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Discriminability measures for predicting readability of text on textured backgrounds

Lauren F. V. Scharff, Alyson L. Hill, and Albert J. Ahumada

Open Access Open Access

Abstract

Several discriminability measures were examined for their ability to predict reading search times for three levels of text contrast and a range of backgrounds (plain, a periodic texture, and four spatial-frequency-filtered textures created from the periodic texture). Search times indicate that these background variations only affect readability when the text contrast is low, and that spatial frequency content of the background affects readability. These results were not well predicted by the single variables of text contrast (Spearman rank correlation=-0.64) and background RMS contrast (0.08), but a global masking index and a spatial-frequency-selective masking index led to better predictions (-0.84 and -0.81, respectively).

©2000 Optical Society of America

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Figures (4)
Fig. 1.
Fig. 1. Six texture samples used to create the backgrounds for the experiment. The lower right pattern shows the original, unfiltered texture. Starting from the top left are the band-pass filtered versions named: Band 1 (0.1875-0.375 cpl), Band 2 (0.375-0.75 cpl), Band 3 (0.75–1.5 cpl), Band 4 (1.5–3 cpl), and a plain texture of the same mean luminance.

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Fig. 2.
Fig. 2. Search times (sec) for all conditions that were readable. The effect of spatial frequency filtering was greatest when the text contrast was low. The text was not readable for the low text contrast using the unfiltered background (all frequencies) and the Band 3 filtered background. Vertical bars represent standard errors of the means.

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Figure 3.
Figure 3. (a,b). Mean search times for each background plotted against text contrast (left) and background RMS contrast (right) and the two rank correlation coefficients (non-parametric). Numbers/letters to the side of each symbol indicate the frequency band (1–4), A (all frequencies), or P (plain texture).

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Fig. 4.
Fig. 4. (a,b). Mean search times for each background plotted against the global masking model equivalent contrast (left) and the frequency-selective model d’ index (right), and the two rank correlation coefficients (non-parametric). Numbers/letters to the side of each symbol are as indicated in Figure 3.

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Tables (1)

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Table 1. Parameter values.

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Equations (5)

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C T = ( L B − L T ) ⁄ L B ,
C RMS = L RMS ⁄ L B ,
L RMS = ( ( Σ ( L i − L B ) 2 ) ⁄ n ) 0.5 ,
d = s n T 0.5 C T ⁄ ( 1 + ( C RMS ⁄ C 2 ) 2 ) 0.5 ,
C M = C T ⁄ ( 1 + ( C RMS ⁄ C 2 ) 2 ) 0.5 .
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