compare image data

[mbostock]

Instead of comparing the encoded images, parse the image data and compare the pixel values; this way we can allow a small (imperceptible) amount of visual error across architectures.

[Fil]

Excellent! I feel that the thresholds are very generous — how much error did you notice in practice?

[mbostock]

In the opacityLegendColor example, which has 256 pixels (1,024 RGBA values), I found:

• 47 values with any error
• 14 values with an error of at least 2
• 6 values with an error of at least 3
• 2 values with an error of at least 4
• no values with an error of more than 4
• a mean error of 0.0673828125
• 95% of pixels had no error
• 96% of pixels had 1 or less error
• 99% of pixels had 2 or less error

We could probably reduce the max error threshold slightly, but that is the noisiest since it can be affected by a single value. I think the other ones are already tight.

[tophtucker]

😻