{"id":26,"date":"2013-06-22T16:59:59","date_gmt":"2013-06-22T16:59:59","guid":{"rendered":"https:\/\/blogs.scummvm.org\/t0by\/?p=26"},"modified":"2022-05-24T17:01:44","modified_gmt":"2022-05-24T17:01:44","slug":"a-plot-twist-or-that-thing-with-muhammad-and-the-mountain","status":"publish","type":"post","link":"https:\/\/blogs.scummvm.org\/t0by\/2013\/06\/22\/a-plot-twist-or-that-thing-with-muhammad-and-the-mountain\/","title":{"rendered":"A plot twist \u2013 or, that thing with muhammad and the mountain."},"content":{"rendered":"

\"\"<\/a><\/p>\n

My faithful readers \u2013 that is, those who have read my one previous post \u2013 will remember how I showed that a surface can be first rotated, then stretched and then correctly positioned by doing a few vector operations..
\nWell, I was lying.
\nThe thing is, while mathematically the idea is perfectly sound, in practice there is a problem.
\nWell, two.<\/p>\n

Problem one: we\u2019d be doing that with integers.
\nThis means that a. we have single pixel accuracy (which, at best, would mean nearest neighbour) b. we have rounding errors building up.
\nWe need subpixel accuracy for this.<\/p>\n

Problem two: I mentioned a magic rotating function that would rotate sprites for me.
\nWell, we do have one, and it\u2019s called SDL_rotozoom.cpp.
\nIt only has one teensy-weensy problem.
\nIt is not as precise as we\u2019d want when it comes to the bounding box around the rotated sprite.
\nBasically, what you\u2019d normally expect is this: a different corner of the original sprite \u2018pushing\u2019 the edges and causing the resulting sprite to be larger.
\nLook at this drawing I made for my reference and posted behind my monitor (I can never remember for which range I must use which corner):<\/p>\n

\"\"<\/a>This (possibly suboptimal but working) code would give you the exact amount of offset thus produced:<\/p>\n

https:\/\/github.com\/tobiatesan\/scummvm\/blob\/d98529c5aa108488399d41ce7125dc1b421c6a14\/engines\/wintermute\/graphics\/transform_tools.cpp<\/a><\/p>\n

Now, look at this instead:<\/p>\n

\"\"<\/a>See that?
\nYep.
\nThere is a 1px offset on top and a 2x offset pushing the sprite right.<\/p>\n

Those are probably due to rounding errors and various stuff going on in the fixed point calculations done in SDL_rotozoom.cpp (home to the magic function I\u2019ve been talking about till now).
\nIt is exceedingly difficult to compensate for those errors on a case by case basis, to the point that, I realized, the code is practically useless, only good for really low-end devices which benefit from some rather lightweight code.<\/p>\n

Could I be completely wrong?<\/p>\n

Possibly. I often am.
\nAt the suggestion of my mentor I looked into how the same feature is done in WME:Lite.
\nTurns out they use SDL to do it \u2013 they specify the hotspot and the positioning, and SDL draws \u201caround\u201d the hotspot using fixed point and possibly hardware rendering.<\/p>\n

We can\u2019t afford this luxury, though; the code being as it is, we cannot draw \u201caround\u201d the origin.
\nAll pixels for a sprite must be below and right of the origin.<\/p>\n

What I\u2019m gonna attempt next will thus be:<\/p>\n

1. Calculate the boundaries of the box first and place it firmly where it will end up in the end.
\n2. In a single pass do an (interpolating?) transform.<\/p>\n

This should save me from all the issues above.<\/p>\n

Stay tune, more to come.<\/p>\n","protected":false},"excerpt":{"rendered":"

My faithful readers \u2013 that is, those who have read my one previous post \u2013 will remember how I showed that a surface can be first rotated, then stretched and then correctly positioned by doing a few vector operations.. Well, I was lying. The thing is, while mathematically the idea is perfectly sound, in practice […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-26","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts\/26","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/comments?post=26"}],"version-history":[{"count":2,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts\/26\/revisions"}],"predecessor-version":[{"id":31,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts\/26\/revisions\/31"}],"wp:attachment":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/media?parent=26"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/categories?post=26"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/tags?post=26"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}