The correct answer is OC. To create a linear perspective, a painter needs to establish a horizon line and a vanishing point.
Linear perspective is a technique used in art to create depth and three-dimensionality on a two-dimensional surface. It involves representing parallel lines as convergent towards a single point on the horizon, known as the vanishing point. The horizon line is a horizontal line that represents the viewer's eye level.
By establishing a horizon line and a vanishing point, the artist can create depth and distance in the painting. Objects closer to the viewer will appear larger and more detailed, while objects further away will appear smaller and less detailed. This manipulation of scale and proportion helps to create depth and spatial relationships within the artwork.
Option A (showing everything in the painting at the same altitude) is incorrect because it does not consider depth and perspective. Option B (making the background smaller than the foreground) is part of the linear perspective technique, but it alone does not encompass the full process. Option D (placing human subjects in both the background and the foreground) is unrelated to linear perspective and more focused on the painting's composition and subject matter.
Therefore, the correct answer is OC. To create a linear perspective, a painter needs to establish a horizon line and a vanishing point.