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Question about Boolean operation (Divide)


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Good day! As you can see, there are 2 objects in the attachment: a circle and a line. I was trying to divide the circle into 2 pieces using the divide Boolean operation but failed. I believe the circle is supposed to be divided into 2 pieces, but that's not happening. But why?

Divide.png

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The "why" is because Affinity boolean operations consider only the area enclosed by paths, so the path of a straight line, no matter how wide its stroke might be, encloses no area & the divide operation simply removes the line.

504850721_booleandivide.jpg.0dacc0f5321059da4bd80579081e5a25.jpg

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35 minutes ago, dutchshader said:

You would need to expand the stroke first, layer>expand stroke

The problem with using Expand Stroke for this is the result will not be two perfect ½ circles because the expanded shape removes a small amount from each of them.

There are several better alternatives if perfect ½ circles are desired. The one I like best is to convert the circle to a Pie shape, set the start angle to 90° & end angle to 270°, then duplicate that & click the Invert angles button.

All 3 1.10.8, & all 3 V2.4.1 Mac apps; 2020 iMac 27"; 3.8GHz i7, Radeon Pro 5700, 32GB RAM; macOS 10.15.7
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1.10.8; Affinity Designer 1.108; & all 3 V2 apps for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 15.7

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42 minutes ago, R C-R said:

The "why" is because Affinity boolean operations consider only the area enclosed by paths, so the path of a straight line, no matter how wide its stroke might be, encloses no area & the divide operation simply removes the line.

504850721_booleandivide.jpg.0dacc0f5321059da4bd80579081e5a25.jpg

Thanks for the explanation. Didn't know that divide command works in this way inside Affinity.

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9 minutes ago, haakoo said:

Or close it to become a shape

Close what? If you mean the line & it is straight, just closing it won't add any area to its path & the divide operation will still remove it.

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13 minutes ago, haakoo said:

Cut with a rectangle
Or close the line to have it become a shape
Better yet,use the pie shape
or the segment shape
or the donut shape
or the crescent shape
or convert to curves and delete the excess nodes

Roads,....Rome.
 

Many thanks for sharing the methods! I know there are several ways to make a perfect half circle. But that's not my concern. I just wanted to know why divide was not working as it's supposed to do.

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@haakoo, in your gif, you do a lot more than just closing the straight line! O.o

All 3 1.10.8, & all 3 V2.4.1 Mac apps; 2020 iMac 27"; 3.8GHz i7, Radeon Pro 5700, 32GB RAM; macOS 10.15.7
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13 hours ago, th_studio said:

Good day! As you can see, there are 2 objects in the attachment: a circle and a line. I was trying to divide the circle into 2 pieces using the divide Boolean operation but failed. I believe the circle is supposed to be divided into 2 pieces, but that's not happening. But why?

Divide.png

Specialized tools would make many workflows faster, easier and more fun.

This would in your simple use case. Draw a rectangle with snapping enabled to align to the centre of the circle easily:

image.thumb.png.5e1be126d71b7fd041235624935b0e22.png

Divide:

image.thumb.png.9878aeffb445848cc51fe42121f48136.png 

Delete the now unwanted rectangular shape (in more complex scenarios you would need to delete more than one shape):

image.png.034aa996cfcd73afefe59ef32d8011e7.png

Recolor the right side half circle:

image.png.b414fb977b91919d2fddde2b0e7b0938.png

Instead of the rectangle you can draw a closed shape with the pen tool instead in other use cases - the poor mans knife tool:

image.png.b864be03c5e296a9f31016f1c825eb13.png

BUT beware - the current bool operations are not great, they can easily result in hundreds of nodes. This example is from a more complex operation than the above:

image.thumb.png.b8ccaae164682ea64c4ed30e292eee97.png

AND even worse, zoom in and find errors like these (it should be a continuous unbroken line):

image.thumb.png.23d92ff83d1c33da21daa8af90cc2faa.png

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