[SOLVED] Need help drawing a rectangle with opacity.
lazarus / 39-Graphics

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knuckles · 2016-05-16 21:05

I need to draw a rectangle that can have a opacity value from 0 to
100.

The below snippet from Delphi Dabbler
(http://delphidabbler.com/tips/203) works but is very slow with larger
rectangles:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
DrawTransparentRectangle(Canvas: TCanvas; Rect: TRect;  Color: TColor;
Transparency: Integer);var  X: Integer;  Y: Integer;  C: TColor;  R,
G, B: Integer;  RR, RG, RB: Integer;begin  RR := GetRValue(Color);  RG
:= GetGValue(Color);  RB := GetBValue(Color);   for Y := Rect.Top to
Rect.Bottom - 1 do    for X := Rect.Left to Rect.Right - 1 do   
begin      C := Canvas.Pixels[X, Y];      R := Round(        0.01 *
(Transparency * GetRValue(C) + (100 - Transparency) * RR)      );    
 G := Round(        0.01 * (Transparency * GetGValue(C) + (100 -
Transparency) * RG)      );       B := Round(        0.01 *
(Transparency * GetBValue(C) + (100 - Transparency) * RB)      );    
 Canvas.Pixels[X, Y] := RGB(R, G, B);    end;end;
I know the reason is because accessing pixels individually is a lot
slower then using scanline but I am unsure how I exactly I could
optimise the above. I thought maybe draw to a offscreen bitmap and use
it's scanline but not exactly sure how I would go about that (the
scanline part), I also need to make sure I can see through the opacity
rectangle at whatever graphic is underneath on the canvas.

Alternatively, Delphi has Windows.AlphaBlend which I believe could
help me greatly but I couldn't find this in Lazarus.

Can anyone advise how to write such a procedure to paint a rectangle
with opacity that is not slow? I need to draw a overlay rectangle on
top of a canvas, it doesn't even need to be based on the example above
I just need a quick way of doing this without any 3rd party solution
such as GR32 etc.

Thanks.

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lainz · 2016-05-16 22:35

Try using BGRABitmap library.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.BGRAVirtualScreen1Redraw(Sender: TObject; Bitmap:
TBGRABitmap);begin  Bitmap.Rectangle(x, y, x2, y2, BGRA(0,0,0,alpha),
dmDrawWithTransparency);end;
x, y, x2 and y2: here put your rect. BGRA function returns a color
with alpha value. In this demo I'm also using BGRAControls
BGRAVirtualScreen component that is a custom panel to draw into it.

Or you can use just a TBGRABitmap object and then draw to your canvas
instead.

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engkin · 2016-05-17 05:24

Quote from: knuckles on May 16, 2016, 11:05:25 pmAlternatively, Delphi
has Windows.AlphaBlend which I believe could help me greatly but I
couldn't find this in Lazarus.

It is in unit JwaWindows.

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knuckles · 2016-05-17 10:33

Quote from: engkin on May 17, 2016, 07:24:07 amQuote from: knuckles on
May 16, 2016, 11:05:25 pmAlternatively, Delphi has Windows.AlphaBlend
which I believe could help me greatly but I couldn't find this in
Lazarus.

It is in unit JwaWindows.

I should have mentioned I tried this, in fact I did a find in files on
the lazarus directory for the keyword AlphaBlend, unfortunately by
adding that unit it messes up my other graphics routines as things
like TBitmap get confused with which unit to use.

EDIT: Moved the unit to be the first in the uses clause and now works,
will experiment with this now thanks.

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knuckles · 2016-05-17 10:37

Quote from: lainz on May 17, 2016, 12:35:45 amTry using BGRABitmap
library.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.BGRAVirtualScreen1Redraw(Sender: TObject; Bitmap:
TBGRABitmap);begin  Bitmap.Rectangle(x, y, x2, y2, BGRA(0,0,0,alpha),
dmDrawWithTransparency);end;
x, y, x2 and y2: here put your rect. BGRA function returns a color
with alpha value. In this demo I'm also using BGRAControls
BGRAVirtualScreen component that is a custom panel to draw into it.

Or you can use just a TBGRABitmap object and then draw to your canvas
instead.

It does simplify things somewhat but at the moment for my purpose I
need to avoid a 3rd party solution.

Thanks though

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circular · 2016-05-17 20:27

Hmmm... it may be faster to use LazIntfImage as an intermediate where
you do the blending and then draw it back on the canvas.