[Moderator note -- This topic was split from another (viewtopic.php?f=5&t=2037). The question about DPI and Export Bitmap became a long discussion and deserves its own topic.]


Thank you brynn.

Question: I come from a raster background. When I create a design in Inkscape and export it as .PNG (for my design to be printed to a print on demand company) would I only change the DPI to what I want (ex: 300 dpi)?

ClearInk

No, in Inkscape, changing the DPI in the Export Bitmap dialog only changes the size of the image, not the quality. To keep the exported image at the same size, keep the DPI at 90.

If you need to change the quality of the image for printing, using DPI, you'll have to use a raster graphics editor for that. (Since Inkscape is a vector graphics editor, the size of the "dots" is irrelevant.)

Also, I may as well say too, that you should use File menu > Export Bitmap to save PNG version of the image, if you want to retain any transparency. If you use Save As Cairo PNG, you will lose any transparency.

[Edit -- as it turns out, this wasn't entirely a correct answer. Please follow the discussion for clarification.]

brynn wrote:No, in Inkscape, changing the DPI in the Export Bitmap dialog only changes the size of the image, not the quality. To keep the exported image at the same size, keep the DPI at 90.

Can't you chnage the DPI in File -> Inkscape Preferences... the Bitmaps tab, to make it permanent?

I'm not sure, shawnhcorey. That's for Edit menu > Make a Bitmap Copy, which I haven't used much. I was talking about File menu > Export Bitmap.

I did a couple of tests. First, I changed that from default 90 to 300. The Export Bitmap dialog still shows 90. So Export Bitmap and Make a Bitmap Copy are apparently not 2 different ways to do the same thing.

Next, I made a star 100 x 100 px. With Bitmap Copy dpi at 90, the status bar reports the result is "Image 100 x 100". When I change dpi to 300, the status bar says "Image 333 x 336". The Selection tool control bar shows both results as 100 x 100 px. So I am indeed perplexed.

I thought that the info in the status bar gave the image dimensions. But in this case, it shows the image as 333 x 336, yet the bounding box indicates 100 x 100. Maybe you or someone can explain that?

(My general understanding is that Inkscape can't change the dpi for printing, and that a raster graphics program is required for that. I know that changing the DPI in Export Bitmap changes the size of the image. But I don't understand why it's called DPI, because usually when someone refers to DPI, it's in the way that raster programs use it, where it affects the quality of the image, not the size. I would truly appreciate if someone could explain )

brynn wrote:No, in Inkscape, changing the DPI in the Export Bitmap dialog only changes the size of the image, not the quality. To keep the exported image at the same size, keep the DPI at 90.

If you need to change the quality of the image for printing, using DPI, you'll have to use a raster graphics editor for that. (Since Inkscape is a vector graphics editor, the size of the "dots" is irrelevant.)

Also, I may as well say too, that you should use File menu > Export Bitmap to save PNG version of the image, if you want to retain any transparency. If you use Save As Cairo PNG, you will lose any transparency.

thanks for your kind sharing, it is really useful, thanks, can I add you to my friend list to ask more questions?

brynn wrote:(My general understanding is that Inkscape can't change the dpi for printing, and that a raster graphics program is required for that. I know that changing the DPI in Export Bitmap changes the size of the image. But I don't understand why it's called DPI, because usually when someone refers to DPI, it's in the way that raster programs use it, where it affects the quality of the image, not the size. I would truly appreciate if someone could explain )

Changing the dpi is the same as changing the size if the program opening it can't read the dpi info out of the png. All changing the dpi does is alter how many pixels it write per inch, so higher dpi = more pixels. If you then still print/display it at the same physical size there's more detail per inch available.

Take a 1"x1" picture, at 25dpi it would be 25 pixels by 25 pixels, increase the dpi to 90 and it would output at 90 x 90, it's still meant to be physically 1"x1" though, and so as log as you print it at that size it will have more detail, although the pixel size is bigger.

