TextExtent(rp, string, count, textExtent)

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void textExtent(struct RastPort *, STRPTR, WORD,

This function determines a more complete metric of the space that a text string would render into than the TextLength() function.

rp: a pointer to the RastPort which describes where the text attributes reside.
string: the address of the string to determine the length of.
count: the number of characters in the string. If zero, there are no characters in the string.
textExtent: a structure to hold the result.

textExtent is filled in as follows:

te_Width: same as TextLength() result: the rp_cp_x advance that rendering this text would cause.
te_Height: same as tf_YSize. The height of the font.
te_Extent.MinX: the offset to the left side of the rectangle this would render into. Often zero.
te_Extent.MinY: same as -tf_Baseline. The offset from the baseline to the top of the rectangle this would render into.
te_Extent.MaxX: the offset of the left side of the rectangle this would render into. Often the same as te_Width-1.
te_Extent.MaxY: same as tf_YSize-tf_Baseline-1. The offset from the baseline to the bottom of the rectangle this would render into.

Before V47, this function suffered from an off-by-one error in case the input font was not fixed width.