The only real thing you need, to get started are what you already have: position, normal, diffuse (texture) color.
1st, your diffuse has all perfectly lit green grass. So if you have a sunlight, you should calculate that first and put that in your diffuse buffer as well. For all other lights (lamposts, vehicles etc): they are contributing (addding) EXTRA light to the scene. So first again, you put your original diffuse on the screen (with or without sunlight), then you use addative blending of each extra light. Again keyword add/extra (addative). For each light you draw a physical model of the actual lights area. For a lampost it might be a 3d cone, a lightbulb would be a sphere. Hopefully you already knew that cuz if not, then your stepping too far.
After that you should have basic lighting. Again with materials like metal you have a certain specular power, as well as certain spots of metal that have different amounts of specular. Imagine a piece of metal with rust, the rust portions have no specular, and the clear ones do because they reflect light back. But even more when the light hits that surface, it has a specular power, either it is really focused and hard metal, or not as focused and soft. So you could (and eventually will), want to have those properties stored either in the color,normal,position, anywhere.
Any property/material that effects light from an object standpoint, is saved. The light properties are just used when you draw those 3d models.
Deferred shading - confusion
1st, your diffuse has all perfectly lit green grass. So if you have a sunlight, you should calculate that first and put that in your diffuse buffer as well.
I think this may potentially just cause more confusion, and in any case I'd say it was a good idea to maintain complete separation of the g-buffer/lighting stages. Are you talking about having a 'sun occlusion' factor (stored in the alpha channel or something), or actually pre-multiplying the diffuse component? The former doesn't work for dynamic objects, and the latter doesn't work at all.
1. The albedo texture should be a combo of the material diffuse property and the objects texture?
2. No ambient material should be needed since it is usually the same as the diffuse property. Later i can add SSAO
3. And now there will be no emissive or specular color but instead a factor that will be multiplied by the light properties?
Only question that remains for me now is if the materials are now like this, will the light sources properties be similar or do they still retain their RGBA values
1. I would say that an objects texture is its material's diffuse property, i.e. the colour which modulates any reflected, diffuse light. So your grass texture represents the final colour you'd see if there was a 100% white light shining on it.
2. You could have a seperate ambient colour if you wanted. But you'd need another set of RGB components in the g-buffer. As you said, however, the ambient reflectance of a material is the same as its diffuse, so you can just use the diffuse property to modulate any ambient light.
3. As with ambient, you could have separate emissive/specular colours but, again, you'd need a bigger g-buffer. For the colour of a specular reflection it's usually good enough to simply use the light's colour.
The light source properties can be as many or few as you need, since these will be passed directly into the shader and not stored in any intermediate buffers. So you could have RGB colours for separate ambient/diffuse/specular light or just a single colour for all three.
I should point out that the example I've given of the g-buffer setup is pretty arbitrary. Basically you need the g-buffer to store whatever you need in order to do lighting. If you look through the various references you'll see lots of different g-buffer configurations, non are the right or wrong way to do it. It entirely depends on what properties you need during the defered lighting pass. Understanding the lighting model you're going to use is key; make sure you're on top of Phong and know what all the inputs are doing.
Author
The only real thing you need, to get started are what you already have: position, normal, diffuse (texture) color.
1st, your diffuse has all perfectly lit green grass. So if you have a sunlight, you should calculate that first and put that in your diffuse buffer as well. For all other lights (lamposts, vehicles etc): they are contributing (addding) EXTRA light to the scene. So first again, you put your original diffuse on the screen (with or without sunlight), then you use addative blending of each extra light. Again keyword add/extra (addative). For each light you draw a physical model of the actual lights area. For a lampost it might be a 3d cone, a lightbulb would be a sphere. Hopefully you already knew that cuz if not, then your stepping too far.
After that you should have basic lighting. Again with materials like metal you have a certain specular power, as well as certain spots of metal that have different amounts of specular. Imagine a piece of metal with rust, the rust portions have no specular, and the clear ones do because they reflect light back. But even more when the light hits that surface, it has a specular power, either it is really focused and hard metal, or not as focused and soft. So you could (and eventually will), want to have those properties stored either in the color,normal,position, anywhere.
Any property/material that effects light from an object standpoint, is saved. The light properties are just used when you draw those 3d models.
Yes i already understand all this except about the sunlighting. Do you mean to calculate phong directional light just as you would in a forward pass renderer and do all lighting calculations on that instead? If so, is there an advantage to this compared to a full screen quad at lighting stage?
1. I would say that an objects texture is its material's diffuse property, i.e. the colour which modulates any reflected, diffuse light. So your grass texture represents the final colour you'd see if there was a 100% white light shining on it.
Ok but in the case which the object i am rendering does not have a texture of its own, the material diffuse will be in this buffer i presume?
Ok but in the case which the object i am rendering does not have a texture of its own, the material diffuse will be in this buffer i presume?
Okay, I see what you mean now. Yes, it will just be the diffuse colour in this case.
Author
Ok i think i have enough info to implement it now, thanks alot for your help and patience 
Edit: Ok, sorry for the silly question. The light_x_level is of course calculated from viewing and light directions etcetera.
Thank you for a great explanation!
The "reduction" of full Phong was logical and implied in the references I have read, but it was good to have someone mention it, giving me confidence to proceed with my code.
In your "property mixing" example, where lights and material multiply, you have separate factors for light color and light level. Why?
I am curious how these factors are calculated. Personally I just have light.diffuse etc.,
Best regards
Tobias
ambient = material_diffuse * light_color * light_ambient_level;
diffuse = material_diffuse * light_color * light_diffuse_level;
specular = light_color * material_specular_level * light_specular_level ^ material_specular_power;
emissive = material_emissiveness * material_diffuse;
result = ambient + diffuse + specular + emissive;
Thank you for a great explanation!
The "reduction" of full Phong was logical and implied in the references I have read, but it was good to have someone mention it, giving me confidence to proceed with my code.
In your "property mixing" example, where lights and material multiply, you have separate factors for light color and light level. Why?
I am curious how these factors are calculated. Personally I just have light.diffuse etc.,
Best regards
Tobias
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