Outputting a transparent PNGimage with BGRABitmap
lazarus / 39-Graphics

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clintonb · 2025-02-03 17:59

I have code that used a dll to output a png with transparent pixels
but i cant find a .so for linux.  I found an article that shows how to
use TBGRABitmap to do this.  I just copied from a control to test it
out by saying :

  tempsaveimage.canvas.copyrect(rect(0,0,panel8.width,panel8.height),p
anelcanvas,rect(0,0,panel8.width,panel8.height));

tempsaveimage.ReplaceColor(BGRA(255,0,0,255),BGRAPixelTransparent);
        

Then i saved it to a file by just calling tempsaveimage.savetofile. 

All works and it actually gives me an output that is a transparent PNG
which is great.  What is strange is that imagemagik doesnt recognize
the transparency but Gimp does.  Ive spent the last 3 hours comparing
a png file that works and the one that i generated with TBGRABitmap
and for the life of me i cant see it.  Is this the wrong way to go
savethe output to a file? Perhaps there is a pixelformat that needs to
be specified?

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clintonb · 2025-02-03 20:00

Its bizarre but i suppose its like saving a jpg with a .bmp extension
and certain programs will tell you its not a valid bitmap where others
look past the obvious and use the header information.  Its not ideal
but opening it in gimp and just re-saving it seems patch what
TBGRABitmap misses and everything works.  I will have to dig through
this a bit closer to figure just what that is.

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TRon · 2025-02-03 20:05

It is impossible to provide any meaningful feedback without knowing
what you actually did in code and what the outcome of it is.

Although there is only a single real one, transparency can mean 2
different things depending on the point of view of whatever program.
Note that most graphical viewers/editor do not have a clue (even if
they are expensive commercial products).

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clintonb · 2025-02-04 08:45

Im not sure i understand why this is an issue given how many programs
had a problem with it but i used the replacecolor to change the
background to BGRAPixeltransparent as per a few tutorials and this
seems to show opaque.  Looking in Gimp, it is simply a pixel color of
black with no opacity or BGRA(0,0,0,0).  Setting this to any non black
color with 0 opacity makes the problem go away.

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H₂SO₄ · 2025-02-04 10:32

You'll need to show your full/exact code and/or upload the broken 
.png  if you want anyone to analyze it further.