1. A display processing method for displaying a shape of an object including a trimmed surface generated based on a base surface, the display processing method comprising: dividing, by using a computer, the base surface into a plurality of triangles; generating, by using the computer, a loop polygon for loops including an outer loop for defining an outer shape of the trimmed surface and an inner loop provided, in the outer loop, depending on the shape of the object; identifying, by using the computer, one or plural sight-line single-valued areas, each of which satisfies a condition that mapping onto a projection surface is bijection, includes plural triangles, and is an area on the base surface, based on a positional relationship between a viewpoint and the base surface; generating, by using the computer, for each of the one or plural sight-line single-valued areas, mask data that is data for controlling whether or not display is performed for each pixel from the sight-line single-valued area and the loop polygon; generating, by using the computer, image data on the projection surface for each of the plurality of triangles; and performing, by using the computer; a drawing processing from the mask data and the image data.