So I did some more tests, and using the "helper" approach to create a layer, bake keyframes and then read the time values on those keyframes seems quite solid. I've tested with different frame rates, different start times etc and all seems to work. It's of course a bit slower, but in my use case it's better to be slow than to fail.
If anyone is intrested, here's a helper class with a few examples at the bottom.
function CompHelper(comp){
// Helper class for working with frames (instead of time) in After Effects.
// Simon Bjork
// December 2020
// bjork.simon@gmail.com
// There are a few bugs in After Effects that prevents an easy frame>time conversion.
// The biggest issue is that there are rounding errors in the internal calculations that make keyframes to be added between frames.
if(!(comp instanceof CompItem)){
alert("No CompItem")
return false;
}
// Initialize the comp.
this.comp = comp
getCompData();
// Class variables.
this.update = update;
this.setStartFrame = setStartFrame;
this.getStartFrame = getStartFrame;
this.getFrameRange = getFrameRange;
this.getTime = getTime;
this.valueAtFrame = valueAtFrame
this.valuesAtFrames = valuesAtFrames;
this.setValueAtFrame = setValueAtFrame;
this.setValuesAtFrames = setValuesAtFrames;
this.setDurationFrames = setDurationFrames;
function getCompData(){
this.width = this.comp.width;
this.height = this.comp.height;
this.par = this.comp.pixelAspect;
this.fps = this.comp.frameRate;
this.duration = this.comp.duration;
this.frameDuration = this.comp.frameDuration;
this.frameRange = getFrameRange(this.comp)
this.first = this.frameRange[0];
this.last = this.frameRange[1];
this.startFrame = getStartFrame();
this.times = generateTimeList()
} // function getCompInfo(){
function update(){
// Better name for external use.
getCompData()
} // function update(){
function generateTimeList(){
// Generate a time value for each frame in the composition.
// Create helper and generate keyframes.
var helper = this.comp.layers.addNull(this.duration);
var prop = helper.rotation;
prop.expression = 1;
prop.selected = true;
app.executeCommand(app.findMenuCommandId("Convert Expression to Keyframes"));
// Loop over the keyframes and collect the time values.
var times = []
for(var i =1 ; i<=prop.numKeys;i++){
// Get the time from index.
t = prop.keyTime(i);
times.push(t);
}
helper.remove()
return times
} // function generateTimeList(frames){
function timeToFrames(time){
return Math.round(time/this.frameDuration);
} // function timeToFrames(time){
function framesToTime(frame){
return frame*this.frameDuration;
} // function framesToTime(frame){
function getStartFrame(){
// Alternative for AE 17.1: startFrame = (app.project.displayStartFrame) + (this.comp.displayStartTime/this.frameDuration)
var startSec = this.comp.displayStartTime
var startFrameCalc = (startSec / this.frameDuration) + app.project.displayStartFrame;
var startFrame = Math.floor(startFrameCalc);
return startFrame;
} // function getStartFrame(){
function getFrameRange(comp){
// Get the first/last frame of the comp.
var first = getStartFrame();
var last = first + timeToFrames(comp.duration, comp) -1;
return [Math.floor(first), Math.floor(last)];
} // function getFrameRange(comp){
function setStartFrame(frame){
if(app.project.displayStartFrame == 1){
var frame = frame-1
}
var startFrameSec = framesToTime(frame, comp) + 0.00001;
if(startFrameSec < 0){
var startFrameSec = 0;
}
this.comp.displayStartTime = startFrameSec;
} // function setStartFrame(comp, frame){
function getTime(frame){
var time = this.times[frame-this.startFrame]
return time;
} // function getTime(frame){
function setValueAtFrame(property, frame, value){
var t = getTime(frame);
property.setValueAtTime(t, value);
} // function setValueAtFrame(property, frame, value){
function setValuesAtFrames(property, frames, values){
// Set values at multiple frames at the same time.
// Much faster than calling setValueAtFrame multiple times.
if(frames.length != values.length){
alert("Frames and values are not the same length");
return
}
var times = []
for(var i=0; i<frames.length;i++){
var frame = frames[i];
var t = getTime(frame);
times.push(t);
}
property.setValuesAtTimes(times, values);
} // function setValuesAtFrames(property, frames, values){
function valueAtFrame(prop, frame){
// Get value at frame.
var t = getTime(frame);
var val = prop.valueAtTime(time, false);
return val
} // function valueAtFrame(prop, frame){
function valuesAtFrames(prop, frames){
var values = []
for(var i=0; i<frames.length;i++){
var frame = frames[i];
var t = getTime(frame);
var value = prop.valueAtTime(t, false);
values.push(value);
}
return values
} // function valuesAtFrames(prop, frames){
function setDurationFrames(frames, reload){
// Set duration to a comp in frames.
this.comp.duration = framesToTime(frames)
if(reload){
update();
}
} // function setDuration(frame, doReload){
// Return the whole thing.
return this
} // function CompHelper(comp){
// ----------------------------------------------------------------
// Example use cases.
var comp = app.project.activeItem;
var c = CompHelper(comp)
var layer = c.layer(1);
// Set start frame of a comp.
c.setStartFrame(36);
// Set a value at frame.
c.setValueAtFrame(layer.rotation, 1005, 20);
// Set values at frames.
c.setValuesAtFrames(layer.rotation, [1003, 1004], [10, 20]);
// And more.
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