I am not sure if it is an option that the user would change a lot, so it’d take room for an option you change only once ?
It can be an option in the Render Slots panel if it’s something you want to control per .blend file, instead of in the user preferences. People can still save that in their startup.blend.
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Sounds as the best fit to me
I changed it, it is now above the new slots list in the N panel of the image editor 
@brecht
I have a few question about the developement now:
At the moment I am storing the char for the compare_with toggle AND an int called compare_slot (which is the number of the slot that is conpared) both in SpaceImage, the problem is that as soon as a new SpaceImage is created, compare_slot goes back to 0, so I have two questions (related hopefully) :
- How can I specify that it has to be equal to 1 by default ?
- Where can I store this variable so that it is not reset every time we close the Image Editor ?
@ideasman42
I would like to start working on a manipulator for the Slider comparison and as you worked a lot on manipulators, I wanted to know if you had a few tips and advices to create a new one, like where to start ? In what file should I write a manipulator that will be in the Image Editor when it is displaying the Render Result ?
For initializing it in new editors, set it in image_new() in space_image.c. The more tricky part is version patching existing files (which includes the startup.blend).
For example see this code for initializing the new texture_paint_mode_opacity property in the 3D view:
https://developer.blender.org/diffusion/B/browse/blender2.8/source/blender/blenloader/intern/versioning_280.c;c974a359531733e02adcd7366d684291b257affa$1517
It seems the entire image editor state is lost when rendering to a new window and closing it. So if you want to avoid that you either need to store it on the “Render Result” Image datablocks, and then it’s shared between all image editors. If it needs to be per image editor, we have to figure out some way to preserve the image editor settings in general.
Hey, I don’t know if @ideasman42 saw this post but if you have an answer for that @brecht, let me know
On top of that I have some questions about drawing the image in the view, to implement the difference mode. I think I found the areas in the hierarchy I have to work on. But I don’t really know if I have to work in the image_draw.c and if there are already functions to do that, or is if I have to go at a lower level maybe in glaDrawImBuf_glsl_ctx_clipping in glutil.c, or even lower to work with the pixel values and in that case I don’t know where this is. Do you have an idea ?
Thanks a lot for your help, hope my code is not too much of a mess, please let ma know if I am really doing things the wrong way
Hey, I was trying to find one of the crash I get when I push the images twice and display the Slot 3, so I am using Microsoft Visual Studio now (was using Sublime Text before) and when I try to run Blender from Visual Studio in Debug mode, I get this :
It works perfectly when I run it in Release mode.
Do you know where that could come from ?
I don’t know if this is the right place (if not, then delete this message), but
Is it possible to implement such feature like render new in new slot, and don’t overwrite existing?
And if render slots is maxed out, then creating empty one for next render or overwrite first one.
And option to store render results in .blend file, because they are disappearing every time you reopen your scene.
And there is no way to compositing render slots?
You still need to save render from slot 1 and attach it as image to compositing with render slot 2?
This is a really interesting topic, I sure hope it is still alive and doing well.
For the render slots themselves, it would be nice if there could be an option to change from thumbnail + text to large/ medium/ small thumbnails with the slot name inside the thumbnail (upon mouse hover). That uses the most screen real-estate, while providing the info if needed.
I’ll also point to: https://blender.community/c/rightclickselect/wGgbbc/ considering it provides further feedback on an eventual frame buffer.
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Hi sorry I know it is a old thread but this functionality would be greatly appreciated and functional, love the slider preview for comparing 2 images, seen it in other 3d software. Any news on a developers thread on it. Thanks
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Hi @thornydre thanks for picking this up!
images are drawn in the image engine. draw/engines/image/image_engine.c the slider overlay could be part of draw/engines/overlay/overlay_image.c. The pixels are drawn inside a shader (engine_image_frag.glsl). Drawing using the glaDrawImBuf leads to alpha artifacts.
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It’s been a while since I last tried to make this work, but I’ll give it another try with these new informations maybe I’ll go a bit further
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Ah, cool that you’re taking a stab at this again! Was wondering though, is your implementation limited to just render slots for comparison? What if I want to take a loaded image, saved from the night before, and compare it with the Slot 5->View layer->Sample Count pass? I’m not sure what the UI could look like (especially with blenders custom widgets; maybe spreadsheet nowadays) but the ability to just pick the exact A and B image data blocks you want to compare might be best.
I am a real beginner in terms of coding on a project of that scale, it sure would be a great think to be able to compare any images between each other, but for now I will stick to the proposal I made, as even that would be an amazing achievement for me. But if I succeed in doing it right, I might try to extend it further But I am already glad some people support this tiny project haha 
Cheers !
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Yup, sounds good. I’m in no position to generate work for you
Thank you for trying to improve this area 
I´m a new Blender user (also using Modo) and I found this thread exactly because I was frustrated with how the render window or the image editor works, I would prefer that the system sorted the slots automatically like in modo.
This is not related maybe but another thing I would like is for everytime I open the render F11, I would like to see the previous renders I did. In Modo because those are stored in a temp folder and can see my previous rendered images, they show up in the frame buffer.
I guess this doesn´t work similarly in Blender because what blender has it´s not really a ipr right?
Thank you.
Sergio
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Not sure if you are still looking for a solution 2 years later 
, but just incase, the renderslot add-on does this.
https://developer.blender.org/diffusion/BAC/browse/master/render_renderslot.py