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How to crop for equal borders using mogrify and the print shop borderless mode

Joined
Nov 1, 2007
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Boucherville, Québec, CANADA
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This can also read: How to crop without being cropped by the print shop borderless mode... ;)

I will use 4" x 6" at 3''ppi in this example but the method is the same for 5" x 7" and others.
  1. First determine how much pixels per inches you will export to.
  2. Photograph two rulers perpendicular to each other in such a way that:
    • They will be respectively parallel to the sides of the image but it is not critical as you can tweak later in Lightroom.
    • You can see the zero inches mark of both rulers with some or little extra.
    • For 4" x 6" prints you can see the the 4" mark of one ruler and the 6" mark of the other. For 5" x 7", replace 4" by 5" and 6" by 7" and so on...
    • The rulers will cross in between the above marks.
  3. Import this photo in Lightroom and select it.
  4. If the rulers are not perfectly aligned with the edges, Press "R" to go into crop mode and click the little ruler which is the straighten tool and click along the longest ruler in the photo and apply the crop to straighten.
  5. Go to crop and choose the wanted aspect ratio. 2:3 is the same as 4:6 as there are no inches involved until "Export".
  6. Crop in such a way that the zero mark of both rulers will be on an edge and the 4" mark of one ruler and the 6" mark of the other will match with the opposite edge. If you can't achieve this, check that the lens used make the less distorsion as possible. Best results can be achieved photographing away from the rulers and zooming or if not possible, including in the image more of the rulers than needed all around the minimum and maximum marks. If you have a lens able to photograph graph paper giving all straight lines and equal squares, you are in luck!
  7. Export this image to 4" x 6" at 3''ppi, resize to fit, uncheck don't enlarge if needed.
  8. Send this to your print shop for borderless printing.
When you get the physical image back, the first thing to check is that the paper used is exactly 4" x 6". If it is not, like for example 3.95" x 5.95", then you will have to substitute 3.95 to 4 and 5.95 to 6 in this whole process including exporting 3.95" x 5.95" in the future...

But lets say we were lucky and it came back 4" x 6". What we want to know is how many pixels were loose by the process to guarantee a 1''% covered print. As they enlarged your image to make it so, you will notice that by putting a real ruler over the printed ones, the printed inches will be slightly longer than real inches. Look to the printed rulers; for example, at the edge it begins at the 1/8" mark and at the opposite edge it ends at the 5_7/8" mark. So, substract 1/8" from 5_7/8" and you get 5¾". But in reality this 5¾" is exactly 6"! Dividing 6 by 5.75, you get a ratio of 1.'43.

So you lost ¼" x 1.'43 . As we sent a 3''ppi file this means that we loss 3''/4 or 75 pixels x 1.'43 = 78 pixels. Do the same calculation for the other perpendicular ruler. For this example, imagine we loss 62 pixels this time (2 x one tenth of an inch x 1.'43).

Lets call it Long Edge Loss or LEL and Short Edge Loss or SEL. If the loss is not the same at each end of the rulers, you will have to use four numbers for Top, Bottom, Left and Right as in Top Loss TL, Bottom Loss BL, LL and RL so you can offset accordingly in the last step.

We now want to compute for the ideal lossless 4" x 6". If you don't use LR/Mogrify to add borders, just crop to the measured paper size and add borders outside the image according to the calculated loss, less a tiny bit to avoid slim white edges.

I you use borders: Total them all in each axis. For example, you use 1' pixels white border all around, another 5 pixels black border over it and 6' white pixels top, left and right plus 135 at bottom to make room for text. The result is 15' pixels horizontally and 225 pixels vertically.

4" x 6" at 3''ppi is 12'' x 18'' post processed pixels. Why I say post processed? Just because we are not stuck cropping that number in the original. We can as easily take 24'' x 36'' or 8'' x 12'' from the original image. Lightroom will take care of the final pixel count at export time. What we want to know in the mean time is the aspect ratio we have to crop to such that by adding borders and taking the loss into account, we will get what we want.

So, in landscape mode, the image will be 18'' - 15' = 165' pixels wide and 12'' - 225 = 975 pixels tall. The portrait mode give 12'' - 15' = 1'5' pixels wide and 18'' - 225 = 1575 pixels tall. We then get a ratio of 975:165' ('.59) for landscape and 1'5':1575 ('.66) for portrait mode.

Knowing all the numbers, the receipe now is:
  1. For portrait crop with an 1'5':1575 (42:63) aspect ratio; For landscape crop with an 975:165' (39:66)
  2. Add a 1' pixels white border all around, another 5 pixels black border over it and 6' white pixels top, left and right plus 135 at bottom to make room for text. Put text in bottom margin.
  3. Add borders for LEL and SEL or TL, BL, LL and RL (see above) of the same color as the last one used.
  4. Export as you did for the rulers photo.
  5. Send to the print shop.
NOTE: Here the calculated crop ratio is different for portrait and landscape because we didn't add borders equally all around! If your borders are all the same around, you will get an identical aspect ratio for both portrait and landscape.

Be happy! :D
 
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