves the alpha channel. The problem is with

PngImage2.Assign(Bmp);

Where the alpha is lost.

I found this library to help with the conversion:

https://github.com/graphics32/GR32PNG

Upvotes: 0

Ken Bourassa
Ken Bourassa

Reputation: 6502

I can only speculate... But PNGs support partial transparency, and 24-bits bitmaps don't. And since the leftover "black" pixels aren't black (they are $000101), I suspect those pixels are semi-transparent in the original png. That, or some antialising effect was applied when converting to bitmap. But I believe semi-transparency is more likely.

I never really worked with 32 bits bitmap, but maybe they could be used to preserve the transparency. (They do have an alpha channel...). But I suspect it might be more tricky than just Bitmap.Assign.

Upvotes: 1

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