You can run multiple filters to bring out the text. Depending on how unimportant the stone itself is, you can take it to some significant extremes. The ultimate key is to reduce noise, as that's largely what makes the text difficult to read (Michael's image is an example of that). Here are a few filters, applied one after the other and possibly to new layers, using Photoshop, showing the results:
High Pass:
Purpose: Reduces large-scale contrast, making it easier to bring out contrast of medium and smaller scale information (i.e. the lettering.)
Settings:
Reduce Noise:
Purpose: Reduces higher frequency information, again making it easier to bring out contrast of medium scale information. Reduce Color Noise eliminates color artifacts that make further processing difficult (color blotches end up getting enhanced with each successive filter or adjustment, making it difficult to see the text.)
Settings:
- Strength: 10
- Preserve Details: 10
- Reduce Color Noise: 100
- Sharpen Details: 0
Solarize:
Purpose: Reduce highlight tones, darken shaded midtones (i.e. eliminate highlights and more high frequency noise, increase contrast of text).
Technique: Copy original layer first, apply Solarize filter, then set layer blending mode to overlay.
Unsharp Mask:
Purpose: Increase local contrast (enhances text futher).
Technique: Select all, copy merged, paste as new layer before applying filter.
Settings:
- Amount: 90
- Radius: 100
- Threshold: 2
Contrast:
Purpose: Increase contrast (enhance text further.)
Technique: Add Brightness/Contrast Adjustment Layer
Settings:
Levels:
Purpose: Increase contrast (enhance text further.)
Technique: Add Levels Adjustment Layer
Settings:
- Black Point: 65
- Gray Point: 1.45
- White Point: 200
Finally, if you do want to preserve some of the nature of the original photo, you can copy-merged, paste the high contrast version as a layer above the original photo, then set the blending mode to color burn; paste another copy of the layer, and set the blending mode to darken:
That enhances the text, without otherwise messing with the original photo.