Although interest in 3D displays that don't require special glasses is growing, developers of 3D displays still face many challenges, such as resolution loss in 3D and 3D/2D convertibility. We have solved these problems with the development of a novel pixel arrangement, called Horizontally Double-Density Pixels (HDDP). In this structure, two pictures (one for the left and one for the right eye) on two adjacent pixels form one square 3D pixel. This doubles the 3D resolution, making it as high as the 2D display and shows 3D images anywhere in 2D images with the same resolution. The display we have developed is lenticular lens based, is 2.5 diagonal inches in size, and has a 320x2 (RL) x 480x3 (RGB) resolution. As a 3D display, the horizontal and vertical resolutions are equal (235 ppi each). A user aptitude investigation was conducted for mobile phone applications. Several kinds of 3D pictures were presented to 345 participants, whose ages ranged from 16 to 54, and their preferences were surveyed. 3D pictures were categorized into three application areas: communication, entertainment, and useful tools in the mobile environment. Eighteen examples of possible 3D applications were listed and put to the vote. The results showed a high acceptance of and interest in this mobile 3D display.