Drawing on 8bpp grayscale bitmap (unmanaged C++) - c++

Question

I've been attempting to draw on an 8bpp grayscale bitmap without success. Here are some of my attempts. Maybe someone can point out what I'm doing wrong.
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Attempt 1: Create, select, and draw:
In constructor:
CBitmap bm;
bm.CreateBitmap (200, 200, 1, 8, NULL);
In OnDraw:
CDC *mdc=new CDC ();
HGDIOBJ tmp = mdc->SelectObject(bm);
Result: tmp is NULL, indicating failure.
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Attempt 2: CreateDIBSection
In constructor:
HBITMAP hbm;
BITMAPINFOHEADER bih;
BITMAPINFO bi;
HANDLE hb;
CDC* myDc = new CDC ();
HDC hdc = myDc->GetSafeHdc ();
void* bits;
RGBQUAD rq [256];
initBi ();
hbm = CreateDIBSection (hdc, &bi, DIB_RGB_COLORS, &bits, NULL, 0);
...
void CEightBitDrawingView::initBi()
{
bih.biSize = sizeof (BITMAPINFOHEADER);
bih.biWidth = 200;
bih.biHeight = -200;
bih.biPlanes = 1;
bih.biBitCount = 8;
bih.biCompression = BI_RGB;
bih.biSizeImage = 0;
bih.biXPelsPerMeter = 14173;
bih.biYPelsPerMeter = 14173;
bih.biClrUsed = 0;
bih.biClrImportant = 0;
memset ((void *) rq, 0, 256 * sizeof (RGBQUAD));
bi.bmiHeader = bih;
bi.bmiColors = rq;
}
Result: This doesn't even compile because the BITMAPINFO bmiColors member is defined as:
RGBQUAD bmiColors[1];
so won't accept more than one RGB color. In fact, nothing I assign to this member compiles! (Could they possibly make it any more complex!?)
Any suggestions would be appreciated! Thanks!
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Answer 1

Here. Code that demonstrates how to - in a not managed world - allocate a dynamically sized structure on the stack, fill it in and pass it to CreateDIBSection.
#include <malloc.h>
HBITMAP CreateGreyscaleBitmap(int cx, int cy)
{
BITMAPINFO* pbmi = (BITMAPINFO*)alloca( sizeof(BITMAPINFOHEADER) + sizeof(RGBQUAD)*256);
pbmi->bmiHeader.biSize = sizeof (pbmi->bmiHeader);
pbmi->bmiHeader.biWidth = cx;
pbmi->bmiHeader.biHeight = cy;
pbmi->bmiHeader.biPlanes = 1;
pbmi->bmiHeader.biBitCount = 8;
pbmi->bmiHeader.biCompression = BI_RGB;
pbmi->bmiHeader.biSizeImage = 0;
pbmi->bmiHeader.biXPelsPerMeter = 14173;
pbmi->bmiHeader.biYPelsPerMeter = 14173;
pbmi->bmiHeader.biClrUsed = 0;
pbmi->bmiHeader.biClrImportant = 0;
for(int i=0; i<256; i++)
{
pbmi->bmiColors[i].rgbRed = i;
pbmi->bmiColors[i].rgbGreen = i;
pbmi->bmiColors[i].rgbBlue = i;
pbmi->bmiColors[i].rgbReserved = 0;
}
PVOID pv;
return CreateDIBSection(NULL,pbmi,DIB_RGB_COLORS,&pv,NULL,0);
}

Answer 2

In both your examples, you created a new CDC with the following line:
CDC* pDC = new CDC();
But there's something missing: This will just create a new CDC object, but without a valid HDC handle attached to it. You need to call CDC::CreateCompatibleDC first, otherwise trying to select any object into this DC will fail.
Regarding the bmiColors: This member is defined as 1 sized array because the data behind it depends on the color depth and type of bitmap. This is documented in the MSDN. For example, if you had a 128x128 pixels 8Bit Bitmap, you would have to allocate the following amount of mem:
128 * 128 * sizeof(WORD) + sizeof(BITMAPINFOHEADER)

Answer 3

I finally resorted to using a .NET graphics tool (Aurigma) to create an 8bpp bitmap, and passed its handle to the unmanaged C++.
Then in C++:
HDC memDc = CreateCompatibleDC (NULL);
HGDIOBJ Obmp = ::SelectObject(memDc, varLayer); // Handle to 8-bit bitmap.
I was able to select the bitmap into a CDC and draw on it. Not 100% unmanaged, but this allowed me to do the drawing in unmanaged code, which gives acceptable speed.

Answer 4

Your bitmap needs to be compatible (same color-depth) as the display context you're going to render it on. Also, 8-bits/pixel bitmaps aren't necessarily grayscale - that's a function of what palette you're using.