Think your confusion is that whatever your opening them in isn't understanding the dpi and so just shows a bigger pic.

OMG! Thanks Sim I can't believe someone's finally explaining this. I thought if I started stating it as a fact, someone would eventually tell me the real truth, lol. What you're saying does make a lot of sense, and it will help me a lot in explaining these questions where they're posted.

I never have even tried to look at an exported image, with anything but 90 dpi, with anything but Inkscape. The reason I think it changes the size is that as soon as you change the dpi from 90 to anything else, the Height and Width in Export Bitmap dialog immediately change. But I did make a test export at 180 dpi, and looked at it with Windows Photo Viewer, and it's shown at the larger size, not the original size. I would think that to be a fairly competent viewer. Maybe not much more than "competent' though. But in case it's a problem with that viewer, I imported back into Inkscape, and the import is also the larger size.

It does make sense that looking at it at the original size, it would be a better quality. But what if you're exporting a selection rather than a page. How does Inkscape know its original size, or tell the viewer the original size? I can see if it's the page size. Actually let me test the Page option....

No, using the Page option, the H and W change when dpi is changed, and the resulting image is the larger size, and not the original size in Windows Photo Viewer. And importing back to Inkscape, still the larger size. So it seems that Inkscape is not reading something right either.

I actually have not tried to print, because I don't have a printer connected to this computer. So I'm not sure if it's possible to tell the printer to print something at a different size than it actually is. Or maybe a professional printer can?

This is a pretty crucial issue. Isn’t it? I’m surprised there isn’t a more definitive answer available.

I can’t believe that people are only using Inkscape to make designs that are to be solely viewed on computer monitors (in which case the default 90 dpi would suffice… or, as has been pointed out, the dpi wouldn’t even matter). A search of the internet shows that a large number of others do seem to be struggling with this same issue (like here:- http://www.inkscapeforum.com/viewtopic.php?f=5&t=11864).

I think (and hope) Simarilius is right. I’ve spent a substantial amount of time over the last year drawing tshirt designs in Inkscape specifically to an A4 size and hoping that I would then be able to export them as a bitmap (png) at 300 dpi or save them as a pdf at 300 dpi (because I’ve been told this is the best resolution for tshirt and other designs) but still maintain the A4 size.

One of my examples is as follows:

I have a 20cm x 20cm (which is 7.874 inches x 7.874 inches) circular tshirt design. When I go to export this as a bitmap (png) at the default 90 dpi, the measurements in pixels is 708.66 pixels x 708.66 pixels (ie 90 x 7.874 inches).

When I go to export this as a bitmap (png) at my desired 300 dpi, the measurement in pixels is 2362.2 pixels x 2362.2 pixels (ie 300 x 7.874 inches). So while the pixel dimensions are larger the actual size in inches of my design is still 7.874 inches x 7.874 inches (or 20cm x 20cm).

When I open up this 300 dpi bitmap (png) in Microsoft Digital Image Suite the image size is 7.874 inches x 7.874 inches (20cm x 20 cm) the pixel size is 2362.2 pixels x 2362.2 pixels and the resolution is 300 dpi. So far so good… although I do admit that in my monitor the the design looks huge!

When I insert this 300 dpi bitmap (png) into a word document the image size is again 20cm x 20cm (7.874 inches x 7.874 inches).

Brynn, can you try opening a word document, inserting your image and then check the size?

Conversely, when I open it in Windows Picture Gallery, Microsoft Office Picture Manager or Microsoft Paint it just looks way too big!

I can’t tell what happens when I save it as a pdf at 300dpi because I only have adobe reader and that doesn’t seem to give me any information about size or resolution – which is a shame.

One day I plan to get the 300 dpi bitmap (png) and the 300 dpi pdf printed to see what happens - but I also don't have my computer connected to a printer.

May be all this is pretty obvious… and I’ve just made a dork out of myself.

