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How to change DPI

Jan 29, 2011, 12:20
Post: #1
How to change DPI
Hi I am trying to make a a graph (in tiff format) for a journal which demands that the "line art must have a resolution of at least 1200 dpi". When i googled i found instructions to click "image' -> scale image and then change the x resolution and y resolution . But the resolutions are not in dpi but in pixels/inch. I read that they are different and 300 ppi is roughly 150 dpi. So what should i do now ? How much x and y resolution should i keep ? Thank you
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Jan 29, 2011, 13:45
Post: #2
RE: How to change DPI
DPI (dots per inch) and PPI (pixels per inch) are two different things. "dots" are what your printer creates on the paper. It will use several dots of the fundamental colors to produce one pixel of the right color. Inkjet printers nowadays print at 2400DPI or more (very often their vertical and horizontal DPI differ) while the human eye won't see any difference past 600PPI. So I'll assume that your publisher really requires 1200PPI (to have some rework margin).

For the effect of PPI settings in Gimp, see my answers in this thread: http://gimpforums.com/thread-resizing-an-image

The net conclusion is that if you intend your final graph to be 10"x10", your publisher wants a 12000x12000 pixels picture.

Note that if your picture is line art, using a vector-graphics-based software like InkScape may be a better solution (unlike bitmap graphics, vector graphics are scalable without loss in resolution).

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Jan 29, 2011, 20:46
Post: #3
RE: How to change DPI
Hmmm. Wondering if this is related to your question with Impress. If this is the case, the published could be very satisfied with the EPS export from Impress.

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Feb 1, 2011, 04:38
Post: #4
RE: How to change DPI
Thank you Ofnuts. It is related to my question on Impress. But I have the graph in Calc and not in Impress. Sorry for the confusion. How do i export the graph as EPS from Calc.

I tried to open the Tiff file (which I had made in Paint) in Gimp and save as EPS. But I want a file with the final size which is half the size of A4 paper. ie about (3"x8"). So I tried to open a new file in Gimp with 36000 x 96000 pixels as suggested in your thread . But Gimp said the file size would be in GBs ! . What could be wrong ?
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Feb 1, 2011, 04:49
Post: #5
RE: How to change DPI
Also, does the graph made in Calc (a bar diagram) qualify as line art ? In that case i will try the other program InkScape which you suggested.
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Feb 1, 2011, 07:33 (This post was last modified: Feb 1, 2011 07:37 by ofnuts.)
Post: #6
RE: How to change DPI
(Feb 1, 2011 04:38)deepla Wrote:  Thank you Ofnuts. It is related to my question on Impress. But I have the graph in Calc and not in Impress. Sorry for the confusion. How do i export the graph as EPS from Calc.
This is something to ask in the OpenOffice/LibreOffice forums. When you "Export as" HTML, the graphs are saved as JPG or PNG (but a bit small for print use, although I've not tried all options).
(Feb 1, 2011 04:38)deepla Wrote:  I tried to open the Tiff file (which I had made in Paint) in Gimp and save as EPS. But I want a file with the final size which is half the size of A4 paper. ie about (3"x8"). So I tried to open a new file in Gimp with 36000 x 96000 pixels as suggested in your thread . But Gimp said the file size would be in GBs ! . What could be wrong ?
Your math... 3"x8" at 1200 PPI is 3600x9600 pixels, not 36000x96000 (100 times more pixels....)

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Feb 1, 2011, 07:47
Post: #7
RE: How to change DPI
(Feb 1, 2011 04:49)deepla Wrote:  Also, does the graph made in Calc (a bar diagram) qualify as line art ? In that case i will try the other program InkScape which you suggested.
Yes, that's line art in my book.

Btw, just found how you can export EPS: in Calc, select the graph. Copy to Clipboard. Open Impress, paste. Then export as EPS.

At that point there is not much purpose for InkScape or Gimp, you can do everything in Impress and export as EPS for the publisher.

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Feb 1, 2011, 10:02
Post: #8
RE: How to change DPI
Quote:Your math... 3"x8" at 1200 PPI is 3600x9600 pixels, not 36000x96000 (100 times more pixels....)

Thanks for being patient . This is really dumb from my side. Any way I am going to try out what you suggested. Thanks again.
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Nov 21, 2011, 20:36 (This post was last modified: Nov 21, 2011 20:36 by ofnuts.)
Post: #9
RE: How to change DPI
36000x96000=3456 megapixels. And in Gimp, one pixel is 4 bytes, so we are looking at 15 Gigabytes. But at 300DPI, this is a 120" by 320" printout...

If you want A4 size at 300PPI (about 8.5"x11.5") this is only 2550x3450 pixels, a bit under 9 megapixels, smaller than the pictures from any recent digital camera. At 600DPI this 4 times as many pixels; getting big, but still manageable.

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