Question

I’m working on making my application DPI sensitive using this MSDN guide where the technique for scaling uses X and Y logical pixels from a device context.

int _dpiX = 96, _pdiY = 96;   
HDC hdc = GetDC(NULL);
if (hdc)
{
    _dpiX = GetDeviceCaps(hdc, LOGPIXELSX);
    _dpiY = GetDeviceCaps(hdc, LOGPIXELSY);
    ReleaseDC(NULL, hdc);
}

Then you can scale X and Y coordinates using

int ScaleX(int x) { return MulDiv(x, _dpiX, 96); }
int ScaleY(int y) { return MulDiv(y, _dpiY, 96); }

Is there ever a situation where GetDeviceCaps(hdc, LOGPIXELSX) and GetDeviceCaps(hdc, LOGPIXELSY) would return different numbers for a monitor. The only device I’m really concerned about is a monitor so do I need to have separate ScaleX(int x) and ScaleY(int y) functions? Could I use just one Scale(int px) function? Would there be a downside to doing this?

Thanks in advance for the help.

Answer 1

It is theoretically possible, but I don’t know of any recent monitor that uses non-square pixels. There are so many advantages to square pixels, and so much existing software assumes square pixels, that it seems unlikely for a mainstream monitor to come out with a non-square pixel mode.

In many cases, if you did have a monitor with non-square pixels, you probably could apply a transform to make it appear as though it has square pixels (e.g., by setting the mapping mode).

That said, it is common for printers to have non-square device units. Many of them have a much higher resolution in one dimension than in the other. Some drivers make this resolution available to the caller. Others will make it appear as though it has square pixels. If you ever want to re-use your code for printing, I’d advise you to not conflate your horizontal and vertical scaling.