`SetupAxisLimitsConstraints()`'s strange behavior?

[Razakhel]

I'm having trouble figuring out the behavior of SetupAxisLimitsConstraints(). As I understood, which may well be entirely wrong, calling SetupAxisLimitsConstraints(...) was roughly equivalent to calling SetupAxisLimits(..., ImPlotCond_Always). However, this does not seem to be the case, at least for the very first frame: using SetupAxisLimits() with the _Always condition is instantly taken into account, while the *Constraints() version needs another frame to work as expected.

const double maxValCount = 4.0;

if (ImPlot::BeginPlot("Overlay", ImVec2(-1, -1), ImPlotFlags_NoMenus | ImPlotFlags_NoBoxSelect)) {
  ImPlot::SetupAxes("X", "Y");

  // Testing either of the following lines:
  ImPlot::SetupAxisLimits(ImAxis_X1, 0.0, maxValCount, ImPlotCond_Always);
  ImPlot::SetupAxisLimitsConstraints(ImAxis_X1, 0.0, maxValCount);

  ImPlot::SetupAxisLimits(ImAxis_Y1, 0.0, 100.0, ImPlotCond_Once);
  ImPlot::SetupMouseText(ImPlotLocation_NorthEast);

  // Plotting 50, 75, 25, 10 & 50 with lines
  // Plotting 15, 30, 0, 60 & 40 in shaded

  ImPlot::EndPlot();
}

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With ImPlot::SetupAxisLimits(ImAxis_X1, 0.0, maxValCount, ImPlotCond_Always);, on the very first frame:

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With ImPlot::SetupAxisLimitsConstraints(ImAxis_X1, 0.0, maxValCount);, still on the very first frame:

Rendering another frame, we get the expected result:

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While it's not critical, it was not something I expected. Is it a bug, or am I missing something?

[Nahor]

calling SetupAxisLimitsConstraints(...) was roughly equivalent to calling SetupAxisLimits(..., ImPlotCond_Always)

This is incorrect (if I'm not mistaken myself).

• SetupAxisLimits(..., ImPlotCond_Always) forces the left/right (bottom/top) values of the axis to be the given values. When called, you cannot zoom or scroll on the affected axis anymore.
• SetupAxisLimits(..., ImPlotCond_Once) only does it on the first frame, i.e. it sets the initial range of the axis. After that, you're free to zoom/scroll as much as you want.
• SetupAxisLimitsConstraints only sets the maximum range of the plot, not the minimum range. So if your constrain is [a..b], then the axis limits can be anything within that [a..b] range. You can't go below a, you can't go above b but you can go anywhere in-between.

So it's pointless to set the "constrains" if you have "limits(...always)". And "limits(...once)" should be within the range specified by "constrains". And in your case, it's actually not expected to go from [0, 1.0] to [0, 4.0] just by setting the constrains, since [0..1] is within the [0..4] range. I suspect that you still have call to SetupAxisLimits(always) somewhere.

Personally, I cannot reproduce the issue. It might have been a bug in implot that has been fixed since (you haven't specified what version you're using, myself I used v0.16) or something else is going on in your code (posting a more complete example would be useful).