Ilia Denisov
45 lines
1.6 KiB
Go
45 lines
1.6 KiB
Go
package world
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// PivotZoomCameraNoWrap adjusts camera center so that the world point under the cursor remains fixed
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// when zoom changes from oldZoomFp to newZoomFp.
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//
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// Coordinate conventions:
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// - CameraXWorldFp/YWorldFp is the center of the viewport in world-fixed units.
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// - cursorXPx/cursorYPx are pixel coordinates relative to the top-left of the viewport.
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// - viewportW/H are viewport size in pixels.
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//
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// This function does not clamp the result; caller should clamp for no-wrap mode using ClampCameraNoWrapViewport.
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func PivotZoomCameraNoWrap(
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cameraXWorldFp, cameraYWorldFp int,
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viewportW, viewportH int,
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cursorXPx, cursorYPx int,
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oldZoomFp, newZoomFp int,
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) (newCamX, newCamY int) {
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if oldZoomFp <= 0 || newZoomFp <= 0 {
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panic("PivotZoomCameraNoWrap: invalid zoom")
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}
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if viewportW <= 0 || viewportH <= 0 {
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panic("PivotZoomCameraNoWrap: invalid viewport")
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}
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// Offset of cursor from viewport center in pixels.
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offXPx := cursorXPx - viewportW/2
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offYPx := cursorYPx - viewportH/2
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// World-fixed per 1 pixel at each zoom.
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// (Conservative: integer arithmetic, consistent with PixelSpanToWorldFixed.)
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oldWorldPerPx := PixelSpanToWorldFixed(1, oldZoomFp)
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newWorldPerPx := PixelSpanToWorldFixed(1, newZoomFp)
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// World point under cursor before zoom:
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// world = camera + offsetPx * worldPerPx
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worldX := cameraXWorldFp + offXPx*oldWorldPerPx
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worldY := cameraYWorldFp + offYPx*oldWorldPerPx
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// Choose new camera so that the same world point stays under cursor:
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// camera' = world - offsetPx * newWorldPerPx
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newCamX = worldX - offXPx*newWorldPerPx
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newCamY = worldY - offYPx*newWorldPerPx
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return
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}
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