Why the measurements of LBearing & RBearing in zero-width glyph are different in Fontforge??
Or is this the same for all font creating software?
LB have negative number while RB has positive number in measurements.
Both points ( left & right most ) are in negative territory of horizontal baseline.
Can someone explain me why and how these are measured? Thanks
Comments
At other times, a designer may want to optically centre the outline on the zero-width, such that the mark anchor sits on that zero-width.
And if you don't place the zero-width glyph( below base ) in the right place you can use mark to Base ( GPOS ) anchor point to position the glyph under preceding glyph, right?. In your image, is "center" is the anchor point of zero-width glyph ( seem like above base )??? Thanks
[Note, I am using FontLab 7, so am not sure how these comments relate to how FontForge handles anchors.]
_____
Technically, if one is relying on anchors—either within the font tool to build composites or in layout via OpenType Layout GPOS anchor attachment positioning—the position of the anchor relative to the 0,0 coordinate of the glyph space is irrelevant: so long as the anchor position relative to the outline is where you want it to be for mark positioning purposes, it can be arbitrarily placed relative to 0,0.
That said, I greatly prefer to keep things tidy, and to have the mark anchor in a consistent x position relative to 0,0 across all combining mark glyphs. This means either on the zero-width, or offset a common distance to the left or right depending on script direction.
wait, is my logic correct? The space to the left of Left edge of the outline is positive and to the right is negative. But the space to the right of right edge is positive and left is negative. I played the mouse dragging the RBearing line to the left and the numbers changes to negative when it passes the right edge of the outline. How do you think?
This well explained it for me, thank you very much.
All coordinates in glyph space are relative to 0,0 and positive values are right and up, and negative values are left and down. That is regardless of the writing direction of the script for which the glyph is designed, so is as true for Arabic glyphs and for Latin ones.
Sidebearing values in editing tools tend to be defined relative to the outline, so if the outline is within the sidebearings the values will be positive, and if either outline edge extends beyond its sidebearing, then that value will be negative.