How to update a user created Bitmap in the Windows API - c++

Question

In my code I quickly generate images on the fly, and I want to display them as quickly as possible. So the first time I create my image, I create a new BITMAP, but instead of deleting the old one and creating a new one for every subsequent image, I just want to copy my data back into the existing one. Here is my code to do both the initial creation and the updating. The creation works just fine, but the updating one doesn't work.
BITMAPINFO bi;
HBITMAP Frame::CreateBitmap(HWND hwnd, int tol1, int tol2, bool useWhite, bool useBackground)
{
ZeroMemory(&bi.bmiHeader, sizeof(BITMAPINFOHEADER));
bi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bi.bmiHeader.biWidth = width;
bi.bmiHeader.biHeight = height;
bi.bmiHeader.biPlanes = 1;
bi.bmiHeader.biBitCount = 24;
bi.bmiHeader.biCompression = BI_RGB;
ZeroMemory(bi.bmiColors, sizeof(RGBQUAD));
// Allocate memory for bitmap bits
int size = height * width;
Pixel* newPixels = new Pixel[size];
// Recompute the output
//memcpy(newPixels, pixels, size*3);
ComputeOutput(newPixels, tol1, tol2, useWhite, useBackground);
HBITMAP bitmap = CreateDIBitmap(GetDC(hwnd), &bi.bmiHeader, CBM_INIT, newPixels, &bi, DIB_RGB_COLORS);
delete newPixels;
return bitmap;
}
and
void Frame::UpdateBitmap(HWND hwnd, HBITMAP bitmap, int tol1, int tol2, bool useWhite, bool useBackground)
{
ZeroMemory(&bi.bmiHeader, sizeof(BITMAPINFOHEADER));
bi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
HDC hdc = GetDC(hwnd);
if(!GetDIBits(hdc, bitmap, 0, bi.bmiHeader.biHeight, NULL, &bi, DIB_RGB_COLORS))
MessageBox(NULL, "Can't get base image info!", "Error!", MB_ICONEXCLAMATION | MB_OK);
// Allocate memory for bitmap bits
int size = height * width;
Pixel* newPixels = new Pixel[size];
// Recompute the output
//memcpy(newPixels, pixels, size*3);
ComputeOutput(newPixels, tol1, tol2, useWhite, useBackground);
// Push back to windows
if(!SetDIBits(hdc, bitmap, 0, bi.bmiHeader.biHeight, newPixels, &bi, DIB_RGB_COLORS))
MessageBox(NULL, "Can't set pixel data!", "Error!", MB_ICONEXCLAMATION | MB_OK);
delete newPixels;
}
where the Pixel struct is just this:
struct Pixel { unsigned char b, g, r; };
Why does my update function not work. I always get the MessageBox for "Can't set pixel data!" I used code similar to this when I was loading in the original bitmap from file, then editing the data, but now when I manually create it, it doesn't work.

Answer 1

to write to a bitmap you have to Select it into a DeviceContext:
hdcMemDC = CreateCompatibleDC(hdc);
... // adjust hdcMem to right sizes (see MSDN : Capturing an Image)
SelectObject(hdcMemDC,bitmap);
... // your Bitmap drawing
BitBlt( hdcMemDC, ... ) // to copy to hdc.
ReleaseDC(hWnd, hdcMemDC);
Also CreateDIBitmap( GetDC(hwnd),... ) needs a ReleaseDC(hWnd ,??? );
You must assign GetDC(...) to a variable so you can release it.

Answer 2

Please, create your bitmap with CreateDIBSection.
This creates a HBITMAP to a bitmap and returns a pointer to a usermode accessible buffer. In the format you specify.