Bitmaps alpha has less effect the whiter a pixel is

[RD_CEDES 0]

Hello everyone

 

Bitmaps do show a weird behavior. The whiter a pixel gets, the less effect setting the alpha has.

 

Here is some code to reproduce the situation. It (should) generate a bitmap that goes from black (top) to white (bottom) and from 0 alpha (left) to 255 alpha (right).

 

{
Should generates a bitmap with:
  - a grayscale gradient on the y-axis from black (top) to white (bottom)
  - an alpha gradient on the x-axis from 0 (left) to 255 (right)

But the whiter a pixel gets, the less effect the alpha has.
}
procedure TfrmMain.FormCreate(Sender: TObject);
const
  RES_X = 512;
  RES_Y = 512;
var
  bmpData: TBitmapData;
  paca: PAlphaColorArray;
  c: TAlphaColor;
begin
  img.Bitmap := TBitmap.Create(RES_X, RES_Y);
  img.Bitmap.Map(TMapAccess.Write, bmpData);
  for var y := 0 to RES_Y - 1 do
  begin
    paca := bmpData.GetScanline(y);
    for var x := 0 to RES_X - 1 do
    begin
      var g := Byte(Round($FF / RES_Y * y)); // Varying gray component based on y coordinate
      var a := Byte(Round($FF / RES_X * x)); // Varying alpha component based on x coordinate
      c := (a shl 24) or (g shl 16) or (g shl 8) or g;
      paca[x] := c;
    end;
  end;
  img.Bitmap.Unmap(bmpData); // Not as expected
  img.Bitmap.SaveToFile('test.png'); // PNGs look exactly as expected
end;

 

When saving the bitmap as PNG, the transparency is exactly as expected (attached "Saved_as_PNG_added_red_background.png"), but the image shown on the form (attached "Form_screenshot.png") behaves differently.

 

Helping words on how to change the opacity of white pixels are highly appreciated.

 

What I'm trying to do is having an image (that is updated every 50 ms) showing the history of a sensor. The data is shown as white pixels, and the further in the past / on the left the data is, the more transparent the pixels shall get.

 

Saved_as_bitmap_no_alpha_add_all.bmp

Edited July 20, 2023 by RD_CEDES

[Alexander Sviridenkov 357]

Bitmaps usually have premultiplied alpha - RGB values should be multiplied to alpha (0-1) value.

[RD_CEDES 0]

Thank you very much Sir! That works!

 

{
Generates a bitmap with:
  - a grayscale gradient on the y-axis from black (top) to white (bottom)
  - an alpha gradient on the x-axis from 0 (left) to 255 (right)
}
procedure TfrmMain.FormCreate(Sender: TObject);
const
  RES_X = 512;
  RES_Y = 512;
var
  bmpData: TBitmapData;
  paca: PAlphaColorArray;
  c: TAlphaColor;
  a, g: UInt8;
begin
  img.Bitmap := TBitmap.Create(RES_X, RES_Y);
  img.Bitmap.Map(TMapAccess.Write, bmpData);
  for var y := 0 to RES_Y - 1 do
  begin
    paca := bmpData.GetScanline(y);
    for var x := 0 to RES_X - 1 do
    begin
      a := Byte(Round($FF / RES_X * x)); // Varying alpha component based on x coordinate
      g := MulDivInt64(a, 255, 255);
      c := (a shl 24) or (g shl 16) or (g shl 8) or g;
      paca[x] := c;
    end;
  end;
  img.Bitmap.Unmap(bmpData);
  img.Bitmap.SaveToFile('test.png');
end;