Firemonkey (FMX) bitmap and colours

Question

Assume I have a small bitmap in Firemonkey (say 32x24 pixels). So I put a TImage onto a form and in the constructor there is this code:

  Image1.Bitmap.Create(32, 24);
  if Image1.Bitmap.Canvas.BeginScene then
  try
    Image1.Bitmap.Canvas.Fill.Color := claBlack;
    Image1.Bitmap.Canvas.Stroke.Color := claYellow;
    Image1.Bitmap.Canvas.FillRect(RectF(0,0,32,24), 0, 0, AllCorners, $FF);
    Image1.Bitmap.Canvas.DrawLine(PointF(1,1), PointF(10,10), $FF);
  finally
    Image1.Bitmap.Canvas.EndScene;
    Image1.Bitmap.BitmapChanged;
  end;

This draws a nice diagonal line on blackground.

What I want to do is now parse the bitmap to determine the pixels affected by the line draw. If I do a basic pixel by pixel check using:

  for y := 0 to 23 do
    for x := 0 to 31 do
      if Image1.Bitmap.Pixels[x,y] <> claBlack then
        memo1.Lines.Add(Format('x=%d. y=%d. c=%x', [x,y,Image1.Bitmap.Pixels[x,y]]));

the output onto my Memo is:

x=0. y=0. c=FF3C3C00
x=1. y=0. c=FF3C3C00
x=0. y=1. c=FF3C3C00
x=1. y=1. c=FFE7E700
x=2. y=1. c=FF3C3C00
x=1. y=2. c=FF3C3C00
x=2. y=2. c=FFE7E700
x=3. y=2. c=FF3C3C00
x=2. y=3. c=FF3C3C00
x=3. y=3. c=FFE7E700
x=4. y=3. c=FF3C3C00
x=3. y=4. c=FF3C3C00
x=4. y=4. c=FFE7E700
x=5. y=4. c=FF3C3C00
x=4. y=5. c=FF3C3C00
x=5. y=5. c=FFE7E700
x=6. y=5. c=FF3C3C00
x=5. y=6. c=FF3C3C00
x=6. y=6. c=FFE7E700
x=7. y=6. c=FF3C3C00
x=6. y=7. c=FF3C3C00
x=7. y=7. c=FFE7E700
x=8. y=7. c=FF3C3C00
x=7. y=8. c=FF3C3C00
x=8. y=8. c=FFE7E700
x=9. y=8. c=FF3C3C00
x=8. y=9. c=FF3C3C00
x=9. y=9. c=FFE7E700
x=10. y=9. c=FF3C3C00
x=9. y=10. c=FF3C3C00
x=10. y=10. c=FF3C3C00

so it's interpreting and "blurring"? my line as the colours (represented by c above) are not equal to claYellow ($FFFF00). If I draw a horizontal or vertical line, the effect is the same. If I change my stroke thickness to 2 and draw a non-diagonal line it draws in claYellow but it covers 2 pixels.

So how can I determine the "true" pixels I've drawn on. In the above sample I would (could) look for $FFE7E700 but how do I know to look for that value (given that if I drew the line in a different colour, that value would be different). I tried to see if there's a consistent "difference" between the colour I drew with and the actual colour rendered but couldn't locate one.

Thanks

not an answer to your question... but another interesting question — Whiler
I don't know FireMonkey, but this sounds like nothing more or less than an ordinary antialiasing. — TLama
This has nothing to do with FireMonkey. It is completely dependent on the underlying graphic system (GDI+, D2D, OpenGL). — Uwe Raabe
If you draw the line using "PointF(1.5,1.5), PointF(10.5,10.5), " there may be no anti aliasing and it will work the way you expect it to. — Giel
@Giel - unfortunately it didn't make much difference. it just returned a different value (in this case $FFEBEB00). — Jason

Yaroslav Brovin Yaroslav Brovin · Accepted Answer · 2013-12-16T13:07:28

FMX use antialiazing for drawing. If you would like draw line without blur effect you should use special function for pixel alignment:

TCanvas.AlignToPixel
TCanvas.AlignToPixelVertically
TCanvas.AlignToPixelHorizontally

This functions automatically calculate pixel position for drawing without blur.

Thank you

Firemonkey (FMX) bitmap and colours

2 Answers