>Psychtoolbox>PsychTests

DriftTexturePrecisionTest([highprecision=0][, verbose=0][, filterMode=1])

This test finds the minimum useful subtexel stepsize of the graphics hardwares
texture coordinate interpolators. The minimum stepsize is the finest
subpixel stepwidth that the gfx-hardware can resolve into an image in a
meaningful way, e.g., for slow subpixel scrolling, drifting gratings and
such.

It draws an alternating black-white pattern, reads it back, then draws
the pattern shifted by some small horizontal amount, reads it back and
compares the two drawn images. If they are different then the hardware
could resolve the subpixel increment into two different, properly shifted
images via bilinear texture interpolation. If they are the same then the
increment was too small to resolve for this hardware. The smallest
stepsize is found in multiple iterations that reduce the stepsize until
no difference is found anymore.

The minimum resolvable stepsize for an ATI Mobility Radeon X1600 (e.g.,
MacBook Pro) is 1/64 th of a pixel, which corresponds to 6 bits subpixel
accuracy.

If the optional flag ‘highprecision’ is set to 1 or 2, then floating
point textures are used, instead of integer textures with 8 bit
resolution. 1 selects 16bpc float textures, 2 selects 32bpc float
textures. A setting of 3 or 4 selects also 16bpc or 32bpc float textures,
but additionally a special PTB GLSL interpolation shader is used instead
of the hard-wired interpolator of your gfx chip. This is computationally
more demanding and therefore slower, but it should provide higher
precision and better resolution.