Canvas efficiency
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From: Jeff Godfrey on 30 Apr 2010 11:26 I'm using a canvas as the core display widget for a geometry viewer. The app supports panning and zooming of the canvas contents. One supported zoom method allows the User to drag out a rectangle over the area to be zoomed, which is then scaled to fit within the canvas view. With large data sets (say, upwards of 25,000 canvas entities), I notice a delay when dragging the zoom rectangle out over the canvas contents. The delay makes the whole operation seem "sluggish", as if running through waist-deep water. The magnitude of the effect is directly related to the density of canvas items at the current mouse location (more items = deeper water). I assume this is related to the scanning that must be going on under to covers in order to keep the item tagged as "current" up-to-date. So, my questions: 1. Am I correct regarding the reason for the delay? 2. Do other factors play a large role? If so, what are those factors? Assuming my guess is correct, it'd be nice to control when the canvas does the automatic scanning. For instance, during a zoom drag operation, I don't care about the current item, and I can't really afford the CPU cycles needed to find it. If I could, I'd turn the scanning off at the beginning of the zoom operation and turn it back on at the end. AFAIK, that's not currently possible, but I wonder if it might be a worthy feature suggestion? Thoughts? Any other suggestions to improve the situation with existing capabilities are appreciated. Heck, if it were possible, I'd think overlaying a temporary, transparent rectangle on top of the data might work nicely too (as it'd always be "current"), but again, AFAIK, I can't do that today. Thanks, Jeff
From: Jeff Godfrey on 30 Apr 2010 11:53 On 4/30/2010 10:26 AM, Jeff Godfrey wrote: > The magnitude of the effect is directly > related to the density of canvas items at the current mouse location > (more items = deeper water). Hmmm... After a bit more testing, the above isn't quite accurate. It seems the effect is more related to amount of canvas geometry found "within" the rectangle I'm dragging out. As the rectangle surrounds more geometry, the effect gets worse. So, now I'm wondering whether the update of the "current" item is even related, since I can make the effect much worse without ever having any geometry at the mouse position. My code is doing nothing but manipulating the coords of the single rectangle during the drag, so I don't think the problem is at the script level. Jeff
From: Alexandre Ferrieux on 30 Apr 2010 12:21 On Apr 30, 5:53 pm, Jeff Godfrey <jeff_godf...(a)pobox.com> wrote: > On 4/30/2010 10:26 AM, Jeff Godfrey wrote: > > > The magnitude of the effect is directly > > related to the density of canvas items at the current mouse location > > (more items = deeper water). > > Hmmm... After a bit more testing, the above isn't quite accurate. It > seems the effect is more related to amount of canvas geometry found > "within" the rectangle I'm dragging out. As the rectangle surrounds > more geometry, the effect gets worse. > > So, now I'm wondering whether the update of the "current" item is even > related, since I can make the effect much worse without ever having any > geometry at the mouse position. > Indeed, this most likely is not the 'current' detection that's hitting you most, but rather the redisplay of the tiny part of the canvas that is marked dirty after you move the rectangle a bit. This operation involves detecting all items intersecting the dirty area, which means a big scan... As a side note, the BLT graph (regardless of the state of the project) is admittedly one of the rare things that can display tens of thousands of dots/lines on a canvas quickly. Individual canvas items don't scale up. If BLT is not an option and you can restrict yourself to single pixels or rectangular dots, then a bitmap/photo image may be the way to go. You can overlay it with your selection rectangle and it will refresh in no time. -Alex
From: Jeff Godfrey on 30 Apr 2010 12:43 On 4/30/2010 11:21 AM, Alexandre Ferrieux wrote: > On Apr 30, 5:53 pm, Jeff Godfrey<jeff_godf...(a)pobox.com> wrote: >> On 4/30/2010 10:26 AM, Jeff Godfrey wrote: >> >>> The magnitude of the effect is directly >>> related to the density of canvas items at the current mouse location >>> (more items = deeper water). >> >> Hmmm... After a bit more testing, the above isn't quite accurate. It >> seems the effect is more related to amount of canvas geometry found >> "within" the rectangle I'm dragging out. As the rectangle surrounds >> more geometry, the effect gets worse. >> >> So, now I'm wondering whether the update of the "current" item is even >> related, since I can make the effect much worse without ever having any >> geometry at the mouse position. >> > > If BLT is not an option and you can restrict yourself to single pixels > or rectangular dots, then a bitmap/photo image may be the way to go. > You can overlay it with your selection rectangle and it will refresh > in no time. Alex, Thanks for the input (as usual)... ;^) Currently, I'd have to say that BLT is not an option, but I'm not following your above suggestion. Do you mind trying again? Thanks, Jeff
From: Will Duquette on 30 Apr 2010 15:13
On Apr 30, 9:43 am, Jeff Godfrey <jeff_godf...(a)pobox.com> wrote: > On 4/30/2010 11:21 AM, Alexandre Ferrieux wrote: > > > > > > > On Apr 30, 5:53 pm, Jeff Godfrey<jeff_godf...(a)pobox.com> wrote: > >> On 4/30/2010 10:26 AM, Jeff Godfrey wrote: > > >>> The magnitude of the effect is directly > >>> related to the density of canvas items at the current mouse location > >>> (more items = deeper water). > > >> Hmmm... After a bit more testing, the above isn't quite accurate. It > >> seems the effect is more related to amount of canvas geometry found > >> "within" the rectangle I'm dragging out. As the rectangle surrounds > >> more geometry, the effect gets worse. > > >> So, now I'm wondering whether the update of the "current" item is even > >> related, since I can make the effect much worse without ever having any > >> geometry at the mouse position. > > > If BLT is not an option and you can restrict yourself to single pixels > > or rectangular dots, then a bitmap/photo image may be the way to go. > > You can overlay it with your selection rectangle and it will refresh > > in no time. > > Alex, > > Thanks for the input (as usual)... ;^) > > Currently, I'd have to say that BLT is not an option, but I'm not > following your above suggestion. Do you mind trying again? > > Thanks, > > Jeff Well, you could try this: 1. Using Img, you can capture the visible area of the canvas as a Tk image. 2. Delete the contents of the canvas. (You're going to redraw it anyway, once you get the rectangle, right?) 3. Position the Tk image on the visible area of the canvas. Now there's no geometry to slow things down. 4. When they've selected the rectangle, delete the image, and redraw. |