Getting Color Depth of any image (again)

Jon E. Scott

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Dec 16, 1998, 3:00:00 AM12/16/98

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I am using D4 C/S on NT4SP3.

I know you can use TBitmap.PixelFormat to determine the number of colors
of an image, but this is "predefined" and we know the # of colors for
each bpp:

if Image1.Picture.Graphic is TBitmap then begin
case Image1.Picture.Bitmap.PixelFormat of //Find color depth
pf1bit : pf := '. Monochrome';
pf4bit : pf := '. 16 Colors';
pf8bit : pf := '. 256 Colors';
pf15bit: pf := '. 32768 Colors';
pf16bit: pf := '. 65536 Colors';
pf24bit: pf := '. 16 Million Colors';
pf32bit: pf := '. Gazillions of Colors!';
else
pf := '. Custom color scheme';
end;
end;

What I want to know is where do these numbers come from? What functions
are there to get these numbers? I'm thinking:

var
DeviceContext: hDC;
NumOfColors: integer;
MyBitmap: TBitmap;

{ Load bitmap into MyBitmap }
...
DeviceContext := GetDC(MyBitmap.Handle);
try
NumOfColors := GetDeviceCaps(DeviceContext, PLANES) *
GetDeviceCaps(DeviceContext, BITSPIXEL);
finally
ReleaseDC(hImage, DeviceContext)
end;

but that doesn't return the correct result. Can anyone enlighten me on
this? What is the formula for getting the number of colors of an image?

TIA,
Jon.

Ian martin

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Dec 16, 1998, 3:00:00 AM12/16/98

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> NumOfColors := GetDeviceCaps(DeviceContext, PLANES) *
> GetDeviceCaps(DeviceContext, BITSPIXEL);

Basically

NumofColors := 1 shl GetDeviceCaps(DeviceContext, BITSPIXEL);

Ian.

Jon E. Scott

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Dec 16, 1998, 3:00:00 AM12/16/98

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I tried that but it returns 1 color for 4-bit and 8-bit bitmaps, which
obviously is incorrect.

Anyone else?
Jon.

David Ullrich

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Dec 16, 1998, 3:00:00 AM12/16/98

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Jon E. Scott wrote:
>
> I tried that but it returns 1 color for 4-bit and 8-bit bitmaps, which
> obviously is incorrect.
>
> Anyone else?

Try changing your

DeviceContext := GetDC(MyBitmap.Handle);

to

DeviceContext := MyBitmap.Canvas.Handle;

GetDC returns a DC for a _window_, not a DC for a bitmap.

> Jon.
>
> Ian martin wrote:
>
> > > NumOfColors := GetDeviceCaps(DeviceContext, PLANES) *
> > > GetDeviceCaps(DeviceContext, BITSPIXEL);
> >
> > Basically
> >
> > NumofColors := 1 shl GetDeviceCaps(DeviceContext, BITSPIXEL);
> >
> > Ian.

--
David Ullrich

sig.txt still not found

Jon E. Scott

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Dec 16, 1998, 3:00:00 AM12/16/98

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OK, this is what I had:

function GetColorDepth(hImage: THandle): integer;
var
DeviceContext: hDC;
begin
DeviceContext := GetDC(hImage);

try
{testing various formulas)

Result := 1 SHL (GetDeviceCaps(DeviceContext, PLANES) *
GetDeviceCaps(DeviceContext, BITSPIXEL));
finally
ReleaseDC(hImage, DeviceContext);
end;
end;

Then I call this function like this:

var
MyBitmap: TBitmap;
NumOfColors: integer;

{ Load image into bitmap here. }
...
NumOfColors := GetColorDepth(MyBitmap.Canvas.Handle);

I've tried what you said and changed the function to:

and call it the same way. Now it returns whatever # colors the comments say
for every bitmap, regardless of the bits. My desktop is set at 24-bit.
Could it be that it GetDeviceCaps is using my desktop instead of the bitmap's
handle?

Any other thoughts/advice?

TIA,
Jon.

