Compositor has a different effect when editing Render Result vs external PNG image?

Question

I get a different resulting image for each of these three situations:

1. I edit the true Render Result directly with "Render Layers" node in the compositor. (The Render Result image was never saved externally.)
2. I save the Render Result as a PNG file, and then edit the externally-saved image with an Image node in the compositor.
3. I save the Render Result as a PNG file, open the image with an Image node in the compositor, and connect the Image node to the Composite node. The external image is now loaded into "Render Result" (kind of like a pseudo render). I then edit "Render Result" with the "Render Layers" node in the compositor.

The second two situations gives me different results from the first.

What I want is the first result, but then I need to re-render if I accidentally close Blender during the process or want to make additional changes with the compositor in the future (since the "true" render is deleted upon closing Blender, and all I can save is the PNG).

Is the saved PNG missing some kind of information that the "true" Render Result image has? How can I remedy this?

(Note that I tested this by using a "Glare" node. This node seemed to have no actual glare effect for situation #2... it only darkened/lightened the whole image. It had a sparkly/glowy effect for situations #1 and #3, but they were very different.)

(I'm new to Blender.)

Answer 1

This issue is that the exported image has been conveted using color management transforms.

In short words, the renderer will determine a light value for each pixel, with range from 0 to infinity in a linear way. When you export an image every value will be converted to fit the 0.0 (black) - 1.0 (white) range, and applies a transform, determined by the settings on the color management section.

The values of the two images (rendered and exported) will be different. In other words: the original values are unbound and linear (also known as Scene Referred), the values on a PNG are limited and a "gamma curve" has been applied so the values are no longer linear (they have been converted to Display Referred)

See this answer for more detailed infos Render with a wider dynamic range in cycles to produce photorealistic looking images.

Saving the image as EXR files, will allow you to retain the original unbound and linear light values!

Answer 2

Never mind! I discovered that if I want to save all the render information, I should save the render as an EXR file. :)

This answer explains how to save it as such.