The following is from 'Wikipedia' and is their take on the term 'Constructive Criticism', and I think is apropos in reply to what you just sent to me.
'The purpose of constructive criticism is to improve the outcome. In collaborative work, this kind of criticism is a valuable tool in raising and maintaining performance standards.
Constructive criticism must always focus on the work rather than the person. Personality issues must always be avoided. Constructive criticism is more likely to be embraced if the criticism is timely, clear, specific, detailed and actionable.'
As concerns that last part about constructive criticism should be: Timely, clear, specific, detailed and actionable, I think the following that was sent to you covers all those categories.
From Drew: "They're floating - No defined shadows. Drew."
Reply from Alan: "There are shadows I can see running SW to NE but they are not very dark."
Reply from Drew: "As I said 'Not Defined'. Drew."
The 'Not Defined' was meant that the shadows weren't dark enough. And in addition the horse and rider had no discernible shadow and it too seemed to be floating – Especially here, no defined shadow.
I'll give you that I'm abrupt, and not that polite. So we're opposites in those traits, but I would expect that someone whom I respect deeply, in both your art and character, would give me some slack for what you consider my shortcomings. I have a thick skin and not easily hurt, and what you have said did not hurt me. It went beyond hurt, it wounded me deeply. And will be reflected in future actions, or inaction.
'Nuff said. Your former friend, but still great admirer, Drew.