How to properly enlarge an image?
lazarus / 39-Graphics

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michi_cc · 2011-10-08 14:13

Hello,

I want to enlarge an image by a variable percentage. I first tried
using TCanvas.StretchDraw, which was very fast, but the resulting
image had very visible resizing artefacts. After that I tried to use
TFPImageCanvas, which not only gave me an image equally bad, but
executing it took like 5 minutes or so (not only the resizing, but the
image loading was comparatively also very slow).

Code extract below, with test.jpg being some random photo. Tested with
0.9.30.2RC1.
Code: [Select]  Img := TFPMemoryImage.Create(0, 0);
  Img.LoadFromFile('test.jpg');

  W := Round(Img.Width * 1.5);
  H := Round(Img.Height * 1.5);

  Dest := TFPMemoryImage.Create(W, H);
  with TFPImageCanvas.Create(Dest) do begin
      StretchDraw(0, 0, W, H, Img);
      Free;
  end;

  Dest.SaveToFile('out.jpg');

Is there something I've missed or done wrong, or do I simply have to
use an external library to get good image resize?

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Shebuka · 2011-10-10 08:18

You need to use external lirary to have really good resample, look at
http://wiki.lazarus.freepascal.org/BGRABitmap_tutorial_2#Stretch_the_i
mage

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michi_cc · 2011-10-10 15:06

Well, if that's the way to go, that's the way to go.

TBGRABitmap looks good, there is only one unfortunate problem: I want
to pass the processed image data on to an external library which
expects the image data to be delivered as RGBA instead of BGRA. Is the
another comparable unit that can provide RGBA? If not, what's the
best/fastest way to convert the data?

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Chronos · 2011-10-10 17:10

TBGRADefaultBitmap already have such method for swapping red and blue
colors.

Code: [Select]procedure TBGRADefaultBitmap.SwapRedBlue;
var
  n:    integer;
  temp: longword;
  p:    PLongword;
begin
  p := PLongword(Data);
  n := NbPixels;
  if n = 0 then
    exit;
  repeat
    temp := p^;
    p^   := ((temp and $FF) shl 16) or ((temp and $FF0000) shr 16) or
      temp and $FF00FF00;
    Inc(p);
    Dec(n);
  until n = 0;
  InvalidateBitmap;
end;