The subpixel rendering technology increases the apparent resolution of an LCD/OLED screen by exploiting the physical property that a pixel is composed of RGB individually addressable subpixels. Due to the intrinsic intercoordination between apparent luminance resolution and color fringing artifact, a common method of subpixel image assessment is subjective evaluation. In this paper, we propose a unified subpixel image quality assessment metric called subpixel image assessment (SPA), which syncretizes local subpixel and global pixel features. Specifically, comprehensive subjective studies are conducted to acquire data of user preferences. Accordingly, a collection of low-level features is designed under extensive perceptual validation, capturing subpixel and pixel features, which reflect local details and global distance from the original image. With the features and their measurements as the basis, the SPA is obtained, which leads to a good representation of the subpixel image characteristics. The experimental results justify the effectiveness and the superiority of the SPA. The SPA is also successfully adopted in a variety of applications, including content adaptive sampling and metric-guided image compression.