How do I show an image into a window's client area in Win32?

Given an RGBA array (24 bpp) of an image that has the same width and height of a window's client area, how do I efficiently show that image on that client area, using the Win32 API?

Can I use the same function(s) to efficiently update part of the window's client area with a changed sub-image? If not, which other functions should I use?

I own the window of which I'm trying to modify the client area.

I tried by using these functions:

1. Creating a compatible device context (with CreateCompatibleDC)
2. Creating a compatible bitmap (with CreateCompatibleBitmap)
3. Selecting the bitmap (with SelectObject)
4. Getting the bitmap object info (with GetObject)
5. Setting the bitmap bits (with SetDIBits) because CreateCompatibleBitmap does not allow to specify bits as a function argument
6. Transferring the color data to the device context (with BitBlt)

But the result I got does not match my expectations at all.

This is what I got:

What I expected was the client area divided in 4 equal parts, each one with a different shade of gray (white, light gray, dark gray, black).

I also tried using SetDIBitsToDevice and removing the alpha channel, but the result is the same.

I suspect that the actual problem is the channel order, as if Win32 didn't use RGB but BGR or others.

Personally, I'd probably use CreateDIBSection.

This creates a DIB Section object, which allows both direct access to its memory and use as a device independent bitmap, so you can create it, copy your data into its memory, then use it with BitBlt (and similar such as StretchBlt, PlgBlt, etc.) just like any other device independent bitmap supports.

A typical call to CreateDIBSection will look something like this:

BITMAPINFO info {};
info.bmiHeader.biSize = sizeof(info.bmiHeader);
info.bmiHeader.biPlanes = 1;
info.bmiHeader.biBitCount = 32;
info.bmiHeader.biCompression = BI_RGB;
info.bmiHeader.biWidth = width;
info.bmiHeader.biHeight = height;

DWORD *pixels;

HBITMAP section = CreateDIBSection(mem, &info, DIB_RGB_COLORS, (void **)&pixels, NULL, 0);

At this point you have an HBITMAP, which you can use to do all the normal bitmap stuff (e.g., select it into a DC,then BitBlit to/from it).

But you also have pixels, which is a pointer directly to the memory that contains the data for that bitmap. The one thing to look out for here is that you need to call GdiFlush() before trying to access that memory directly (to give GDI a chance to finish if it's already using it).

Now don't get me wrong: if you get the format correct, it's certainly entirely possible to make SetDIBIts work as well. But at least personally, I find DIB Sections less annoying to deal with.

Oh, one other minor detail: I'd advise always using 32 bits per pixel instead of 24. This eliminates any need to pad the data (which can be mildly annoying), and usually blits faster as well (one dword per pixel is easy to manipulate, display, etc.)