Footage of a man slapping and kicking a 15-month-old baby he is accused of murdering have been shown to a court.
Jurors watched as the video clips taken on a mobile phone by Darren Newton, of Earby, Lancashire, were played to Manchester Crown Court.
They were told the 32-year-old recorded the footage of Charlie Hunt for his own "pleasure" while the baby's mother, Laura Chapman, was at work.
Newton has pleaded guilty to 12 counts of child cruelty but denies murder.
Newton, of Warwick Drive, Earby has also pleaded not guilty to a further two counts of child cruelty.
The court heard how police found a series of clip titles stored on his mobile phone, including one entitled "happy slap".
In the clip, taken two weeks before Charlie died, Newton is seen to aim 12 slaps around the head of the crying youngster.
Charlie was taken to hospital and died from head injuries on 19 November 2009 after what the prosecution claims was a final fatal attack.
Dennis Watson QC, opening the prosecution case, told the jury Newton "took pleasure from the fact that Charlie was suffering from cruel and wicked behaviour".
He said Newton had repeatedly assaulted the child in June and November 2009.
"Not only did he do that he filmed himself doing it on his mobile phone. The assaults and cruelty increased in seriousness over those months," added Mr Watson.
Ms Chapman met Newton in early 2009, believing he was a "quiet man, a caring man and a loving man" who got on well with her baby son.
"The sad truth was whatever the outward appearances, they hid a dark and we submit wicked side to the defendant which only surfaced when he was alone with Charlie," said Mr Watson.
On 19 November, Newton returned home after his shift at a local factory to look after Charlie.
At 1630 GMT he went next door to his parents' home for help with the youngster, who appeared limp, was struggling to breath and had a tremor down his left-hand side.
An ambulance was called and Charlie was taken to hospital, where he died at 1745 GMT.
Doctors later noticed he had bruises on his right eyelid, right cheek and right big toe, the court heard. Post-mortem examinations later revealed the extensive damage to his brain and head.
Newton told police he had noticed Charlie's eyes were glazed when he tried to change his nappy, and believed he was about to have a fit.
A Home Office pathologist concluded the injuries were non-accidental and deliberately inflicted.
"Those head injuries are completely inconsistent with the defendant's explanation of a fit," Mr Watson added.
"On the contrary, the injuries are consistent with Charlie being struck on the head in a very similar manner to that seen in the previous video clip, 'Happy Slap' two weeks earlier."
The trial is expected to last three weeks.