Viewport zoom scale factor

To get the viewport zoom scale factor, you can query for the ‘CustomScale’ property on the paper space viewport entity. Alternatively, get the viewport paperspace height, which is DXF group code 41. The model space viewport height is contained in the viewport’s extended entity data and is the 2nd DXF group code1040. Extended entity data can be extracted into ‘resbuf’ result buffers, however, ObjectARX provides AcDbViewport::viewHeight() that gets this value directly. Therefore, the viewport entity (AcDbViewport) zoom scale factor is calculated as :

group_41 / 2nd_group_1040 (or pspace_height / mspace_height).

The following code fragments demonstrates how to calculate the zoom scale factor for an AcDbViewport entity:

(defun c:ZoomScaleFactor (/ a_app a_doc obj a_pvp msp_ht )
    (vl-load-com)
    (setq    a_app (vlax-get-acad-object)
            a_doc (vla-get-ActiveDocument a_app)
            obj   (vlax-ename->vla-object (car (entsel "Select a pviewport entity: ")))
    )
    (vla-put-mspace a_doc :vlax-true)
 
    (setq    a_pvp  (vla-get-ActivePViewport a_doc)
            pvp_ht (vla-get-height a_pvp)
            msp_ht (GETVAR "VIEWSIZE")
            zsf   (vla-get-customscale a_pvp)
    )
 
    (princ (strcat "pvp height : " (rtos pvp_ht)))
    (princ (strcat "msp height : " (rtos msp_ht)))
 
    (vla-put-mspace a_doc :vlax-false)
 
    (print (strcat "Viewport Zoom scale factor = " (rtos (/ pvp_ht msp_ht) 2 2)))
 
    (print (strcat "Viewport Zoom scale factor = " (rtos zsf)))
 
    (princ)
)

Here is the ObjectARX equivalent of the same code :

ads_name ename;
ads_point pt;
int rc = acedEntSel(_T("nSelect Viewport entity "), ename, pt);
if(rc != RTNORM)
{
    acutPrintf(_T("nError selecting entity "));
    return;
}
 
AcDbObjectId entId;
Acad::ErrorStatus es = acdbGetObjectId(entId, ename);
 
AcDbEntity* pEnt;
es = acdbOpenObject(pEnt, entId, AcDb::kForRead);
AcDbViewport* pVPEnt = AcDbViewport::cast(pEnt);
if(!pVPEnt)
{
    acutPrintf(_T("nEntity is not a Viewport entity "));
    pEnt->close();
    return;
}
 
// Get the paper space viewport's height
// DXF group code 41
double psht = pVPEnt->height();
 
// Get the model space viewport's height
// Extended Entity Data 2nd 1040 DXF group code under 'ACAD'
double msht = pVPEnt->viewHeight();
 
// Calculate the viewport entity zoom factor
// The ZOOM factor is calculated with the following formula:
// group_41 / 2nd_group_1040 (or pspace_height / mspace_height).
double xpsf = psht / msht;
 
acutPrintf(_T("nViewport Zoom scale factor = %.2lf "), xpsf);
 
// OR
 
acutPrintf(_T("nViewport Zoom scale factor = %.2lf"), pVPEnt->customScale());
 
pVPEnt->close();