Joe C. Hecht

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Dec 16, 1998, 3:00:00 AM12/16/98

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Jon E. Scott wrote:
>
> I am using D4 C/S on NT4SP3.
>
> I know you can use TBitmap.PixelFormat to determine the number of colors
> of an image, but this is "predefined" and we know the # of colors for
> each bpp:
>
> if Image1.Picture.Graphic is TBitmap then begin
> case Image1.Picture.Bitmap.PixelFormat of //Find color depth
> pf1bit : pf := '. Monochrome';
> pf4bit : pf := '. 16 Colors';
> pf8bit : pf := '. 256 Colors';
> pf15bit: pf := '. 32768 Colors';
> pf16bit: pf := '. 65536 Colors';
> pf24bit: pf := '. 16 Million Colors';
> pf32bit: pf := '. Gazillions of Colors!';
> else
> pf := '. Custom color scheme';
> end;
> end;
>
> What I want to know is where do these numbers come from? What functions
> are there to get these numbers? I'm thinking:

Dude. If you want to know where these numbers come from, then
look at the VCL code that ships with Delphi Professional or
Delohi Client Server.

In short. these numbers come from:

1) If the source is a DIB (device independent bitmap) then the dibheader
contains the pixel format. You can write oodles of code to read your
own dib from various sources (file, clipboard, resource file, stream),
if you like. Its a good education. Start out with the WinAPI help file,
and the graphics functions related to bitmaps.

2) If the source is not a DIB, then the source is either monochrome
or the color depth of the screen.

One of the reasons that GetDeviceCaps is not working for you is that
you are working with a MemoryDC. MemoryDC are not devices, and this call
does not work well with all drivers when used with a memory dc.

Hope that helps!

Joe
--
Joe C. Hecht
http://home1.gte.net/joehecht/index.htm

Jon E. Scott

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Dec 17, 1998, 3:00:00 AM12/17/98

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I now realize these values come from this formula: 2 to the BitsPerPixel'th
power. Example:

if 8 bit, then Power(2, 8) = 256.

Does this sound correct for DIBs? if PixelFormat returns pfDevice, then we're
dealing with a DDB. Therefore the number of colors would be at what your
graphics card is set at. Am I getting this right?

I know I am a pain in the butt when dealing with graphics, but I have to start
learning somewhere.

Thanks,
Jon.

David Ullrich

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Dec 17, 1998, 3:00:00 AM12/17/98

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Jon E. Scott wrote:
>
> OK, this is what I had:

> [old version]

>
> Then I call this function like this:
>
> var
> MyBitmap: TBitmap;
> NumOfColors: integer;
>
> { Load image into bitmap here. }
> ...
> NumOfColors := GetColorDepth(MyBitmap.Canvas.Handle);
>
> I've tried what you said and changed the function to:
>
> function GetColorDepth(DeviceContext: THandle): integer;
> begin
> { Test various formulas }
> Result := 1 SHL (GetDeviceCaps(DeviceContext, PLANES) *
> GetDeviceCaps(DeviceContext, BITSPIXEL)); { Returns
> 16M colors for any bitmap. }

this version of the formula makes no sense - it's giving the
"right" answer only because PLANES happens to be 1. What you
actually want is PLANES * (1 SHL BITSPIXEL).

> //Result := 1 SHL GetDeviceCaps(DeviceContext, BITSPIXEL); { Returns 16M
> colors for any bitmap. }
> //Result := GetDeviceCaps(DeviceContext, PLANES) *
> // GetDeviceCaps(DeviceContext, BITSPIXEL); { Returns
> 24 colors for any bitmap. }

These last two are doing the same thing, right? Because
1 SHL 24 is 16M, and PLANES = 1.

> end;
>
> and call it the same way. Now it returns whatever # colors the comments say
> for every bitmap, regardless of the bits. My desktop is set at 24-bit.
> Could it be that it GetDeviceCaps is using my desktop instead of the bitmap's
> handle?

I can't tell exactly because of the parts you're leaving out - "load
image into bitmap" could mean various things, and the results will be different.
Assuming you're talking about D3 or higher, a TBitmap can be a DDB or a DIB.
If it's a DDB then you get the number of colors for your display - of it's
a DIB you should get something consistent with the value of PixelFormat.

Try explicitly _setting_ the value of PixelFormat before calling
your function. Immediately before.

Colin Wilson

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Dec 18, 1998, 3:00:00 AM12/18/98

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In article <367840...@badsector.com>, Joe C. Hecht wrote:
> You can write oodles of code to read your
> own dib from various sources (file, clipboard, resource file, stream),
> if you like. Its a good education.
>

.. but it will probably send you mad...

Colin
e-mail :co...@wilsonc.demon.co.uk
web: http://www.wilsonc.demon.co.uk/delphi.htm