DCG discussion thread

munchie14

you know in those perspective questions, how do you get and when do you use does unusual vanishing points ,the ones that are not in line with the first 2 vanising pts,
like on page 179 on the red dcg book

mikeyb09

munchie14 wrote: »

you know in those perspective questions, how do you get and when do you use does unusual vanishing points ,the ones that are not in line with the first 2 vanising pts,
like on page 179 on the red dcg book

its something to do with wen there are angles! U cant get the point from the normal 2 so u use auxiliary vanishing points! I dont really like those questions!

Wat questions out of section B are people answering?
Im doin roag geometry & surface geometry!

munchie14

roads and mechanisms
which i had to teach myself

croker1

auxillary vanishing points are used when ther is a sloped surface such as a roof of a building.. not a hundred percent sure how to gwt them i must look over it myself soon and il try get back to ya..

im doing the dynamic mechanisms question and the assemles question for section C but i dont really like them tbh.. have to do em tho coz there the only ones we'v studied.

what do ye think will come up for section B?? im kinda banking on the intersecting planes question and the interpenetration question...

also does anyone know where i can get some sample questions on interpenetration? iv done all the ones in the book and in the sample papers...

croker1

croker1 wrote: »

also does anyone know where i can get some sample questions on interpenetration? iv done all the ones in the book and in the sample papers...

There's two decades of past papers from the old course. Interpentration was question 4 I think. God, I miss the old course.

munchie14

could someone give me a link to does folens sample papers that ever1 else seems to have except me,,,our school wasnt sent anything like it ,i need to see what standard of question is likly to come up, the past papers i think are harder and have stuf on them that we dont need
can some one help me out

aido179

auxiliary vanishing points are used when the surface (or the edge of the surface) is not parallel to the horizontal plane. Think of it like all "flat" surfaces (flat being hozontal) extending as far as the horizon, so the edge meets on it. so in fact all parallel lines have the same vanishing point.

when you want to find an auxiliary vanashing point, you need a few things:
1. if the line is on the same verical plane as a horizontal line, its aux vanishing point will be some height directly above the regular van. point.
2. you can get the height of it by two ways , one is kind of cheating, but makes it a lot less technical...so i'll explain that one first.
2.1 you take a point on each end of the line and join them...(you might not be able to
though, so this is not a certainty) then you continue the line until it is directly above
a regular van. point from a line thats on the same vertical plane.
other method
2.2 you use a view of the object (elevation most of the time) and you find the line in that. then you extend the line until it touches the horizontal plane (ground). you bring that point onto the plan and then join it to the point you already have of the plane in the perspective view giving you a line which goes back onto the vanishing point form here you find a regular vanishing point as if the point you had on the plane was horizontal and draw straight up from that. where your vertical line you've just drawn intersects the line i wrote in bold gives you the vanishing point...


thats long winded i'm sure and the second bit possibly might have some mistakes as i'm just writing it without drawing it...but it'll get the idea across

What question do auxiliary vanishing points come up in? Is it one of the options?

Delta Kilo

Getting auxiliary vanishing points is very straight forward.

At the spectator, you have a 90 degree angle. The horizontal line from S, where it cuts the picture plane, go straight up, vertical. With your 30 degree set square, go from S all the way back to the vertical line. Measure how high up the vertical line it cuts with your compass. This is the same height as the Auxiliary Vanishing point from the horizon line.

Give me 5 mins and ill sketch it and put it up in a post.

aido179

it will be a long question so you will have a choice. comes up in perspective

aido179

aido179 wrote: »

it will be a long question so you will have a choice. comes up in perspective

Ah right. I haven't done anything on perspective yet. I've kind of neglected DCG this year as I'm repeating, so I'm just hoping my knowledge of the old course will drag me through.

aido179

im just doing the new course first time, but from what i know it should be alright...once you payed even a small bit of attention in class...

Fince

for the options i'm doing geometry and structural (with the hyperbolids of revolution, hyberbolic parabolas etc), think thats the nicest question on the paper, two of three things have to come up on it.

