Revelers of the Rosas de Ouro samba school perform atop of a float during the first night of carnival parade at the Sambadrome in Sao Paulo, Brazil.
Most of the more than two million revelers at the Rio carnival are simply out to have a good time.
But for the city's top samba groups, it's a prime opportunity to put their best foot forward as they chase the honor of being selected Special Group winners.
The slightest error can cost them dear before a 40-strong jury.
- What's at stake? -The top schools work for months perfecting their parade routine, which must last no longer than 82 minutes.
The title of carnival champions brings prestige. But, as might be expected in a country fanatical about football, status is also at stake because relegation from the special group befalls the school least up to scratch on the day.
Likewise, there's all to play for in the second-tier groups, with their own champion earning promotion.
- How many schools? -This year sees 12 special schools battle it out -- six on Sunday; six more on Monday.
The action starts at 9:00 pm and can go on almost until dawn as a packed 70,000 crowd soaks up the festivities at the city's famed Sambadrome.
Millions more are glued to the television screen awaiting the outcome.
- Parades, costumes: How much? -Big money is involved with each parade costing between two and five million dollars. Where once the cash was raised largely through illicit gambling, nowadays both domestic and foreign companies have a stake in what has become big business.
The lovingly created sparkling costumes worn by participants as they gyrate on or around their respective group's richly decorated floats cost up to $35,000 and can weigh up to 40 kilos (88 pounds) just for the material.
Making them can take up to eight months.
- Spectator prices -A Brazilian earning a low wage can net a basic ticket for $15 but the best ones fetch as much as $900.
- On your marks -Scoring rules are carefully laid down in the league of samba schools (LIESA) juror's handbook.
There are 40 judges carefully assessing every last aspect of the Special Group schools' showing.
The marks are published the Wednesday after the parade. Thousands of fans of each troupe gather at their respective headquarters to hear the verdict.
For the winners, a huge party awaits. For the losers, there's always next year -- assuming they have avoided relegation.
- How are marks assessed ? -Judges working in four groups carefully assess ten aspects of performance, from percussion elements, such as the squeaking cuica drum or agogo bell, to how well the groups can sing their school's theme song.
Also important is their choreography and to what extent they can perform as if with one voice.
As they make their way along the 720 meters leading into the sambadrome, their coordination, emotion and fluidity are also carefully noted.
Missteps and gaps in the ranks can cost title hopes dear, while the groups' visual expression, their floats and costumes also play a role in the final mark awarded.
A special commission which presents each group to the watching public makes a separate assessment. In addition, the judges assess the coordination between and elegance of the first dancing master and his partner, who carries the school flag. The skills of those two individuals can make all the difference.
- Reigning champs -Last year's title went to Unidos de Vila Isabel, who just saw off Beija Flor by three-tenths of a point.
The top six finishers will perform again the Saturday following carnival in the Champions' Parade.
- And elsewhere in Brazil -Globally, Rio is the byword for carnival in Brazil, but other cities push deep into the festive spirit, not least Salvador de Bahia in the northeast.
There, giant lorries transport groups of samba-playing musicians whom millions of residents follow on foot. Recife further north also has no shortage of revelry.
Their popular street groups or "blocos" are also to be seen by the hundred in Rio, away from the elite schools, as just about everybody slips into the samba rhythm.