Create a transparent bitmap and UpdateLayeredWindow - delphi

Question

I'm trying to create a mask window with "loading" text, to alert the user about a busy state of my application.
For this, I first created a single form with:
BorderStyle = bsNone
Color = clBlack
AlphaBlend = True
AlphaBlendValue = 180;
As the second step, I want to create another Form, but this one will has dynamic content.
I need to create a transparent bitmap with some status text and use the UpdateLayeredWindow to draw the window as the text.
Take a look and my desired result:
Remember: the text will be different in some cases, like:
Recalculating
Loading resources
Loading report
That's the reason what I need a dynamic bitmap generation.
QUESTION
How can I create a transparent bitmap with text and use it on a form with UpdateLayeredWindow?
I'm trying this, but without success ( to try the code put a Button5 and Label2 on a form):
procedure Inc(var p: pointer);
begin
p := Pointer(Integer(p) + 1);
end;
var
s: string;
frm: TForm;
f: HFont;
tx: HDC;
bmp, old: HBITMAP;
rc: TRect;
h: BITMAPINFOHEADER;
pvBits: Pointer;
t: tagBITMAPINFO;
x,y: integer;
a, r, g, b: byte;
sz: TSize;
p: tpoint;
BlendFunction: TBlendFunction;
begin
tx := CreateCompatibleDC(0);
s := label2.Caption;
f := SelectObject(tx, label2.Font.Handle);
fillchar(rc, SizeOf(rc), 0);
DrawText(tx, PChar(s), length(s), rc, DT_CALCRECT);
fillchar(h, SizeOf(h), 0);
pvBits := nil;
h.biSize := SizeOf(h);
h.biWidth := rc.Right - rc.Left;
h.biHeight := rc.Bottom - rc.Top;
h.biPlanes := 1;
h.biBitCount := 32;
h.biCompression := BI_RGB;
FillChar(t, SizeOf(t), 0);
t.bmiHeader := h;
bmp := CreateDIBSection(tx, t, 0, pvBits, 0, 0);
old := SelectObject(tx, bmp);
if old > 0 then
begin
SetTextColor(tx, $00FFFFFF);
SetBkColor(tx, $00000000);
SetBkMode(tx, TRANSPARENT);
DrawText(tx, PChar(s), length(s), rc, DT_NOCLIP);
r := GetRValue($FF);
g := GetGValue($FF);
b := GetBValue($FF);
for x := 0 to h.biWidth-1 do
for y := 0 to h.biHeight-1 do
begin
a := Byte(pvBits^);
Inc(pvBits);
Byte(pvBits^) := (b * a) shr 8;
Inc(pvBits);
Byte(pvBits^) := (g * a) shr 8;
Inc(pvBits);
Byte(pvBits^) := (r * a) shr 8;
Inc(pvBits);
Byte(pvBits^) := a;
end;
SelectObject(tx, old);
end;
SelectObject(tx, f);
deleteDC(tx);
sz.cx := h.biWidth;
sz.cy := h.biHeight;
p := Point(0,0);
BlendFunction.BlendOp := AC_SRC_OVER;
BlendFunction.BlendFlags := 0;
BlendFunction.SourceConstantAlpha := 255;
BlendFunction.AlphaFormat := AC_SRC_ALPHA;
frm := TForm.CreateNew(self);
frm.BorderStyle := bsNone;
frm.Position := poOwnerFormCenter;
frm.Show;
UpdateLayeredWindow(frm.Handle, 0, nil, #sz, bmp, #p, 0, #BlendFunction, ULW_ALPHA);
end;

Answer 1

Assuming you can use Windows API functions from Delphi (since you've tagged winapi), one easy way is:
CreateDIBSection() to create a 32 bit bitmap
FillRect() to fill the background, DrawText() to draw the text
Fix up the alpha
Use that bitmap with UpdateLayeredWindow()
Fixing up the alpha is done by directly modifying the bitmap bits that you get back from CreateDIBSection(). Note that UpdateLayeredWindow() requires pre-multiplied alpha, so you have to multiple the RGB components by the alpha value in advance.
The following code is C but hopefully will give you the idea:
LPVOID pBits; // bits from CreateDIBSection
const int width, height; // size of bitmap
const int alpha; // level of transparency
RGBQUAD* pPtr = (RGBQUAD*)pBits;
for (int y = 0; y < height; ++y)
{
for (int x = 0; x < width; ++x, ++pPtr)
{
pPtr->rgbBlue = (alpha * pPtr->rgbBlue) / 255;
pPtr->rgbGreen = (alpha * pPtr->rgbGreen) / 255;
pPtr->rgbRed = (alpha * pPtr->rgbRed) / 255;
pPtr->rgbReserved = alpha;
}
}