Results get overly exposure when rendering with AOIntegrator and RGBFilm with ISO larger than 100

[w3ntao]

Rendered results are getting brighter and brighter as ssp increases when rendered with Integrator "ambientocclusion" and RGBFilm with ISO larger than 100.

These images are what I got when rendering sssdragon/dragon_10.pbrt using Integrator "ambientocclusion" and changing output format from .exr to .png with ssp 1, 4, 16, 64:

ssp: 1

ssp: 4

ssp: 16

ssp: 64

Is this what it suppose to be?

---

modified sssdragon/dragon_10.pbrt:

Sampler "halton"
    "integer pixelsamples" [ 1024 ]
PixelFilter "gaussian"
Film "rgb" "float iso" 400
    "string filename" [ "dragon_10.png" ]
    "integer yresolution" [ 1024 ]
    "integer xresolution" [ 1366 ]
Scale -1 1 1
LookAt 3.69558 -3.46243 3.25463
    3.04072 -2.85176 2.80939
    -0.317366 0.312466 0.895346
Camera "perspective"
    "float fov" [ 28.841503 ]

Integrator "ambientocclusion"

WorldBegin


AttributeBegin
    LightSource "infinite" "string filename" "textures/small_rural_road_equiarea.exr"
AttributeEnd

AttributeBegin
    Translate 0.2 0.3 0.78
    Rotate 90 1 0 0
    Rotate -90 0 1 0
    Scale 0.02 0.02 0.02
    Material "subsurface"
        "float scale" [ 10 ]
        "string name" [ "Skin1" ]
        "float eta" [ 1.5 ]
    Shape "plymesh"
        "string filename" [ "geometry/dragon.ply.gz" ]
AttributeEnd

AttributeBegin
    Material "coateddiffuse" "float roughness" .2
    Shape "plymesh"
        "string filename" [ "geometry/meshes_0.ply" ]
AttributeEnd

pbrt-v4 version: 2b5837a