It seems crucial to me. But then I understand so little anyway. I'm not sure if I still have the image I originally tested with. But if I can't find it, I could make another. However, I don't have Word on this computer. I'm using Open Office Writer here. Will that be good enough for whatever you have in mind?

Edit
Ok, I made a new test. A 100 x 100 px object, exported at 90 and also at 270 dpi. I imported both in to an OO Writer doc, and they both appear to be the same size, pretty close to 100 x 100 px, I'd say. However, the lower dpi object retains Inkscape's anti-aliasing, while the 270 dpi object show the individual pixels quite obviously! So if having a higher dpi is supposed to improve the quality, it surely does not appear to have in this case. Although I understand photos are a different story than a line drawing. I'm not sure if attachments are allowed in this subforum, and if they are, I'm not sure if ODT is accepted. But I'll try to attach it.

Well, unfortunately it says ODT not allowed. Actually that doesn't tell us whether attachments aren't allowed, or only ODT, because it would say they same thing either way Ok, here's a screenshot, to show the anti-aliasing issue:

One of the ancillary chunks of the PNG specification is pHYs, which specifies the physical attributes of the image. I don't know if Inkscape adds this chunk to exported images or if the viewing software (like Word) honour it but it could explain why images have the same physical size regardless of their pixel resolution. It seems to me that you have stumbled across this.

Brynn, with your example what is the actual size in inches/cms/mms of your two images? That’s the key question - whether the actual size of the image stays the same no matter whether you export as a bitmap (png) at 90 dpi or 300 dpi (irrespective of what happens to the pixel dimensions) - isn’t it? The only reason I asked you to try and insert your image into a Word document was that Word seems to be one of those programs which Similarius suggests can “read the dpi info out of the png” (see page one of this thread)… and I just thought that everyone has Word!!

I’ve also opened my 300 dpi bitmap (png) in GIMP and, as with Microsoft Digital Image Suite and Word, whereas the pixel size has increased to 2362.2 pixels x 2362.2 pixels the actual image size remains at 7.874 inches x 7.874 inches (20cm x 20 cm), ie the actual same size as the 90 dpi bitmap (png). Brynn, do you have GIMP?

In this thread here http://www.inkscapeforum.com/viewtopic.php?f=5&t=11864 people seem to suggest that you can export as a bitmap (png) at the 90 dpi default and then take the image to a raster editor, ie GIMP and change the resolution. Can you really do this? I would’ve thought that once you had exported it at a 90 dpi you couldn’t increase it – you’re stuck with 90 dpi.

On my point above about saving as a pdf at 300 dpi – my Inkscape drawings are all just vector drawings. They don’t involve any raster images. Am I correct in thinking that, because of this, when I save as a pdf it doesn’t matter what I set the dpi at?

The pdf format confuses me a bit. If I draw a vector image in Inkscape and save it as a pdf, what’s it doing? Is it just interpreting my vector image (which I assume is a SVG) in a manner which will enable it to display in various pdf readers (like Adobe)? If so, what happens when I give someone a pdf of my drawing to print out? Am I correct in thinking that it won’t print out my actual vector image but the pdf “interpretation” of this vector image? In which case I don't really have any control over the quality of the pdf image that will be printed out.

Answers on a postcard to New Zealand

PS I sound terribly demanding in this post don't I - maybe I should change my user name to Veruca Salt

Yes, shawncorey, it seems you've found a clue That info is at a technical level that a bit beyond my understanding. But I'm guessing that must be somewhat of a raging debate among the pros and experts -- should the creating software dictate the size or should the display software take care of it?

richie8,
Uuhhmm....100 px = 1.111 inches. When I look in Export Bitmap dialog, 90 DPI makes a 100 px image; 270 DPI makes a 300 px image. 300 px = 3.333 inches. When I open the 90 dpi PNG in Windows Picture Viewer, Properties shows 100 px (and looks to be about an inch). When I open the 270 dpi PNG, it shows 300 px (and looks to be about 3 inches). But when I inserted them into OO Writer, then both are the same size (1.12 inches). What I don't get, it why the quality is so much worse with the higher dpi?!

