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How would I go about making lines in a jpg thicker?

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I'm imagining some type of 'grow' function that selects a color and expands it.  I have a black and white jpg with a simple design using thin black lines - I'd like to make those lines thicker. 

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Depends on the amount of lines, if straight ones or not, possibly overlapping etc. There are generally several possibilities to do this, though for many lines I would trace them into vectors instead, expand their contour/stroke and then export it back as a bitmap/raster JPG format.


☛ Affinity Designer 1.6.1 ◆ Affinity Photo 1.6.7 ◆ OSX El Capitan

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There will be lots of ways to do this, this one is based on using the selection menu


Select > Tonal Range > Select Shadows
Select > Grow / Shrink

add 3px
Select > Smooth
add 4px

Use paint brush (set to black) to paint in the selection

1. Vary the above pixel values depending on how much you want to increase the thickness of the lines

2. If using Designer switch to the Pixel Persona first


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Simplest trick would be to use Levels or Curves command and drag black point towards middle. All antialiased grey pixels will get darker, some of them all to way to black.

However, example image is rather weakly antialiased so it may not help much here.

Right way to do this would be to trace the image to vectors. That is what we do a lot in our publishing jobs.

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I would also say vectorizing is a better way to go. Expanding the bitmap will inevitably make for jaggy lines. But Photo will do a decent quick and dirty. Use the magic wand, select the sine form, and expand the selection a few px. Edit/Fill/Secondary color.


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