So far this week, we’ve
separated the DIB metadata (BITMAPINFO)
from the pixels of a DIB section.
But there’s really no need for the DIB section at all!
As long as you have the pixels and the metadata, you
can draw bits.
We demonstrate this by drawing a rather stupid-looking bitmap
onto the screen, but doing so without the use of HBITMAPs
at all!
Start with a brand new
scratch program and make these changes:
const BYTE g_rgbPixels[16][8] = {
{ 0xDD, 0xDD, 0xDD, 0xDD, 0xDD, 0xDD, 0xDD, 0xDD },
{ 0xDD, 0xDD, 0xD0, 0x00, 0x00, 0xDD, 0xDD, 0xDD },
{ 0xDD, 0xD0, 0x01, 0x11, 0x11, 0x00, 0xDD, 0xDD },
{ 0xDD, 0x01, 0x11, 0x11, 0x11, 0x11, 0x0D, 0xDD },
{ 0xD0, 0x11, 0x11, 0x11, 0x11, 0x11, 0x10, 0xDD },
{ 0xD0, 0x11, 0x00, 0x00, 0x00, 0x01, 0x10, 0xDD },
{ 0x01, 0x11, 0x09, 0x99, 0x99, 0x01, 0x11, 0x0D },
{ 0x01, 0x11, 0x09, 0x99, 0x99, 0x01, 0x11, 0x0D },
{ 0x01, 0x11, 0x09, 0x99, 0x99, 0x01, 0x11, 0x0D },
{ 0x01, 0x11, 0x09, 0x99, 0x99, 0x01, 0x11, 0x0D },
{ 0x01, 0x11, 0x09, 0x99, 0x99, 0x01, 0x11, 0x0D },
{ 0xD0, 0x11, 0x00, 0x00, 0x00, 0x01, 0x10, 0xDD },
{ 0xD0, 0x11, 0x11, 0x11, 0x11, 0x11, 0x10, 0xDD },
{ 0xDD, 0x01, 0x11, 0x11, 0x11, 0x11, 0x0D, 0xDD },
{ 0xDD, 0xD0, 0x01, 0x11, 0x11, 0x00, 0xDD, 0xDD },
{ 0xDD, 0xDD, 0xD0, 0x00, 0x00, 0xDD, 0xDD, 0xDD },
};
struct BITMAPINFO16 {
BITMAPINFOHEADER bmiHeader;
RGBQUAD bmiColors[16];
} g_bmi = {
{ sizeof(g_bmi.bmiHeader), // biSize
16, // biWidth
16, // biHeight
1, // biPlanes
4, // biBitCount
BI_RGB, // biCompression
0, // biSizeImage
0, // biXPelsPerMeter
0, // biYPelsPerMeter
16, // biClrUsed
16, // biClrImportant
},
{
{ 0x00, 0x00, 0x00, 0x00 },// bmiColors[0]
{ 0x80, 0x00, 0x00, 0x00 },// bmiColors[1]
{ 0x00, 0x80, 0x00, 0x00 },// bmiColors[2]
{ 0x80, 0x80, 0x00, 0x00 },// bmiColors[3]
{ 0x00, 0x00, 0x80, 0x00 },// bmiColors[4]
{ 0x80, 0x00, 0x80, 0x00 },// bmiColors[5]
{ 0x00, 0x80, 0x80, 0x00 },// bmiColors[6]
{ 0xC0, 0xC0, 0xC0, 0x00 },// bmiColors[7]
{ 0x80, 0x80, 0x80, 0x00 },// bmiColors[8]
{ 0xFF, 0x00, 0x00, 0x00 },// bmiColors[9]
{ 0x00, 0xFF, 0x00, 0x00 },// bmiColors[10]
{ 0xFF, 0xFF, 0x00, 0x00 },// bmiColors[11]
{ 0x00, 0x00, 0xFF, 0x00 },// bmiColors[12]
{ 0xFF, 0x00, 0xFF, 0x00 },// bmiColors[13]
{ 0x00, 0xFF, 0xFF, 0x00 },// bmiColors[14]
{ 0xFF, 0xFF, 0xFF, 0x00 },// bmiColors[15]
}
};
void
PaintContent(HWND hwnd, PAINTSTRUCT *pps)
{
StretchDIBits(pps->hdc, 0, 0,
g_bmi.bmiHeader.biWidth*4,
g_bmi.bmiHeader.biHeight*4,
0, 0,
g_bmi.bmiHeader.biWidth,
g_bmi.bmiHeader.biHeight,
g_rgbPixels, (BITMAPINFO*)&g_bmi,
DIB_RGB_COLORS, SRCCOPY);
}
We are drawing a bitmap without using an HBITMAP!
The technique is the same one we’ve been using all week:
Building a BITMAPINFO and using it
to guide the interpretation of a chunk of memory containing
pixels.
Just to make things easier to see, I magnified the image,
but the point is the same.
Okay, now let’s look at the limitations and caveats of this technique.
First, of course, is that this works only when the memory contains
the source bitmap for a
BitBlt or
StretchBlt operation.
It’s nice when you have a bunch of bitmaps that you only intend to
draw from,
but
you can’t use this technique to draw into a chunk of memory.
For that, you’ll want to create a DIB section and draw into that.
Another problem with this technique is that it doesn’t
play friendly with
remote desktops,
one of those Windows programming taxes.
After all, pumping a chunk of pixels to the screen is the
logical equivalent of a BitBlt of a bitmap
nobody has ever seen before.
You’ve got pixels just coming out of nowhere.
There’s nothing to cache since there is no HBITMAP
to associate the pixels with.
On remote desktops, it’s probably okay to
use this technique for small bitmaps
(like ours) where the cost of sending the pixels is insigificant,
or if your render target is not the screen (for example,
if you are rendering to an off-screen bitmap because you’re doing some
image manipulation behind the scenes).
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