DG_GetSurfaceDesc (m_hSurface, &surfDesc);
DG_Lock(m_hSurface, 0, 0, unitRect.width, unitRect.height, 0);
x = horizonal value from 0 - 1920 (for 1080I)
y = vertical value from 0 - 1080 (for 1080I)
*((uint16_t *)((uint8_t *)surfDesc.lpsurface + (y*surfDesc.lPitch)) + x) = 0xffff;
// format is r5, g6, b5
edit: It was quite easy to switch it to a higher color format by just changing it from Format_16 to Format_32 when the surface was being created. It does not affect the video, it only applies to the overlay surfaces. But the SDK has Format_16 as the default, possibly inserted there by Ellion.