In my case OBS displays captured window, but box with directshow stream is completely black(mask color from above).An application using a hardware overlay gets a completely separate section of video memory that belongs only to that application. Because nothing else uses it, the program never needs to waste resources in checking whether a given piece of the memory belongs to it, nor does it need to monitor whether the user moves the window and changes the location of the video memory. To get the image from the separate video memory to display in tandem with the remaining shared elements on the display, the graphical subsystem associates a certain attribute (for example, a particular color) as a "mask" for that overlay, which the graphics card understands to mean that it is to draw from the separate overlay buffer onto the screen. (This technique has become known as "chroma key".)