Resolution

[Patrick Dillon]

I need to tile an image for a wall mural that will be 48" x 30". Could someone explain the resolution I would need for a raster image. Its a
photograph that was scanned at 600 dpi so it may not be a high enough
resolution. Thanks

[Matty McQuilkin]

If you have scanned the image at 600 dip that will be ok, just make sure that you engrave the image at the same dip as it was scanned

[Rodne Gold]

You need to divide the amount of pixels by the inches of output.
Lets say you have an image 4800 pixels by 3000 pixels (DPI is not at all relevant) you will only have 100 pixels per inch (PPI) to work with.
Large graphics should not be engraved at high dpi or ppi as the viewing distance is large too (at least the diagonal of the graphic)
A pixel too is not equivalent to a dot the laser can engrave , a pixel with shades of grey is actually a matrix of dots which simulate the shade via spacing.
In reality , I would use a very low DPI , not more than 100 or so for this and as the amount of pixels you have to work with drops if the pic was small , the DPI also needs to drop. A whole lot depends on the material , how the laser reacts with it , spot size of the beam and so on.
Your BEST bet is to use Photograv on this , resize the image to actual size (48x30) without any interpolation , export it as a 8 bit greyscale BMP and run it thru photograv using your lasers parameters and their material paramaters. engrave the result using your normal DPI , the result is NOT a 1/2 tone image , it is a single colour bitmap image that has a whole lot of different operations applied (which you could do yourself in something like photoshop) and is totally optimised for your application , makes life real easy.

[Al Curatolo]

My personal opinion is that photograv sucks for this type of work...

I'm guessing that you want to do something like the River Wall Mural here:
http://www.handsonimaging.com/Gallery.htm

The original picture came in as a 2k digital image. I brought it into photoshop and resized it to 60"x48" at 100dpi. Then I darkened the image as things tend to wash out when you're not using photograv. Convert to greayscale and save. Brought it into Corel and "printed" at 200dpi. By using the lower res photo (100dpi) and printing at a higher resolution (200dpi) you get a nice dithering of the grayscale and pick up much more detail than with photograv and it doesn't have that wierdo photograv look to it.

Al

[Barbara Buhse]

Al,
Although I'm not doing a mural, I am curious if your process can be applied to "normal" size engravings (I engrave mostly on wood).
Does printing in a dpi larger than the original photo replace the photograv process? Your mural was beautiful, but if I were to try a picture without photograv, it usually comes out blurry. Your mural did not. ???

Barbara

[Al Curatolo]

Sorry it's taken so long to respond, I only chime in here about once a month.

Anyway...The only thing I use photograv for is smaller items (less than 12" square) if I'm working on granite I never go higher than 200 dpi. Marble I usually cut at 300 dpi and glass at 300 dpi as well.

In all cases I find that photos have to be darkened by at least 10% if you're not using photograv because things tend to look overexposed/washed out, you loose all the detail. I'll sometimes print at a larger dpi than the photo because it allows Corel to do better dither to simulate the grey's of the original image, I find that it helps a poor photograph. I don't downsample things to try to get a better image, that just loses detail. If I have to upsample I try to do as little upsampling as possible, hence the image at 100dpi, it's really the lowest workable rez for an image. Granite is such a coarse grained stone that you really don't lose much detail by going low-rez to those giant sizes plus people generally don't stand 2" away from a big mural so you don't need as much detail.

Works great for granite but results are less spectacular for marble or other fine grained materials.

Al