Flipping after glReadPixels
lazarus / 70-OpenGL

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gilsonalencar · 2012-07-19 12:18

After many efforts, i can to put a OpenGLControl graphics in a bitmap
image. But it is inverted from original source, when glReadPixels is
used. After weeks searching in the web,  i can´t to find any solution
to flip it using gl functions. Anybody knows how to do it? Technics
using Canvas.Pixels[X,Y] are very slow.

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Leledumbo · 2012-07-19 12:39

What format parameter (5th position) do you give?

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gilsonalencar · 2012-07-19 13:31

The complet code is here.

Code: [Select]
function BitmapImage(c:TOpenGLControl):TBitmap;
var
  data,rdata: pbyte;
  w,h,s: integer;
  bitMap: TBitmap;

begin
  c.Repaint;
  w := c.Width;
  h := c.Height;
  s := w*h*4;
  result := TBitmap.Create;
  bitMap:=result;   try
    bitMap.SetSize(w,h);
    bitMap.PixelFormat:=pf32bit;
    getMem(data,s);     try
      glPixelStorei(GL_PACK_ALIGNMENT, 4);
      pixel}
      glReadPixels(0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE, data);     
      rdata := bitMap.RawImage.Data;
      Move(data^,rdata^,bitMap.RawImage.DataSize);
    finally
      freemem(data);
    end;
  except
    bitMap.free;
  end;
end;

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KpjComp · 2012-07-19 13:39

When you say inverted, do you mean the R & B are swapped.

I wrote a routine here that might help ->
http://80.123.225.56/index.php/topic,17383.msg95733.html#msg95733

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User137 · 2012-07-19 13:55

If it's about colors, then this code:
Code: [Select]glReadPixels(0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE,
data);you can switch to GL_BGR.

If it's graphics upside down, then you need to draw then in OpenGL
differently. If you use ortho (2D) mode, you should set the screen so
that origo (0, 0) is in top left.

Or that may not help at all, who knows. I haven't used glReadPixels, i
hear it's slow operation. Even slower than canvas operations.

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KpjComp · 2012-07-19 14:09

QuoteGL_BGR

I think GL_BGR is an extension, so some video cards might not support
it.  Maybe most video cards will have this option now, so it might not
be an issue.  

Quotei hear it's slow operation

I suppose it depends on what your doing, I use it for doing image
analysis and for this it seems to work pretty well.  Also if you do
use it, one idea is to only grab the area of interest.

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gilsonalencar · 2012-07-19 14:15

QuoteWhen you say inverted, do you mean the R & B are swapped.

I wrote a routine here that might help ->
http://80.123.225.56/index.php/topic,17383.msg95733.html#msg95733

A 180 degrees inversion in position, not color.

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KpjComp · 2012-07-19 14:36

QuoteA 180 degrees inversion in position, not color.

One idea is maybe try changing the HandleType to bmDIB, as I think
there always top to bottom.  Not sure if this has an effect on Linux.

Another idea is copy Scanlines at a time.

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gilsonalencar · 2012-07-19 14:37

The image file attached, shows the problem. On the left, the original
graphics from OpenGLControl. On the right, the result of bitmap image
(3D axis).

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KpjComp · 2012-07-19 15:02

That is strange, it's not just top to bottom, but left to right.

Are you calling this during your render while the context is still
valid, and straight after you do the swapbuffers?

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gilsonalencar · 2012-07-19 15:27

QuoteThat is strange, it's not just top to bottom, but left to right.

Are you calling this during your render while the context is still
valid, and straight after you do the swapbuffers?

I'm calling this after do swapbuffers.

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KpjComp · 2012-07-19 15:50

QuoteI'm calling this after do swapbuffers.

Oh, in that case is there any reason your doing ->
Code: [Select]c.Repaint;
If your doing this during the Paint Event, then your telling your
control to repaint during your paint event, doesn't really make
sense.  The way I captured the screen is set a flag to say then next
render will be captured, and after the render I would reset.   Not
sure if this is your problem though, I'm not an OpenGL expert.

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User137 · 2012-07-19 18:19

Some options:
1) You could try using Canvas.Copyrect() to flip the image to another
TBitmap. It should support negative width and height in TRect.

2) Draw them 180 degree rotated when taking screenshot.

3) Same as 1), but copy pixel by pixel to inverse coordinates.