Since the inflate command can not know how much memory will be required to store the result of inflation, EVE writes back the last memory location used in a unconventional place. In the "EVE School" example from Matrix Orbital, you can see this in action on the last line of the LoadJPG() function in "process.c".
https://github.com/MatrixOrbital/EVE-School
Well, the question was about CMD_INFLATE, not CMD_LOADIMAGE.
And you are using CMD_GETPTR after CMD_LOADIMAGE in your example.
The Programming Guides up to 1.2 for BT81x state for CMD_GETPTR:
"Get the End Memory Address of Data Inflated by CMD_INFLATE"
For CMD_LOADIMAGE we have CMD_GETPROPS - although this is kind of useless.
The pointer returned is the start-address which we already fed as parameter into CMD_LOADIMAGE.
And width and height are not helpfull without the format of the decoded image.
I just gave it a quick try with a FT813 and CMD_GETPTR does not work for CMD_LOADIMAGE.
EVE_cmd_inflate(MEM_LOGO, logo, sizeof(logo));
test_pointer1 = EVE_cmd_getptr();
EVE_cmd_loadimage(MEM_PIC1, EVE_OPT_NODL, pic, sizeof(pic));
test_pointer2 = EVE_cmd_getptr();
EVE_cmd_dl(DL_COLOR_RGB | BLACK);
EVE_cmd_number(120, 300, 28, EVE_OPT_RIGHTX, test_pointer1);
EVE_cmd_number(120, 330, 28, EVE_OPT_RIGHTX, test_pointer2);
This prints the same number twice and it is the end-address of the deflated logo image.
Hmm, well, it actually is the first address after the deflated data.