Document that -J is not required anymore for modern mode?

[Esteban82]

As I understand it, -J is an optional argument for all GMT modules. And from what I see in the following code, the default values are defined by these lines in src/gmt_inti.c:

gmt/src/gmt_init.c

Lines 16127 to 16148 in 82a5564

	if (got_J == false) { /* No history, apply default projection, but watch out for subplots and time-axis */
	unsigned int geo = gmtinit_is_region_geographic (GMT, *options, mod_name);
	if (geo) /* Max dimension lon/lat plot of 15 cm */
	snprintf (scl, GMT_LEN64, "Q15c+du");
	else { /* Use 15cm square but watch out for panels and time-axes */
	char *Tcode[2] = {"", "T"};
	unsigned int xy[2];
	xy[GMT_X] = (gmt_get_column_type (GMT, GMT_IN, GMT_X) == GMT_IS_ABSTIME);
	xy[GMT_Y] = (gmt_get_column_type (GMT, GMT_IN, GMT_Y) == GMT_IS_ABSTIME);
	if (P && (P->dir[GMT_X] == -1 || P->dir[GMT_Y] == -1)) /* Nonstandard Cartesian axes directions */
	snprintf (scl, GMT_LEN64, "X%gi%s/%gi%s", P->dir[GMT_X]*P->w, Tcode[xy[GMT_X]], P->dir[GMT_Y]*P->h, Tcode[xy[GMT_Y]]);
	else if (GMT->common.R.aspect == 1) /* Want the same scale in x and y and x should be 15 cm */
	snprintf (scl, GMT_LEN64, "X15c%s/0", Tcode[xy[GMT_X]]);
	else if (GMT->common.R.aspect == -1) /* Want the same scale in x and y and y should be 15 cm */
	snprintf (scl, GMT_LEN64, "X0/15c%s", Tcode[xy[GMT_X]]);
	else
	snprintf (scl, GMT_LEN64, "X15c%s/15c%s", Tcode[xy[GMT_X]], Tcode[xy[GMT_Y]]);
	}
	if ((opt = GMT_Make_Option (API, 'J', scl)) == NULL) return NULL; /* Failure to make option */
	if ((*options = GMT_Append_Option (API, opt, *options)) == NULL) return NULL; /* Failure to append option */
	GMT_Report (API, GMT_MSG_DEBUG, "Modern: Adding -J%s to options since there is no history available.\n", scl);
	}

However, I cannot find this mentioned in the documentation anywhere. Is this the case? Or is there any good reason not to do it?
I offer to document it if needed.

This is related #8694.

[seisman]

Is it still required in classic mode?

[Esteban82]

Is it still required in classic mode?

Yes, I think is required for classic mode. (I will update the name of the issue).

[joa-quim]

No, it's not required in classic neither.

[seisman]

No, it's not required in classic neither.

I believe it's still required in classic mode.

$ gmt grdimage @earth_relief_01d > map.ps
grdimage [ERROR]: Must specify a map projection with the -J option

[Esteban82]

Weird. For me, this command worked:
gmt grdimage @earth_relief_01d > map.ps

But I got this wrong:

gmt psbasemap -R0/15/0/15 -Baf > plot.ps
psbasemap [ERROR]: Must specify a map projection with the -J option

I am using GMT 6.6.0_82a5564_2025.02.18 on linux.

[joa-quim]

Dongdong must be using an older GMT version. When not provided grdimage sets a default -J

https://github.com/GenericMappingTools/gmt/blob/master/src/grdimage.c#L1705-L1707

[Esteban82]

https://github.com/GenericMappingTools/gmt/blob/master/src/grdimage.c#L1705-L1707

In that line, I see that Q15c+ is used while Q15c+du is used in gmt_init.c. Shouldn't be the same?

[seisman]

Dongdong must be using an older GMT version. When not provided grdimage sets a default -J

https://github.com/GenericMappingTools/gmt/blob/master/src/grdimage.c#L1705-L1707

You're right that I'm using GMT 6.5.0, but those lines were added in #8440.

[joa-quim]

+du Honestly I don't remember what that does and seems a very minor thing.