> On 26 Oct 2015, at 10:52, Kimmo Lindholm <kimmo.lindh...@eke.fi> wrote: > > I think I found a way to do this. > > I draw the painting tool to a separate preview canvas, and use this as a > alpha-mask input for the shader. Then combine this with processed drawing > canvas contents and view the shader output in realtime. > When user accepts the result, the whole thing is grabbed as an image and > drawn on top of the main drawing canvas. > > PoC with simple shader that swaps RGB --> BRG channels on user selected > area:https://twitter.com/LiKimmo/status/658559356221440000/photo/1 > > shader code: gl_FragColor = texture2D(source, qt_TexCoord0).brga * > texture2D(mask, qt_TexCoord0).a; > where source is whole drawing canvas, mask is preview canvas where the actual > drawing happens – hidden during this process.
Nice one :) > > grapToImage is still slow, but that delay is suffered only when applying the > results. Maybe this is place for a Qt feature request, Item::toCanvasImageData Yeah grabToImage() is pretty costly due to the pixel readback. What you want is to draw your ShaderEffect into a texture and then draw that texture directly onto the canvas without going through the pixel readback / CPU memory code path that grabToImage would imply. Not sure how exactly we would swing that with the current Canvas design (at least if we take threaded/non-threaded and Image codepaths into account), but feel free to open a suggestion for that on bugreports.qt.io. Something might happen :) > > -kimmo > > From: devel-boun...@lists.sailfishos.org > [mailto:devel-boun...@lists.sailfishos.org] On Behalf Of Kimmo Lindholm > Sent: 26. lokakuuta 2015 8:55 > To: devel@lists.sailfishos.org > Subject: [SailfishDevel] Drawing ShaderEffect to Canvas > > Hi, > > This relates to my Paint app. Doing tools to modify image, blur, invert, > colorchannel swap etc. from area under your finger, like realtime brush. > > Doing the bitmanipulation with javascript in canvas-onPaint works but even > simple things like invert are utterly slow. > > Then I decided to give ShaderEffect a try. I can successfully get image data > under mouse from Canvas; > > ShaderEffectSource { > sourceItem: myCanvas > sourceRect: Qt.rect(x,y,w,h) > … > > The shader outputs the selected part of canvas nicely on screen with > fragmentShader applied. Looks great. > > Now I would like to draw the shader output to my 2nd canvas, which is later > drawn on top of the actual canvas if accepted by user. > > But putImageData(myShader,x,y) does not draw on canvas, and does not give any > error. (other trials complain that ImageDataObject is invalid) > > I managed to do this by calling myCanvas.grabToImage(…) and > drawImage(result.url) but this is again as slow as the Javascript > implementation. > Nothing that I can use on realtime-editing. This could be used after masking > an area where to apply the shader. > > If someone has any ideas? > > -kimmo > > _______________________________________________ > SailfishOS.org Devel mailing list > To unsubscribe, please send a mail to devel-unsubscr...@lists.sailfishos.org _______________________________________________ SailfishOS.org Devel mailing list To unsubscribe, please send a mail to devel-unsubscr...@lists.sailfishos.org
