GIMP Version: 2.6.11
Operating System: Windows
OS Version: Windows 7
GIMP Experience: Basic Level

List any relevant plug-ins or scripts:
upsize

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Hi all,

As you can see I'm new to the forum, but I've been using GIMP for a few years (at beginner skill level).
Ok, so I want to take this image (1st pic) and resize it to print on a tshirt.
I downloaded the upsize plugin and upsized it.
Thing is, it's now got jagged lines all over the place (2nd pic)

I tried checking out some tutorials and forum threads but they dont give me the desired result.
I've tried:
1. selection to path, then use stroke
2. blur image, then use curves from the colours menu
3. using the magic selection tool with feathering

none of these got it right.
Can you please help me!

Try antialiasing. Those jagged lines should smooth out.

You mean Filters> Enhance > Antialias ?
Tried it now... didn't do anything to the image
Looks exactly like before.

Select your image > Alpha to selection > Select NONE > open your path dialogue click the stroke button at the bottom pick a width then click stroke.
If this doesn't work then we can try something else.

when you said "Alpha to selection" Did you mean "Selection to path"?
Cos if i follow the above exactly, there is no path object to work on

upscaling it always problematic
i think the solution is doing it with Inkscape:
vectorize and then scale

very nice picture btw !

How to did you scale up the pic?.

This is a redraw with inkscape, tracing the bitmap, save as *.svg and upload in Gimp

,i too agree that will be better use inkscape
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No she mean Layer menu/Transparency/alpha to selection that will work if there is alpha and transparency.

But i suppose she miss a step somewhere, between "alpha to selection" and "select none"

More the image you posted is a jpg it could not have transparency so i can't see how that could work ("alpha" indicate transparency if no transparency "alpha to selection" cannot work)

The best way to deal with a similar image would be convert to vector , vectors may be scaled at any size without quality loss ,but you would need a vector program (inkscape or illustrator )


With gimp you may try to

1 open the image go to Layers /transparency/add alpha channel
2 now get the color selector tool
3 chose white (click with the tool on a white spot in the image)
4 look well the selection you need all white selected
5 to really look the selection you should click that tiny rectangle at the lower left bottom of gimp image windows, that it is the Quickmask will show the selection as a red overlay on the image, it is more accurate because the "marching ant " display can't show partially selected pixels.
You may correct the quickmask with any painting tool, including eraser, and clicking again the quickmask toggle you correction will be applied to the selection

6 This is not strictly needed but will help to check for defects in your selection:
create a new layer below of a strong color (red, green), then on your image above do Edit/clear
You should now see clearly if are "white fringes" left or part of the black was eroded in case , correct
removing the white or filling the black

7 do again "alpha to selection" , "Selection to Path" and now you should have a clean path to enlarge and stroke as you wish

I am with the vector opinion, to print you probably need an image about 2000 pix wide With a vector you can scale up or down.

I did after a little Gimp processing with autotrace (svg attached)

Thanks to all for the input and for takign the time to respond!!
@molly : tried it and the result is quite decent, but nothing like the 'vectorise and scale' solution presented above.
@Esper : Thanks... when I search for pics to print on tshirts, the Line Drawing filter on Google Image Search is very handy. You cracked it first... 'vectorise and scale' is the way to go! 10/10!
@jamac4k : I used a plugin called upsize to scale it uphttp://registry.gimp.org/node/19582 Wow your result is impressive!
@PhotoComix : I will definitely give it a try, even if it is just to learn more about GIMP and the stroke feature
@rich2005 : Super result! Very impressive! Thank you.

WOW! You are all legendary! Busy installing Inkscape right now. New things to try/learn/break!

You may well give a try but better use the right tool for the right task, to enlarge THAT type of images most neat and simple solution is to convert in Vector,as example use Inkscape to convert in svg

In this case is so simple, only black on white, no gradient fill no photographic details ideal to be painlessly converted in a vector

That i suggested would be more useful to clean the outer edges of images that could not be so easily converted in Vectorial .

Also about the filter you used first i believe that was designed for more "photographic " image and the method to use to scale at best a photography is very different for that would be needed to scale 1bit image as was your, i mean there you should consider only Black OR White , one OR the other, with no intermediate tones , no gray in the middle, not even dithering to emulate gray (if scaled dithering easy create sort of moire effect ) ...that is just the contrary of what was designed to scale :

For photography or other type of graphic are millions of colours that need to be considered and that require a totally different approach, and in case that filter should work much better (but upscaling is always critical for image based on pixels,because the more you enlarge a pixel more will tend to look blocky and squared, and so oblique lines will tend to become more and more jaggy.

There are methods to minimize the visible defects but anyway scaling is always critical and scaling up the most critical