figure 4. Making a drawing. with the drawing page folded in half,
the artist makes a freehand drawing while referring to the prompt
page (left). the completed drawing page (right) contains a freehand
drawing and a registered drawing.
1024 × 768 pixel binary image containing a single pixel wide
approximation of the human artist’s lines. While this procedure takes up to twice as long as a single drawing (e.g., it
requires the artist to draw every line twice), it achieves a nice
balance between the design trade-offs: the line drawings are
composed in a freehand manner familiar to artists, while
the intended locations of every line on the 3D surface can be
inferred with great accuracy.
2. 4. Data collection
This line drawing and registration procedure was repeated
for 29 artists, most of them were enrolled in one of four art
classes (two composed of middle and high school students,
one of adult evening students, and another of college students). Two of the participants were professional artists.
Each artist completed up to 12 prompts. Every participant
completed a questionnaire listing his/her gender, age, and
number of years of art training. In all, there were 22 females
and 7 males. The ages ranged from 10 to 54 years, with an
average of 22; and the participants reported an average of
6 years of art training (this number should be taken with a
grain of salt, as some participants reported only training at
the college level, while others reported all art classes).
Every artist was provided a folder with 1 page of instructions, 12 prompt pages, and 12 corresponding drawing
pages (one for each model). The folders were arranged such
that no artist could draw the same model more than once,
and prompts for models, viewpoints, and lighting conditions were arranged in shuffled order to reduce effects of
training on our analysis.
The artists were given brief verbal instructions (“draw
lines that convey shape” and “be sure to copy every line from
your freehand drawing over the faded image below”) and
then told to complete line drawings at their own pace for as
long as they had time. Most of the art classes were scheduled
for a 2-h block, and each line drawing took 10–15min, on
average (with time split around two-thirds for drawing freehand and one-third for tracing lines over the faded image).
Each participant completed an average of 7. 5 drawings—
only one participant (a professional artist) completed all
12 available in his folder.
In all, 208 line-drawing images were collected. Generally
speaking, the artists followed the directions well, produced
figure 5. Example drawings. three drawings of the screwdriver
model from the same view (a–c), and the average of 14 drawings of
the same view (d).
line drawings that convey shape effectively, and were careful
when tracing lines over the faded image (some example line
drawings are shown in Figure 5). However, in some cases,
the artists clearly were not careful in the registration step,
failing to follow even the exterior outline of the shape. Since
accurate registration of lines to image features is essential
for meaningful results, we cull these tracings from our analysis. To do this in an unbiased way, we assume that inclusion
of the exterior outline is common to all human line drawings and eliminate from our data set any drawings where
less than 90% of the exterior is within 1mm of a human-drawn line. The remaining 170 line drawings form the basis
for our analysis.
We can investigate a number of questions by comparing the
captured line drawings and the synthetic images provided
as prompts to the artists. We ask not only how artists’ drawings overlap with one another but also how they overlap with
lines generated by CG algorithms and how they can be predicted from local properties of the underlying surface and
3. 1. how similar are the artists’ drawings?
The first and most basic analysis we perform is to measure the similarity between artists’ drawings of the same
prompts. We can show consistency between artists qualitatively by superposing drawings on top of each other and
visualizing how much they overlap. For example, Figure 6a
shows each artist’s drawing in a separate color. In this example, the artists agree very closely with each other in most
areas, especially along obvious features such as boundaries and occluding contours, but differ in exactly where they
place lines in the right part of the rockerarm.
In order to quantify consistency, we compute a histogram of pairwise distances between artists’ drawings
(Figure 6b). For every pixel in every drawing, we record the
distance to the closest pixel in every other drawing of the
same prompt and then observe how often these distances
lie within the tolerance of the tracing procedure ( 1 mm).
Across all prompts, approximately 75% of human drawing