Making Bitmap Strokes

By Jessica Slater

1) Draw or paint your strokes in black on white paper. Mine were 1 cm thick and 3 - 4 cms long.

2) Scan in full colour and save in a lossless format such as .bmp, .tif, or .png. It should be 24 bit and I chose 200dpi. If your scanner has exposure adjustment, play with the shadow and highlight sliders until the paper appears completely white, and there is the right amount of solid black on the stroke. Later, the solid black will give the main area of colour to the stroke. I chose .png as the save format.

3) I used Paint Shop Pro to introduce the alpha channel, but if you're familiar with Photoshop I would love to know how to go about it in that program too.

In PSP, rotate the stroke until it is horizontal if you wish it to lie directly along your lines. Or leave it slightly skewed if you are aiming for a quirky off-centered look.

Now with the magic wand and a very low tolerance (I used 3), select the white area around the stroke. Select similar if necessary, then invert the selection. The marching ants will reveal if you need to clean up the edges a little. Choose a paintbrush with white and 100% hardness, opacity and density. The size will vary depending on the stroke. Make sure you have no background or foreground textures. Zoom in on the stroke (I found 3:1 good) and draw carefully around it. Deselect and repeat the selection process followed by the paintbrush touch-up until you are happy with the result.

Now with the stroke selected, it's a good time to work any contrast or filter magic on your stroke, or touch up the texture with a transparent black brush. Bear in mind that the end result will appear a lot lighter once it's transparent. Next making sure the stroke is still selected, Image>crop to selection, followed by Masks> New> From Image. A dialog will appear. Choose the following: Source window = this window. Create mask from... source luminance. And tick Invert mask data.

Save as .png but if a dialog comes up warning you that the image will be flattened and the alpha channel will be lost, use File> Export> PNG optimiser instead. On the various tabs, I have these options selected: 16.7 million colours. Alpha Channel transparency. Existing Image or Layer transparency... and that's it for PSP.

4) In Expression, File> Insert Bitmap. I found it best to do this one at a time. Some of the strokes wouldn't work when I attempted small batches and had to be redone. A dialog will come up enquiring about Alpha. I used unassociated alpha. If you didn't already invert mask data in PSP you may need to tick "Inverted" If you use associated alpha, the resulting stroke (.sks file) will be much smaller, maybe half the size, but it will look strange unless you're going to use it colourised. Once it's colourised there's no obvious difference.

You're on the home run now. Stroke> Define bitmap stroke and in the box give it a name and a size. I liked 150 for the size but I was making it up as I went along. You must choose Use Original Alpha though. Click OK and you should have your new stroke. Don't forget to share it with the rest of us!