If clicks work while the camera is at (0, 0) but stop working after it moves, check whether the pointer and the component are being compared in different coordinate spaces. In the reported Limn Engine case, the correction was to add the camera’s x and y offsets to the pointer coordinates before hit-testing. That camera-offset adjustment is the reported fix; it does not by itself account for canvas positioning, CSS scaling, or zoom.
Why camera movement breaks clicks
A hit test can only succeed when the pointer and the target are expressed in the same coordinate space. A component may be positioned in world coordinates, while a mouse event reports a position relative to the page or viewport. When the camera is at the origin, those values can appear to line up; once the camera moves, they no longer describe the same point.
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Limn’s intermediate guide describes the camera as x/y offsets applied to rendering by translating the drawing context by -camera.x and -camera.y. The scene is drawn shifted in the viewport, but world-positioned components retain their world coordinates. Input therefore needs to be mapped back through the camera transform before it is compared with those components.
What Limn’s input API expects
Limn’s beginner documentation says display.x and display.y hold pointer coordinates while a mouse button or finger is pressed, then become false on release. It describes clicked() as checking whether those coordinates fall inside a component’s bounding box, accounting for component rotation. The documented usage pattern is:
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if (display.x && btn.clicked()) {
// Handle the click
}The documentation describes the canvas origin as its top-left corner, with x increasing to the right and y increasing downward. The important question when using this pattern is whether display.x and display.y use the same coordinate space as the component checked by clicked().
The reported camera-offset correction
In the reported scrolling-map bug, clicks worked before the camera followed the player and failed afterward. The article summary describes storing the pointer position with the camera offsets added:
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window.addEventListener('mousedown', (e) => {
this.x = e.pageX + this.camera.x; // World space
this.y = e.pageY + this.camera.y; // World space
});Adding the camera offsets maps a viewport position back toward world space when rendering subtracts those offsets. Use the pattern only when the event coordinates are already in the coordinate system expected by the canvas and the camera applies the described translation. The available report is a secondary summary of the fix, not an independently reproduced patch, and it does not establish handling for other transforms.
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Check every transform between the event and the hit test
If adding the camera offsets does not resolve the issue, trace the full coordinate pipeline rather than changing the hit-test bounds at random. Convert the pointer through the inverse of each transform that separates the event position from the component’s coordinates:
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- Page or viewport to canvas: account for the canvas element’s position. The reported snippet uses
pageXandpageY; these are not automatically canvas-local coordinates when the canvas is offset on the page. - CSS size to canvas coordinates: if CSS resizes the displayed canvas relative to its drawing buffer, scale the local pointer position to the coordinate system used by rendering and components.
- Camera translation: for the documented render translation by
-camera.xand-camera.y, add the camera offsets when mapping a canvas-space point back to world space. - Zoom or other transforms: if rendering also scales or applies additional transforms, invert those in the correct order according to the engine’s actual render pipeline.
This is general coordinate-conversion guidance, not a claim that the reported Limn patch implements all these steps. The engine version, complete patch, and handling of canvas offsets, CSS scaling, device-pixel ratio, and zoom are not established by the available report.
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How to narrow down the mismatch
- Log the pointer coordinates immediately before hit-testing and compare them with the component’s position and bounds.
- Repeat the comparison with the camera at the origin and after it moves. A discrepancy that tracks camera movement points toward a missing or incorrectly applied camera transform.
- Check that the canvas origin and display size are reflected in the pointer conversion before camera offsets are applied.
- Confirm whether the hit test expects world coordinates or canvas coordinates; do not apply the camera transform twice.
- Keep the pointer and component values in one documented space throughout the call to
clicked(), including any rotation-aware bounds calculation.
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