Yeah, when I bought this computer, I needed to cut some corners to afford it. I had heard a lot about the Open Office Suite, so I decided to try it, and save the cost of buying Word. I do have GIMP. Do you want me to open the exports in GIMP?

To my understanding, PDF can contain and display vector images (and/or raster). So if you save a vector image as PDF, it's still a vector image. But I don't know what happens as far as dpi. I think, in the Save As PDF dialog, there's an option to change the dpi....yes, "Resolution for rasterization (dpi)". But I guess if it's a vector image, dpi doesn't matter. So if you send a vector PDF to someone, they will be printing the same vector image as you drew it in Inkscape. ....unless you've used a raster type of filter....if you use some filters, they will be rasterized in the PDF, and then the dpi would matter.

I'm thinking about splitting out this discussion on dpi into another topic. Of course I'll make sure there's a link in place. But we are veering off topic I'll re-read through the thread, and if I can make a clean split, that's what I'll do

brynn wrote:But I'm guessing that must be somewhat of a raging debate among the pros and experts -- should the creating software dictate the size or should the display software take care of it?

No debate. Show the image at the size it was created at.

brynn wrote:What I don't get, it why the quality is so much worse with the higher dpi?!

Because the software is averaging 2 or more pixels into one. This gives the image a blurry look, as though the camera lens is not quite focused correctly.

brynn wrote:To my understanding, PDF can contain and display vector images (and/or raster). So if you save a vector image as PDF, it's still a vector image. But I don't know what happens as far as dpi.

PDF can handle vector images but I don't know if Inkscape exports vectors to PDF. Also, PDF can not handle gradients.

shawnhcorey wrote:

brynn wrote:What I don't get, it why the quality is so much worse with the higher dpi?!

Because the software is averaging 2 or more pixels into one. This gives the image a blurry look, as though the camera lens is not quite focused correctly.

No, the one on top, that looks slightly blurry, is the 90 dpi. It appears to retain Inkscape's anit-aliasing. But the one on the bottom is the 270 dpi, and has no anti-aliasing.

I thought the higher dpi made for a better quality image. But I think the 90 dpi one looks better in the ODT file (screenshot above).

I never said the 270 dpi one used anti-aliasing. Simply because it doesn't. I said it average two or more pixels causing blurring.

Well maybe our unique eyes are seeing different things. I don't see the 270 dpi one is blurry at all.

brynn wrote:Uuhhmm....100 px = 1.111 inches. When I look in Export Bitmap dialog, 90 DPI makes a 100 px image; 270 DPI makes a 300 px image. 300 px = 3.333 inches.

I don’t quite follow this. Can’t you just open your drawing in Inkscape, go up to the Tool Controls Bar and check what the width and height are in inches? I suspect that, if you export your drawing first as a bitmap at 90 dpi and then as a bitmap at 300 dpi, when you open the two drawings in GIMP they will both be the same physical size (width x height in inches) despite having different pixel sizes. Brynn, I think that if you do open them in GIMP (and compare their width x height in inches) then that might go some way to answering some of the issues raised in this thread.

As to which of the drawings in your screenshot is better quality, I think that both you and shawnhcorey just have a difference of opinion over the use of the term “blurry”.

On my monitor the top drawing (with the lower dpi) looks better. The bottom drawing (with the higher dpi) looks brighter but has a bit of a “shimmer”/”fuzziness” around the black lines (I suspect this is what shawnhcorey calls “blurry” – and is possibly this anti-aliasing factor which you talk about). As to why the bottom drawing (with the higher dpi) looks like this I’m going to subscribe to shawnhcorey’s theory, ie “Because the software is averaging 2 or more pixels into one. This gives the image a “blurry” look, as though the camera lens is not quite focused correctly.” The top drawing (with the lower dpi) does look a tad “dull” and a tad “smudgy”(possibly what Brynn calls “blurry”) – but is, as I say, better than the bottom drawing (with the higher dpi). But we’re not interested in how they look on screen are we - because surely the quality of the image is always going to depend upon what monitor, equipment, program we’re viewing them on/in – what we’re interested in is how they look when printed out, in particular whether they maintain the same physical size when you export them as a bitmap at a higher resolution.

