After looking for a way to resize TPngObject and maintain the transparency + alpha channels to no avail, I'm trying to use GDI+
Here is my code, and it seems to work fine. it will down/up scale a PNG. Tested on XP so far:
uses GDIPAPI, GDIPOBJ, GDIPUTIL;
procedure TForm1.Button1Click(Sender: TObject);
var
encoderClsid: TGUID;
stat: TStatus;
img, img_out: TGPImage;
begin
img := TGPImage.Create('in.png'); // 200 x 200
img_out := img.GetThumbnailImage(100, 100, nil, nil);
GetEncoderClsid('image/png', encoderClsid);
img_out.Save('out.png', encoderClsid);
img_out.free;
img.Free;
end;
My question: is using GetThumbnailImage the correct way of doing this? I did not find any other method.
I don't think that GetThumbnailImage method is a good way to go because I doubt that you will get a high quality resampled image. In this article you can find how to rescale the image. They're using the DrawImage method for that, so I would do the same. Just before that I would set also the high quality graphics modes to get high quality output. Here is an example:
procedure TForm1.Button1Click(Sender: TObject);
var
Input: TGPImage;
Output: TGPBitmap;
Encoder: TGUID;
Graphics: TGPGraphics;
begin
Input := TGPImage.Create('C:\InputImage.png');
try
// create the output bitmap in desired size
Output := TGPBitmap.Create(100, 100, PixelFormat32bppARGB);
try
// create graphics object for output image
Graphics := TGPGraphics.Create(Output);
try
// set the composition mode to copy
Graphics.SetCompositingMode(CompositingModeSourceCopy);
// set high quality rendering modes
Graphics.SetInterpolationMode(InterpolationModeHighQualityBicubic);
Graphics.SetPixelOffsetMode(PixelOffsetModeHighQuality);
Graphics.SetSmoothingMode(SmoothingModeHighQuality);
// draw the input image on the output in modified size
Graphics.DrawImage(Input, 0, 0, Output.GetWidth, Output.GetHeight);
finally
Graphics.Free;
end;
// get encoder and encode the output image
if GetEncoderClsid('image/png', Encoder) <> -1 then
Output.Save('C:\OutputImage.png', Encoder)
else
raise Exception.Create('Failed to get encoder.');
finally
Output.Free;
end;
finally
Input.Free;
end;
end;
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