Due to hardware limitations, some devices restrict how wide a destination rectangle can be compared with the corresponding source rectangle. DirectDraw communicates these restrictions as stretch factors. A stretch factor is the ratio between the widths of the source and destination rectangles. If the driver provides information about stretch factors, it sets the DDCAPS_OVERLAYSTRETCH flag in the DDCAPS structure after you call the IDirectDraw4::GetCaps method. Note that stretch factors are reported multiplied by 1000, so a value of 1300 actually means 1.3 (and 750 would be 0.75).
Devices that do not impose limits on stretching or shrinking an overlay destination rectangle often report a minimum and maximum stretch factor of 0.
The minimum stretch factor tells you how much wider or narrower than the source rectangle the destination rectangle needs to be. If the minimum stretch factor is greater than 1000, then you must increase the destination rectangle's width by that ratio. For instance, if the driver reports 1300, you must make sure that the destination rectangle's width is at least 1.3 times the width of the source rectangle. Similarly, a minimum stretch factor less than 1000 indicates that the destination rectangle can be smaller than the source rectangle by that ratio.
The maximum stretch factor tells the maximum amount you can stretch the width of the destination rectangle. For example, if the maximum stretch factor is 2000, you can specify destination rectangles that are up to, but not wider than, twice the width of the source rectangle. If the maximum stretch factor is less than 1000, then you must shrink the width of the destination rectangle by that ratio to be able to display the overlay.
After stretching, the destination rectangle must conform to any size alignment restrictions the device might require. Therefore, it's a good idea to stretch the destination rectangle before adjusting it to be size aligned. For more information, see Boundary and Size Alignment.
Hardware does not require that you adjust the height of destination rectangles. You can increase a destination rectangle's height to preserve aspect ratio without negative effects.