To examine how the visual system uses horizontal and vertical disparities and eye-position signals to estimate surface slant, we need to be able to manipulate those signals independently. We did so by using a haploscope. The sketch below is a plan view of this instrument. The observer's eyes are placed directly above the two pivot points. The CRTs and mirrors are rigidly attached to armatures that pivot about those points. Thus, the left eye views the left CRT in the left mirror and so forth. The virtual image of the Fixation Point is shown in the sketch. You can simulated conditions of the experiment by dragging the fixation point left and right or up and down. When you drag left and right, you're simulating a version eye movement; up and down simulates a vergence eye movement. Now consider given images on the left and right CRTs. Note that the retinal images created always remain the same for all positions of the two armatures. Thus, we can dissociate changes in retinal images from changes in eye position. That's key to the issues under examination and the experiments described next.

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the device

The picture below shows the major parts of the haploscope. The observer's head is stabilized by use of the bite bar. The mirrors are visible just above the bite bar. The two CRT monitors are visible to the left and right. The black planes with square apertures are used to reduce stray light. The PowerPC that drives both monitors is visible in the rear.

anti-aliasing and calibration

Because each haploscope arm, mirror, and CRT rotates about the center of rotation of the appropriate eye, we can change the headcentric azimuth of the stimulus without altering the retinal images. Despite the short viewing distance of 42.5 cm, the visual locations of the dots in our displays are specified to within ~30 seconds of arc. This high level of spatial precision is achieved by use of two procedures: anti-aliasing and spatial calibration.

Anti-aliasing allows dot (or line) placement at arbitrary positions between integral pixel locations. Each dot is composed of four adjacent pixels whose intensities are adjusted to place the center of brightness at the desired location taking into account the adjacent-pixel nonlinearity (Klein, Hu, & Carney, 1996).

Spatial calibration is accomplished as follows. A flat loom strung with fine nylon filament is placed in front of the appropriate CRT at the image plane; the loom creates a 36-by-26-cm grid with 1-cm spacing (see figure below). The loom is mounted 1 cm in front of the CRT to be calibrated, and is viewed in the mirror from the standard viewing position. An observer positions a dot to be coincident with specified intersections in the grid. When the dot is aligned with an intersection, its coordinates are recorded. The procedure is repeated for whichever dot was most poorly aligned until the interpolated positions of all the dots are correct (about 70 explicit settings). The loom is removed during experiments. Its former location defines a virtual plane onto which stimuli were projected. The calibrated area is 47 x 35 deg on each monitor. The spatial calibration achieves unmatched precision in the positioning of dots and lines on the two CRTs and creates stereoscopic percepts that are not affected by the standard distortions on CRTs.