On my pdf point: all my drawings are just basic vector drawings so I assume they don’t involve filters, gradients etc. In fact, I’m not even entirely sure what filters and gradients are – so I’m going to go away and swot up on them... and anti-aliasing as well, because I’m not 100% sure what that is either!!

Thanks for helping me try and get to the bottom of this DPI/resolution/export issue.

brynn wrote:

shawnhcorey wrote:

brynn wrote:What I don't get, it why the quality is so much worse with the higher dpi?!

Because the software is averaging 2 or more pixels into one. This gives the image a blurry look, as though the camera lens is not quite focused correctly.

No, the one on top, that looks slightly blurry, is the 90 dpi. It appears to retain Inkscape's anit-aliasing. But the one on the bottom is the 270 dpi, and has no anti-aliasing.

I thought the higher dpi made for a better quality image. But I think the 90 dpi one looks better in the ODT file (screenshot above).

The higher res is looking worse as the device your looking at it on cant cope with the resolution
if you print it on a printer capable of printing the higher resolution image it should look better, if you look at it on a screen that is only 90dpi, it wont as it cant render the extra info, and in fact ends up having to scale the bitmap down. a better demo might be to export one at 10dpi, you should still get an image that appears the same size, but the difference in amount of info should be fairly obvious.

the fundamental thing you need to get down is that pixel size is not a measure of physical size until you associate a dpi with it. a lower dpi will make a bitmap of a given size bigger, and the pixels more obvious.
most of the time we infer a dpi (the physical dpi of our screen generally) and so we get used to treating a pixel like a unit of size.

richie8 wrote:

brynn wrote:Uuhhmm....100 px = 1.111 inches. When I look in Export Bitmap dialog, 90 DPI makes a 100 px image; 270 DPI makes a 300 px image. 300 px = 3.333 inches.

I don’t quite follow this. Can’t you just open your drawing in Inkscape, go up to the Tool Controls Bar and check what the width and height are in inches? ....

Yeah, that's how I got the inch units. I wasn't sure why you were asking, so I included the dpi values for clarity.

Seeing the difference between how Windows Picture Viewer displays them and how they're displayed in OO Writer really helped me to understand. I don't know why Windows would use a viewer that doesn't recognize the image size info, but I guess MS cuts corners where they think they can. **

Thanks Simarilius. I understand much better now! And thanks shawncorey and richie8 too

**Can anyone recommend a better image viewer, preferably free? OH -- actually I think Irfanview has a handy viewer.... Hhm, no, it doesn't appear to recognize the size info either. It displays the higher res image at the larger size too! Maybe I should finally install my printer software into this computer. It gives me the HP Photo something or other. Well, if anyone can recommend a viewer that recognizes the size info, I'd appreciate it

brynn wrote: if anyone can recommend a viewer that recognizes the size info

I think there is still some confusion here. what 'size info' and what do you mean by 'recognize'? Are you looking for a viewer that has onscreen rulers? are you looking for a viewer that calculates zoom based on print resolution, if it is available? are you just looking for a viewer thaat can edit/add print resolution values? GIMP is a fabulous editor but 100% zoom is still shown as 1 pixel per screen pixel... FastStone will resize and resample quickly and easily and change print resolution but it's not at all clear that that's what you want. are you looking for some program that has no zoom feature?

I thought the reason that Windows Picture Viewer shows the larger dpi image at a larger size, was because the viewer couldn't read the size info (per shawncorey's and Sim's comments). Or is the problem that Inkscape doesn't export the size info? Well, it must export the size info because the correct size is shown when inserted into OOW doc. So I thought the viewer couldn't read it.

