When a community is set up with access to multiple IDP with SSO. The standard login page will disply buttons for each of the allowed IDP's and the user can make a choice on who to autenticate with.

How to get this functionality working on a custom button in visual force as a SP-initiated request. I am able to get a button working as a idp-initiated request .

With the standard page i have found the following it  makes a call to the salesforce SAML request generator '/saml/authn-request.jsp' with  parameters picked from SSO setup.  This in turn generates a SAML assertion make a call to the idp sending through the  saml request and relay state as a POST message.

I have not found anything that salesforce provides that can do this for custom visualforce. ie: a function that can pick values from SSO setup and make a call to the SAML generator. Something that can be invoked into the button click event for the button  . Is there something or is there any other way 

When a community is set up with access to multiple IDP with SSO. The standard login page will disply buttons for each of the allowed IDP's and the user can make a choice on who to autenticate with.

How to get this functionality working on a custom button in visual force as a SP-initiated request. I am able to get a button working as a idp-initiated request .

With the standard page i have found the following it  makes a call to the salesforce SAML request generator '/saml/authn-request.jsp' with  parameters picked from SSO setup.  This in turn generates a SAML assertion make a call to the idp sending through the  saml request and relay state as a POST message.

I have not found anything that salesforce provides that can do this for custom visualforce. ie: a function that can pick values from SSO setup and make a call to the SAML generator. Something that can be invoked into the button click event for the button  . Is there something or is there any other way