Delta Kilo

OK here we go:
Hope this helps!

munchie14

Delta Kilo wrote: »

OK here we go:
Hope this helps!

but what about the unusual ones where the "AVP" is not above the vanishing points

Delta Kilo

munchie14 wrote: »

but what about the unusual ones where the "AVP" is not above the vanishing points

Don't worry about them. They would never ask a question that difficult in the exam because you would need about an hour to do it. I haven't much of a clue either.

Is it bad that I've never seen anything about perspective before? I've never done anything about vanishing points, or anything on perspective for that matter...

Delta Kilo

-JammyDodger- wrote: »

Is it bad that I've never seen anything about perspective before? I've never done anything about vanishing points, or anything on perspective for that matter...

Perspective is fairly handy. Takes a few minutes to set up though. If you are good at orthographic, you would be fairly good at it. Can get a little confusing around the spectator/picture plane area because of the amount of lines. Can get a little tricky with angles and curves aswell. Similar degree of difficulty, maybe slightly easier, as interpenetration.

Delta Kilo

Delta Kilo wrote: »

Perspective is fairly handy. Takes a few minutes to set up though. If you are good at orthographic, you would be fairly good at it. Can get a little confusing around the spectator/picture plane area because of the amount of lines. Can get a little tricky with angles and curves aswell. Similar degree of difficulty, maybe slightly easier, as interpenetration.

I'm not sure if I'll bother covering it...

I know everything else pretty perfectly (i.e. interpenetration, solids in contact etc. etc.) as I did the old course last year.

I might have a look over it though. Tricky to teach to yourself?

munchie14

i found one thatll test yas
1997 perspective q
the vanising points wont work!!
try it
http://www.examinations.ie/archive/exampapers/1997/LC028ALP210EV.pdf

Delta Kilo

You need 4 AVPs for this, one above and below each vanishing point. Same height for all of them. may be a little confusing for what to join to what. Very time consuming though.

timmywex

munchie14 wrote: »

i found one thatll test yas
1997 perspective q
the vanising points wont work!!
try it
http://www.examinations.ie/archive/exampapers/1997/LC028ALP210EV.pdf

Lovely question; points above and below the horizon line.

shanethemofo

Can anyone tell me when doing the dihedral angle between 2 planes, which angle is it or does it matter?

Delta Kilo

shanethemofo wrote: »

Can anyone tell me when doing the dihedral angle between 2 planes, which angle is it or does it matter?

I don't think so. I usually mark the big angle but I wouldn't say it matters.

galway.gaa

are most people using the old exam papers to practice questions? ive been doin them from the sample paper and the book cause i thaught that the old papers qs were way harder cause there were more marks goin for each question

Delta Kilo

galway.gaa wrote: »

are most people using the old exam papers to practice questions? ive been doin them from the sample paper and the book cause i thaught that the old papers qs were way harder cause there were more marks goin for each question

We mainly used the old questions in class and left out the bits that were too hard for the current course. Them questions are a lot longer! We used them because there was nothing else after the book at the time!

munchie14

ive nearly finshed it and i havent used any vp below the horizon
i hope theres more than one way to do it
i used height lines aswel

can some one give me a link for the sample papers ,we didnt get any

timmywex

munchie14 wrote: »

ive nearly finshed it and i havent used any vp below the horizon
i hope theres more than one way to do it
i used height lines aswel

can some one give me a link for the sample papers ,we didnt get any

Ya dont have to, but the more vanishing points make it easier!

Fince

shanethemofo wrote: »

Can anyone tell me when doing the dihedral angle between 2 planes, which angle is it or does it matter?

it doesn't matter which angle

Fince

munchie14 wrote: »

can some one give me a link for the sample papers ,we didnt get any

as far as i know the papers are not available on t'internet(i'm no George Lee but i'm fairly sure no one would pay for them if they were). the solutions are on folens website but thats not much good to you if you don't have the questions.

Prowetod

In the official sample paper in the intersecting planes question it says: Determine the angle of inclination of the line of intersection to the vertical plane.
Does this mean you look perpendicularly at the line of intersection in plan? or am I completely off!?

Anyone else think that time could be a huge factor? How long are you giving to each question?