The size of both the 90 dpi and 270 dpi are 100 px (i.e. 1.1 inch square). I was asking for a viewer that would show the higher dpi image at the correct size.

brynn wrote:The size of both the 90 dpi and 270 dpi are 100 px (i.e. 1.1 inch square).

There is a serious contradiction in what you are saying here, can you see it?
if a 270 DPI object (i am going with the flow here and not using the 'correct' term PPI* which i would prefer) is 1.1 inch in a dimension it must be 297 pixels. if a 270 dpi object is 100 px in a dimension it must be 0.370 in. It simply cannot be both 100px AND 1.1 in.

The problem when you 'scale' an object - the application you are scaling it in may or may not tell you what the new pixel dimensions and print resolution are. It may not even know these things itself, leaving the final figuring out for your operating system's print routines.

In Inkscape, are you using the info bar and the control bar numbers interchangeably? They're not the same. is your control bar set to give dimensions in inches? Fine and dandy, but for bitmaps you have to multiply by 90 to get the output dimensions... Change it to px to see the difference when you scale a bitmap, or fiddle with the export dialog...

Not trying to pick on you here, and as soon as you get things worked out just say so and no more need be said...

*About PPI and DPI (also LPI and SPI even though you don't want to know about those ... yet ...): A nice short explanation and a nice long one but very clear if you read all five pages...

You like to do this, don't you druban? Just when I think I'm finally starting to understand something, you come around and say the exact opposite thing I'm just kidding. I really appreciate your help

Ok, then let's forget about using px as a unit of measure. The image is 1 inch square, and 270 dpi. Windows Picture Viewer shows it at 3 inches square. It shows a 1 inch square image at 90 dpi as the correct 1 inch. The size of both images is 1 inch square, but one of them is displayed 3 inches square. However, in the OO Writer doc, they're both displayed at 1 inch square.

How would you explain that?

I thought that the point of increasing dpi was to put more "dots" "pixels" whatever into an image, therefore making it a higher quality or resolution. If the 1 inch/270 dpi image is shown at 3 inches, then isn't that extra quality lost? Don't all those extra dots, pixels, whatever need to be crammed into the same size image, to make the quality better? I just want a viewer that can show the proper size in the proper resolution.

(I'm not sure what you mean by "info bar" and "control bar". Normally I use px units, but I set the Selection tool control bar units to inches for this test. I'm not sure what you mean about the "info bar". The status bar doesn't give size info....OH, well if I had imported a raster image, it would show the size. So no, I'm not using the status bar at all, because this is originally a vector image, exported to raster.)

I'll look at the links you gave, and let you know if it helps. Thanks again

Just when I think I'm finally starting to understand something...

... the first step on the road to understanding is knowing that you don't understand! Look at me - I think i know it all, so probably I'm completely clueless. You're further ahead!

Anyway, to go on, OO has a ruler across the top and you are seeing a page preview with a reference for size. WPV is just trying to show you all the pixel info. OO does what it does because when you have several elements being displayed together their relative sizes do matter. WPV is only showing you one item at a time so there is no relative size to refer to. Different purposes, different methods.
AFAICT any program I have that displays more than one element on the same page observes the embedded resolution. GIMP is a bit of a special case when it comes to placing imported images on their own layers so let's not get into that just now...

Hhmmm.....well I do understand what you mean. But in some ways, it doesn't make sense that an image viewer would ignore the physical size of the image. Does that mean the WPV cannot display different resolutions? I guess not. So if someone is concerned about making or working with a high resolution raster image, they pretty much have to view it within a graphics program? Or well.....no....unless Irfanview is a special case. It can't display the higher resolution either.

I didn't use the ruler in OO to know the size. I looked at the properties (because I was trying to learn if an image could have size properties correct, yet display a different size).

Well anyway, thanks again