Create HBitmap in IExtractImage - winapi

Question

I'm developping a thumbnail creator as a shell extension.
To do that, I choosed to implement interface IExtractImage.
My dll is loaded and correctly called, but, the thumbnail is always black, instead of being red.
What am I missing?
class MyShellPreview : public IExtractImage, IPersistFile
// set by IExtractImage::GetLocation
SIZE m_size;
IFACEMETHODIMP Extract(HBITMAP *phBmpImage)
{
size_t size = m_size.cx * m_size.cy * 3;
// alloc buffer
BYTE *buffer = (BYTE*)malloc(size);
// fill buffer
for (k = i = 0; i < m_size.cx; ++i)
{
for (j = 0; j < m_size.cy; ++j, ++k)
{
buffer[k] = 128;
buffer[k+1] = 0;
buffer[k+2] = 0;
}
}
*phBmpImage = CreateBitmap(m_size.cx, m_size.cy, 3, 8, buffer);
free(buffer);
return S_OK;
}
};
I know that for performance reason, I should use CreateCompatibleBitmap and SetDIBits, but I'm not sure from where I should get an HDC.

Answer 1

As pointed on comments, the solution was to use:
32 bits images
1 color plane
class MyShellPreview : public IExtractImage, IPersistFile
// set by IExtractImage::GetLocation
SIZE m_size;
IFACEMETHODIMP Extract(HBITMAP *phBmpImage)
{
size_t size = m_size.cx * m_size.cy * 4;
// alloc buffer
BYTE *buffer = (BYTE*)calloc(size, 1);
// fill buffer
for (k = i = 0; i < m_size.cx; ++i)
{
for (j = 0; j < m_size.cy; ++j, k+=4)
{
buffer[k] = 128;
}
}
*phBmpImage = CreateBitmap(m_size.cx, m_size.cy, 1, 24, buffer);
free(buffer);
return S_OK;
}
};