u8g2_UserInterfaceSelectionList text centering is sometimes inconsistent

[dkonigsberg]

The u8g2_UserInterfaceSelectionList() function creates a menu where each line on the display is supposed to be centered.

When using this function, I frequently want lines to instead appear "fully justified" by using a monospace font and having all of them be the exact same number of characters.

I recently discovered a bug where these lines can end up out of position by one pixel:

(If you look closely at the "]" characters on the end of the first two option lines, it should be easy to see.)

I did some investigation, and I think I found the cause (and perhaps a simple fix).

The lines of text on the menu are drawn using the u8g2_DrawUTF8Line() function, which centers the text based on a simple calculation using u8g2_GetUTF8Width(). However, if the first character of the provided string has an X rendering offset, then the returned width will include this offset and do the centering calculation with it. Meanwhile, the rest of the drawing code likely isn't taking that offset into account.

Here's a useful code snippet to show this for reference:

size_t len1;
size_t len2;
u8g2_uint_t width1;
u8g2_uint_t width2;

u8g2_SetFont(&u8g2, u8g2_font_pxplusibmvga9_tf);

len1 = strlen(str1);
width1 = u8g2_GetUTF8Width(&u8g2, str1);

len2 = strlen(str2);
width2 = u8g2_GetUTF8Width(&u8g2, str2);

printf("len1 = %d, width1 = %d\n", len1, width1);
printf("len2 = %d, width2 = %d\n", len2, width2);

u8g2_ClearBuffer(&u8g2);
u8g2_DrawUTF8Line(&u8g2, 0, 10, 128, str1, 0, false);
u8g2_DrawUTF8Line(&u8g2, 0, 22, 128, str2, 0, false);
u8g2_SendBuffer(&u8g2);

You'll note that both strings are the same length (14 chars), return different widths (123 and 125 pixels), and the u8g2_DrawUTF8Line() function doesn't render them properly aligned with each other.

A simple fix for this problem is to make u8g2_DrawUTF8Line() subtract the X offset for a string before doing its centering calculation. This means to change the code at the top of this function (which is in u8g2_selection_list.c, around line 63) from:

  /* calculate the width of the string in pixel */
  str_width = u8g2_GetUTF8Width(u8g2, s);

to

  /* calculate the width of the string in pixel */
  str_width = u8g2_GetUTF8Width(u8g2, s) - u8g2_GetXOffsetUTF8(u8g2, s);

Its possible that there could be side-effects to this change, or that its relevance depends on whether U8G2_BALANCED_STR_WIDTH_CALCULATION is defined, so I'll let someone else take the lead in actually fixing it in the repo.

[olikraus]

Nice finding... However, I now added MUI (https://github.com/olikraus/u8g2/wiki/muimanual), which should replace the UserInterface functions...

[dkonigsberg]

I do need to look into the MUI stuff for the future, but right now I have a lot of code built on top of the UserInterface functions (either directly, or by re-implementation of portions). So even though MUI exists, I still need UserInterface to work as expected.

[olikraus]

Maybe you can sent the PR also to this repo. I could then include your suggestion.