get-client-screenshot.lua

local DATA_STEP = 0xffff - 100

if CLIENT then
	local function get_screenshot_data(on_data_extract)
		hook.Add("PostRender", "dataextraction", function()
			hook.Remove("PostRender", "dataextraction")
			local data = render.Capture
			{format = "jpeg", quality = 70,h = ScrH(), w = ScrW(),x = 0, y = 0}
			on_data_extract (data)
		end)	
	end
	
	local function send_image_data(data, ply)
		data = util.Compress (data)
		for i = 1, #data - 1, DATA_STEP do
			local part = data:sub(i, i + DATA_STEP - 1)
			net.Start "GetClientScreenshot"
				net.WriteEntity(ply)
				net.WriteUInt(#part, 32)
				net.WriteData(part, #part)
			net.SendToServer()
		end
	end 
	
	if LocalPlayer():IsAdmin() then
		local DATA = [[]]
		net.Receive("GetClientScreenshot", function()
			local datapart = net.ReadData(net.ReadUInt(32))
			DATA = DATA .. datapart
			if #datapart == DATA_STEP then return end
			DATA = util.Decompress (DATA)
			-- opening image data.
			local frame = vgui.Create "DFrame"
				frame:SetSize(ScrW(), ScrH())
				frame:Center()
				frame:MakePopup()
			local html = frame:Add "DHTML"
				html:Dock(FILL)
				html:SetHTML("<img src=\"data:image/jpeg;base64,"
					.. util.Base64Encode(DATA) .. "\"/>")
			DATA = ""
			
		end)
	end
	usermessage.Hook("extract_client_image", function(um)
		local ply = um:ReadEntity()
		get_screenshot_data(function(data) send_image_data(data, ply) end)
	end)
else
	util.AddNetworkString "GetClientScreenshot"
	
	net.Receive("GetClientScreenshot", function(_, ply)
		local subject = net.ReadEntity()
		local length = net.ReadUInt(32)
		local data = net.ReadData(length)
		net.Start "GetClientScreenshot"
			net.WriteUInt(length, 32)
			net.WriteData(data, length)
		net.Send(subject)		
	end)
	
	concommand.Add ("sv_screenshot_get_all", function(ply)
		if not ply:IsAdmin() then return end
		umsg.Start("extract_client_image")
		umsg.Entity(ply)
		umsg.End()
	end)
	
	concommand.Add ("sv_screenshot_get", function(ply, _, args)
		if not ply:IsAdmin() then return end
		local p
		for k, v in pairs(player.GetAll()) do
			if v:Nick():find(args[1]) then
				p = v
				break	
			end
		end
		if not IsValid(p) then
			return ply:ChatPrint("Invalid player!")	
		end
		ply:ChatPrint("Getting screenshot from player: " .. tostring(p))
		umsg.Start("extract_client_image", p)
		umsg.Entity(ply)
		umsg.End()
	end)
end