Thread: Coloring single pixels - performance killer

Instead of multiple calls to SetPixel, you should use a dib section.

Since the GDI works best with BMPs it is best to load your format into a buffer and then convert it into the correct format that Windows likes. GDI+ will greatly aid you in this.

Well, thanks for the answers! Creating a dib section seemed to be the simplest choice. As I need to implement alpha blending I am using the BITMAPV4HEADER.

Code:

memset (&bmiHeader, 0, sizeof (BITMAPV4HEADER));
bmiHeader.bV4Size        = sizeof(BITMAPV4HEADER);
bmiHeader.bV4Width       = 300;
bmiHeader.bV4Height      = 300;
bmiHeader.bV4Planes      = 1;
bmiHeader.bV4BitCount    = 32;
bmiHeader.bV4V4Compression = BI_BITFIELDS; //BI_RGB;
bmiHeader.bV4RedMask = 0x00FF0000;
bmiHeader.bV4GreenMask = 0x0000FF00;
bmiHeader.bV4BlueMask = 0x000000FF;
bmiHeader.bV4AlphaMask = 0xFF000000;

The RGB masks do work but changing the Alpha value has no effect at all...
What could I've done / not done that it won't blend?

In order to alpha blend you must call AlphaBlend() which takes yet another structure in order to work. GDI is sort of a huge mess.

Is there an other way of alpha blending? Why's there a header made for 32 bit bmps if it needs a GDI function to display them?
Sadly I can't get AlphaBlend() to work.
code:

Code:

WINGDIAPI BOOL WINAPI AlphaBlend(HDC,int,int,int,int,HDC,int,int,int,int,BLENDFUNCTION); // used global

and inside the WndProc():

Code:

BLENDFUNCTION bld;
bld.BlendOp = AC_SRC_OVER;
bld.BlendFlags = 0;
bld.SourceConstantAlpha = 255;
bld.AlphaFormat = AC_SRC_ALPHA;

...

	case WM_PAINT:
		BeginPaint(hwnd, &Ps);
			BitBlt(Ps.hdc, 40, 40, 200, 200, hdc, 0, 0, SRCCOPY);
			AlphaBlend(Ps.hdc, 40, 40, 200, 200, hdc, 0, 0, 200, 200, bld);
		EndPaint(hwnd, &Ps);
		break;

The result is a:

[Linker error] undefined reference to `_Z10AlphaBlendP5HDC__iiiiS0_iiii14_BLENDFUNCTION@ 44'
ld returned 1 exit status

Also, rather than declaring the function yourself (your declaration won't work because it is not extern "C"), use this above your includes:

Code:

#define WINVER 0x0500

It's going on well, the program is likely done, but I have still 2 problems left.
The main problem is that I can't get AlphaBlend to function. Thanks for the two last posts, I can compile and link with no errors now, but using the AlphaBlend() function right after BitBlt() just has no effect, independant of SourceConstantAlpha and AlphaFormat.

Here is the code:

Code:

HDC        hdc = NULL;
PAINTSTRUCT Ps;
BLENDFUNCTION bld;

...

hdc = CreateCompatibleDC(NULL);

...

bld.BlendOp = AC_SRC_OVER;
bld.BlendFlags = 0;
bld.SourceConstantAlpha = 0; //Should make the creating bmp completely invisible
bld.AlphaFormat = AC_SRC_ALPHA; // Also tried 0

Code:

	case WM_PAINT:
		BeginPaint(hwnd, &Ps);
			BitBlt(Ps.hdc, 0, 0, 512, 512, hdc, 0, 0, SRCCOPY);
			AlphaBlend(Ps.hdc, 0, 0, 512, 512, hdc, 0, 0, 512, 512, bld);
		EndPaint(hwnd, &Ps);
		break;

I am directly drawing the pic in the main window with CreateDIBSection. The area is black. Does DIB_RGB_COLORS restrict anything?

Code:

hBmp = CreateDIBSection(NULL, (BITMAPINFO *)&bmiHeader, DIB_RGB_COLORS, (void**)&pBits, NULL, 0);

As long as I am not working on an editor for these file formats, non working Alpha blending is not that tragic...

My second problem now is that after opening the file it's not actually drawn inside the window, I have to lay another window over the application window or I have to wipe or mop the target area with another windows window before anything is being drawn...
I've tried out UpdateWindow() but it had no effect, i tried sending a WM_PAINT message with SendMessage() but it also didn't help.

The purpose of the AlphaBlend function is to draw the source image over the destination image with a specified amount of transparency. In your case, the destination(Ps.hdc) is undefined and it is therefore unclear what you are trying to achieve. Are you trying to make the image look transparent over white or another colour or are you trying to make the window transparent?

>> I have to lay another window over the application window or I have to wipe or mop the target area with another windows window before anything is being drawn. <<

You need to call InvalidateRect when the window needs repainting.

Originally Posted by JaWiB

> In your case, the destination(Ps.hdc) is undefined and it is therefore unclear what you are trying to achieve.

Shouldn't Ps.hdc contain a handle to a DC for the current window after calling BeginPaint (unless it failed)? Did you mean the source (hdc) is undefined?

Yep, Ps.hdc is defined. I meant to say that the contents of the window (Ps.hdc) is undefined, which is why it needs painting. Although, I guess if you are using a background brush (WM_ERASEBKGND), then it might be defined to be the background colour.