I'm very experienced with Qt's QGraphicsScene, but I'm hoping someone can clarify a detail with regard to the boundingRect and shape methods for a QGraphicsItem. As far as I can find, the documentation doesn't address this specific concern.
I have a situation where I need to calculate a shape for many complex paths with the shape including a slight buffer zone to make the paths easier for the user to click on and select. I'm using the QPainterPathStroker, and it's expensive. I'm currently attempting to delay the shape calculation until the shape method is actually called, and that's helping with the performance.
The situation now is that the bounding rectangle is calculated from the path boundaries plus any pen width, which is correct for enclosing the painted area. However, when the shape result is calculated, it is larger than the bounding rectangle because the selection buffer zone is larger than the drawing area.
Is that a problem? Is it acceptable that the boundingRect does NOT enclose the shape result area? Or do I need to recalculate the boundingRect when I recalculate the shape?
Thank you.
Doug McGrath
QGraphicsItem::shape is used for object collision detection, hit tests and knowing where mouse clicks occur.
In contrast, QGraphicsItem::boundingRect is used when drawing an object, knowing when an object is off the screen, or obscured. As the documentation states for boundingRect: -
QGraphicsView uses this to determine whether the item requires redrawing.
Therefore, the boundingRect should wholly encompass the QPainterPathreturned from the shape function.
Related
Using moveto and lineto to draw various lines on a window canvas...
What is the simplest way to determine at run-time if an object, like a bit map or a picture control is in "contact" (same x,y coordinates) with a line(s) that had been drawn with lineto on a window canvas?
A simple example would be a ball (bitmap or picture) "contacting" a drawn border and rebounding... What is the easiest way to know if "contact" occurs between the object, picture or bitmap and any line that exists on the window?
If I get it right you want collision detection/avoidance between circular object and line(s) while moving. There are more option to do this I know of...
Vector approach
you need to remember all the rendered stuff in vector form too so you need list of all rendered lines, objects etc ... Then for particular object loop through all the other ones and check for collision algebraically with vector math. Like detecting intersection between bounding boxes and then with particular line/polyline/polygon or what ever.
Raster approach
This is simpler to mplement and sometimes even faster but less acurate (only pixel precision). The idea is to clear object last position with background color. Then check all the pixels that would be rendered at new position and if no other than background color present then no colision occurs so you can render the pixels. If any non background color present then render the object on the original position again as collision occur.
You can also check between old and new position and place the object on first non collision position so you are closer to the edge...
This approach need fast pixel access otherwise it woul dbe too slow. Standard Canvas does not allow this without using BitBlt from GDI. Luckily VCL GRaphics::TBitmap has ScanLine[] property allowing direct pixel access without any performance hit if used right. See example of it in your other question I answered:
bitmap rotate using direct pixel access
accessing ScanLine[y][x] is as slow as Pixels[x][y] but you can store all the pointers to each line of bitmap once and then just use that instead which is the same as accessing your own 2D array. So you really need just bitmap->Height calls of ScanLine[y] for entire image rendering after any resize or assigment of bitmap...
If you got tile based scene you can use this approach on tiles instead of pixels something like this:
What is the best way to move an object on the screen? but it is in asm ...
Field approach
This one is also considered to be a vector approach but does not require collision checks. Instead each object creates repulsive force the bigger the closer you are to it which is added to the Newton/D'Alembert physics driving force. When coefficients set properly it will avoid collisions on its own. This is used also for automatic placement of items etc... for more info see:
How to implement a constraint solver for 2-D geometry?
Hybrid approach
You can combine any of the above approaches together to better suite your needs. For example see:
Path generation for non-intersecting disc movement on a plane
I asked a question yesterday about a problem I was having displaying a level in a C++ 2D GameBoy Advance game when the level is larger than the screen size. However, I may have been slightly too specific, and so I want to ask this more generally.
What is the simplest way to go about trying to display a scrolling level which is large (512x512 pixels) on a screen which is much smaller (240x160 pixels)?
A brief description of my code structure so far: I have a base class called Object which defines (x, y) position and a width and height, there is an Entity class which inherits from Object and adds velocity components, and a Character class which inherits from Entity and adds movement functions. My player is a Character object, and boxes I want the player to pick up are an array of Entity objects. Both the player and cubes array are members of the Level class, which also inherits from Object.
So far I have implemented a game which works very well when the level is the same size as the screen - all the objects' positions are stored relative to their position in the level. However, I am having serious trouble trying to work out how to display the objects in the correct place when the level is offset on the screen. I want the viewport to never extend out of the level, and if the viewport is not against the edge of the level, display the player in the middle of the screen.
