Question

I’m in the process of converting a custom CAD software from GDI to Direct2D. I’m having issues when panning the drawing. What I would like to do is to create a bitmap that’s 3x as wide as the drawing window & 3x as high. Then, when the user begins to pan, I would render the part of the bitmap that should be visible.

Trouble is, it doesn’t appear as though you can have bitmaps bigger than your render target. Here’s approximately what I’ve done so far:

// Get the size of my drawing window.
RECT rect;
HDC hdc = GetDC(hwnd);
GetClipBox(hdc, &rect);

D2D1_SIZE_U size = D2D1::SizeU(
    rect.right - rect.left,
    rect.bottom - rect.top
);

// Now create the render target
ID2D1HwndRenderTarget *hwndRT = NULL;

hr = m_pD2DFactory->CreateHwndRenderTarget(
    D2D1::RenderTargetProperties(),
    D2D1::HwndRenderTargetProperties(hwnd, size),
    &hwndRT
    );

// And then the bitmap render target
ID2D1BitmapRenderTarget *bmpRT = NULL;
// We want it 3x as wide & 3x as high as the window
D2D1_SIZE_F size = D2D1::SizeF(
    (rect.right - rect.left) * 3, 
    (rect.bottom - rect.top) * 3
);
hr = originalTarget->CreateCompatibleRenderTarget(
        size,
        &bmpRT
        );

// Now I draw the geometry to my bitmap Render target...

// Then get the bitmap
ID2D1Bitmap* bmp = NULL;
bmpRT->GetBitmap(&bmp);

// From here I want to draw that bitmap on my hwndRenderTarget.
// Based on where my mouse was when I started panning, and where it is
// now, I can create a destination rectangle. It's the size of my
// drawing window
D2D1_RECT_U dest = D2D1::RectU(x1, y1, x1+size.width, y1+size.height);
hwndRT->DrawBitmap(
    bmp,
    NULL,
    1.0,
    D2D1_BITMAP_INTERPOLATION_MODE_LINEAR,
    dest
    );

So when I check the size of my bitmap, it checks out OK – it’s the size of my bitmap render target, not my hwnd render target. But if I set x1 & y1 to 0, it should draw the top left-hand corner of the bitmap (which is some geometry off the screen). But it just draws the top left-corner of what is on the screen.

Does anyone have any experience with this? How can I create a fairly large bitmap, and then render a portion of it on a smaller-sized render target? Since I’m panning, the render will take place on every mouse-move, so it has to be reasonably performant.

Answer 1

Sorry for the trouble – the code above works OK. The problem was elsewhere in my codebase.

Not sure how to mark this question as “Nevermind,” but that’s likely what it should be.