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Bug 361615

Summary:	[Canvas] Implement SWT.NO_BACKGROUND to avoid clearing the canvas rectangle
Product:	[RT] RAP	Reporter:	Arian Storch <storch>
Component:	RWT	Assignee:	Project Inbox <rap-inbox>
Status:	ASSIGNED ---	QA Contact:
Severity:	enhancement
Priority:	P3	CC:	linuxhippy, tbuschto
Version:	unspecified
Target Milestone:	---
Hardware:	All
OS:	All
Whiteboard:
Bug Depends on:	370313
Bug Blocks:

Description Arian Storch

2011-10-21 04:23:45 EDT

Build Identifier: 

Everytime the canvas is rendered, it's first clearing its rectangle. This is not a good default strategy, if you want to perform selective drawing updates later.

Special case:
I render a grid which leads to more than 4000 drawing commands the first time the canvas is painted. Later (because of actions) only special parts of the canvas need to be redrawn. But this selective drawing is senseless because the whole canvas is cleared and filled with a background color.

Reproducible: Always

Steps to Reproduce:
Look at details

Comment 1 Tim Buschtoens

2011-10-21 04:33:46 EDT

So you would like to draw on top of everything else, never clearing anything you have drawn. Is that correct? Could you use two Canvas on top of each other, one for unchanging background, the other for dynamic content?

Comment 2 Arian Storch

2011-10-21 04:38:04 EDT

(In reply to comment #1)
> So you would like to draw on top of everything else, never clearing anything
> you have drawn. Is that correct? Could you use two Canvas on top of each other,
> one for unchanging background, the other for dynamic content?

This could be a solution. But nevertheless the foreground canvas needs to have
a transparent background otherwise the background canvas isn't visible.

Comment 3 Tim Buschtoens

2012-02-01 05:42:55 EST

This would be very useful for clientside scripted painting. The only issue i see is that for IE7/8, the number of shapes would continue growing. To solve this, IE could detect if a rectangle is drawn over the entire area, effectively clearing the canvas.

Comment 4 Ivan Furnadjiev

2016-03-01 05:47:36 EST

Arian, could you try RAP 3.1M5? We don't clear the complete canvas anymore if redraw(x, y, width, heigth) method is used.

Comment 5 Linuxhippy

2017-02-11 17:11:19 EST

I guess I am also bitten by this issue ... I am currently trying to write a widget and ran into this issue too. I would like to do most of the rendering on the server, but allow client-side selection of objects in a MouseDown ClientListener (overdraw the content created on the server-side).

Inside the MouseDown handler I do not have access to a gc (event.gc is null), so I call redraw() there - which clears the canvas no matter what arguments I pass and regardless of SWT.NO_BACKGROUND set.

I worked arround the issue by preserving the GC during paint, and performing draw calls inside the mouse handler, but I would prefer a proper way of solving this issue...