• Ensure the sphere targets are firmly fixed in the scene and cannot move/roll between photograph captures.
• Ensure the background is not the same color as the spheres themselves. The whole background does not need to contrast with the sphere but most of it should.
• The image of the spheres should not be overly large in the photos (up to 1/50th of the image size in pixels)
• Use sphere targets that are solid in color (i.e., no pattern on the sphere), and matte (so no bright specular reflections).
• Use sphere targets that cannot be easily deformed. For example, soft foam balls or squash balls may depress against a hard surface under the force of their own weight and warp, and therefore may not be a consistent shape across all images.
• Use sphere targets that have smooth surfaces. For example, a tennis ball has a coarse and jagged surface (from the fuzz) which results in noisy edges and therefore less precise marking.
• Use sphere targets with flat lighting to ensure that there are no specular reflections on the sphere target nor strong shading (half the sphere in shadow for example). A specular reflection can appear in different positions depending on the camera angle, and this can confuse the detector.
• Examine the red outline of the detected ellipse after the sphere has been marked. A problem with the sphere detection can often be seen in how this red outline matches the edge of the sphere image. In the case of a poor fit, delete the mark and re-mark it by ensuring that the region tightly encompasses the sphere with little extra background.