Should I try to work it out using a couple of simple offset variables to move the background? If so, in what order should the offsets be calculated and applied? How would the offsets differently apply to the player and boxes? Or instead, should I try creating another Object as a member of the Level class for the viewport? How would I go about calculating the offsets using that?
Any advice provided wilil be greatly appreciated.
I'm self learning C++ and playing around 2D tile mapping.
I have been reading through this scrolling post here, which is based on this tiling tutorial.
Using the above tutorial and some help from the Pearson, Computer Graphics with OpenGL book I have written a small program that draws a 40x40 tiled world and a Sprite (also a tile).
In terms of drawing/render order, the map(or world) itself is that back layer and the Sprite is the forward most (or top) layer. I'm assuming that's a good way of doing it as its easier for 2 tiles to interact than a tile and a custom sprite or rectangle. Is that correct?
I have implemented a Keyhandling() function that lets you move the map inside the viewport using the keyboards arrow keys. I have a variable called offsetx, offsety that when a key is pressed increases or decreases. Depending on whether I assign the variable to the map or sprite, I can more one or the other in any direction on the screen.
Neither seems to work very well, so I assigned the variables to both (map and sprite) but with positive values for the sprite, and negative for the map. So upon a key press, this allows my Sprite to move in one direction whilst the map moves in the opposite direction.
My problem is, the sprite soon moves enough to leave the window and not enough to bring the more of the map into the scene. (The window only shows about 1/8th of the tiles at any one time).
I've been thinking all day, and I think an efficient/effective way to solve this issue would be to fix the sprite to the centre of the screen and when a key is pressed the map moves around... I'm unsure how to implement this though.
Would that be a good way? Or is it expected to move the viewport or camera too?
You don't want to move everything relative to the Sprite whenever your character moves. Consider a more complicated world where you also have other things on the map, eg other sprites. It's simplest to keep the map fixed, and move each sprite relative to the map, (if it's a movable sprite). It just doesn't make much sense to move everything in the world whenever your character moves around in the world.
If you want to render your world with your character always at the center, that's perfectly fine. The best thing to do is move the camera. This also allows you to zoom your camera in/out, rotate the camera, etc. with very little hassle in keeping track of all the objects in the world.
Be careful with your usage of the word "viewport". It means a very specific thing in OpenGL. (ie, look at the function glViewport). If your book uses it differently, that's fine. I'm just pointing this out because it's not 100% clear to me what you mean by it.
I have a sprite with a triangle shape and I want to know when this triangle intersects with another object (a CCSPrite). I have been using CGRectIntersectsRect but it is not accurate because it take the bounding box of the the two sprites and not the actual shape.
--Edit
I think one way is to define several points around the actual triangle and check intersection between those points with another sprite. I am just wondering if there is an easier way to do this.
You can use box2d to detect collisions accurately. It can be useful if you have many different complicated shapes. Or you can just check intersections of shape's edges.
If there is many objects to detect collisions with, I offer to use box2d. It has good internal optimizations to be able to work with large amount of objects. In this case you will just have to create physical body equal to the your sprite's shape before adding object to your game layer.
I have been able to find a lot of information on actual logic development for games. I would really like to make a card game, but I just dont understand how, based on the mouse position, an object can be selected (or atleast the proper way) First I thought of bounding box checking but not all my bitmaps are rectangles. Then I thought f making a hidden buffer wih each object having a different color, but it seems ridiculous to have to do it this way. I'm wondering how it is really done. For example, how does Adobe Flash know the object under the mouse?
Thanks
Your question is how to tell if the mouse is above a non-rectangular bitmap. I am assuming all your bitmaps are really rectangular, but they have transparent regions. You must already somehow be able to tell which part of your (rectangular) bitmap is transparent, depending on the scheme you use (e.g. if you designate a color as transparent or if you use a bit mask). You will also know the z-order (layering) of bitmaps on your canvas. Then when you detect a click at position (x,y), you need to find the list of rectangular bitmaps that span over that pixel. Sort them by z-order and for each one check whether the pixel is transparent or not. If yes, move on to the next bitmap. If no, then this is the selected bitmap.
Or you may use geometric solution. You should store / manage the geometry of the card / item. For example a list of shapes like circles, rectangles.
Maybe triangles or ellipses if you have lots of time. Telling that a triangle has a point or not is a mathematical question and can be numerically unstable if the triangle is very thin (algorithm has a dividing).. Fix: How to determine if a point is in a 2D triangle?
I voted for abc.