{"action":"created","issue":{"url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/issues/581","repository_url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs","labels_url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/issues/581/labels{/name}","comments_url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/issues/581/comments","events_url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/issues/581/events","html_url":"https://github.com/KhronosGroup/Vulkan-Docs/issues/581","id":263930174,"number":581,"title":"[Specification] Elliptical footprint approximation","user":{"login":"Retrofractal","id":2734645,"avatar_url":"https://avatars0.githubusercontent.com/u/2734645?v=4","gravatar_id":"","url":"https://api.github.com/users/Retrofractal","html_url":"https://github.com/Retrofractal","followers_url":"https://api.github.com/users/Retrofractal/followers","following_url":"https://api.github.com/users/Retrofractal/following{/other_user}","gists_url":"https://api.github.com/users/Retrofractal/gists{/gist_id}","starred_url":"https://api.github.com/users/Retrofractal/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/Retrofractal/subscriptions","organizations_url":"https://api.github.com/users/Retrofractal/orgs","repos_url":"https://api.github.com/users/Retrofractal/repos","events_url":"https://api.github.com/users/Retrofractal/events{/privacy}","received_events_url":"https://api.github.com/users/Retrofractal/received_events","type":"User","site_admin":false},"labels":[{"id":327327932,"url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/labels/Resolving%20Inside%20Khronos","name":"Resolving Inside Khronos","color":"ed458b","default":false}],"state":"open","locked":false,"assignee":{"login":"TomOlson","id":17166411,"avatar_url":"https://avatars1.githubusercontent.com/u/17166411?v=4","gravatar_id":"","url":"https://api.github.com/users/TomOlson","html_url":"https://github.com/TomOlson","followers_url":"https://api.github.com/users/TomOlson/followers","following_url":"https://api.github.com/users/TomOlson/following{/other_user}","gists_url":"https://api.github.com/users/TomOlson/gists{/gist_id}","starred_url":"https://api.github.com/users/TomOlson/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/TomOlson/subscriptions","organizations_url":"https://api.github.com/users/TomOlson/orgs","repos_url":"https://api.github.com/users/TomOlson/repos","events_url":"https://api.github.com/users/TomOlson/events{/privacy}","received_events_url":"https://api.github.com/users/TomOlson/received_events","type":"User","site_admin":false},"assignees":[{"login":"TomOlson","id":17166411,"avatar_url":"https://avatars1.githubusercontent.com/u/17166411?v=4","gravatar_id":"","url":"https://api.github.com/users/TomOlson","html_url":"https://github.com/TomOlson","followers_url":"https://api.github.com/users/TomOlson/followers","following_url":"https://api.github.com/users/TomOlson/following{/other_user}","gists_url":"https://api.github.com/users/TomOlson/gists{/gist_id}","starred_url":"https://api.github.com/users/TomOlson/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/TomOlson/subscriptions","organizations_url":"https://api.github.com/users/TomOlson/orgs","repos_url":"https://api.github.com/users/TomOlson/repos","events_url":"https://api.github.com/users/TomOlson/events{/privacy}","received_events_url":"https://api.github.com/users/TomOlson/received_events","type":"User","site_admin":false}],"milestone":null,"comments":1,"created_at":"2017-10-09T15:11:39Z","updated_at":"2017-10-16T19:09:15Z","closed_at":null,"author_association":"NONE","body":"From the Vulkan (1.0.62) specification (15.6.7 Scale Factor Operation, Level-of-Detail Operation and Image Level(s) Selection):\r\n> The _scale factors_ ρx and ρy, calculated from the magnitude of the derivatives in x and y, are used to compute the minimum and maximum scale factors.\r\n\r\nThis isn't correct; you cannot determine the maximum and minimum scale factors with just these scale factors.\r\n> ρx and ρy **may** be approximated with functions fx and fy\r\n\r\nThis should probably read\r\n\r\n>ρmin and ρmax **may** be approximated with functions fx and fy\r\n\r\nwith the following subsequent alteration:\r\n\r\n> The minimum and maximum scale factors (ρmin,ρmax) are given, under this approximation, by\r\nρmax = max(fx, fy)\r\nρmin = min(fx, fy)\r\n"},"comment":{"url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/issues/comments/337000492","html_url":"https://github.com/KhronosGroup/Vulkan-Docs/issues/581#issuecomment-337000492","issue_url":"https://api.github.com/repos/KhronosGroup/Vulkan-Docs/issues/581","id":337000492,"user":{"login":"pdaniell-nv","id":10520383,"avatar_url":"https://avatars1.githubusercontent.com/u/10520383?v=4","gravatar_id":"","url":"https://api.github.com/users/pdaniell-nv","html_url":"https://github.com/pdaniell-nv","followers_url":"https://api.github.com/users/pdaniell-nv/followers","following_url":"https://api.github.com/users/pdaniell-nv/following{/other_user}","gists_url":"https://api.github.com/users/pdaniell-nv/gists{/gist_id}","starred_url":"https://api.github.com/users/pdaniell-nv/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/pdaniell-nv/subscriptions","organizations_url":"https://api.github.com/users/pdaniell-nv/orgs","repos_url":"https://api.github.com/users/pdaniell-nv/repos","events_url":"https://api.github.com/users/pdaniell-nv/events{/privacy}","received_events_url":"https://api.github.com/users/pdaniell-nv/received_events","type":"User","site_admin":false},"created_at":"2017-10-16T19:09:15Z","updated_at":"2017-10-16T19:09:15Z","author_association":"NONE","body":"When you read the spec, did you assume that the scale factors ρx and ρy were the magnitude of the derivatives in x and y? We didn't make this assumption, so that's why we thought your proposal was not a significant change to the spec.\r\n\r\nI think ρx and ρy should be seen as the major and minor axes of the ellipse that cover the region you are sampling. You do a Max() to get the major axis and a Min() for the minor\r\n\r\nThe equations to compute those 2 axes could be quite complex, and I'm sure everyone as its own technique, but in the end, I expect implementations to use derivatives in x and y to compute those 2 values.\r\n"}}
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