First I create a scene, the object uses PBR material, and imageProcessing is enable. When exposure and contrast are turned on, and the value is 1, the final render color of opaque objects remains unchanged.
However, the final render color of transparent objects change .
How can I avoid the color change of transparent objects when the values of exposure and contrast equal 1.0.
PG: https://playground.babylonjs.com/#17VHYI#55
Hello !
Just for the sake of clarity, I fixed your playground because on sliders there were an error (postProcess not defined) and also, I added a checkbox to enable / disable the PostProcessing, in order to be able to test the actual issue :
Indeed I admit that there is a strange behavior here 
Since post prod runs on the final render, I would expect that transparency of a material has no effect (In this scene I would expect final render should be alpha == 1.0 everywhere…)
[EDIT] : Have a look at image processing alphaMode I suspect the fix will come from there 
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There is not much we can do here 
What is happening is through the use of post process, the alpha blending math happens on linear space whereas without post pro they happen in gamma space which won t be resulting in the same value.