> So the default xfermode param to draw color is kSrcOver. This means 1 *
> Src + (1-Src_Alpha) * Dst. So when you pass 0 for the source alpha this
> simplifies to just Dst. It's as though nothing was drawn. To replace the
> dst color with the src color you'd use the kSrc mode. However, clear() is
> basically a shortcut for drawing a color in kSrc mode while ignoring the
> clip. So it's best to just use clear().

> On Tue, Oct 9, 2012 at 9:07 AM, Anthony Catel <a.c...@weelya.com<javascript:>
> > wrote:

>> Brian,

>> I used canvas->drawColor(SkColorSetARGB(0, 0, 0, 0));

>> On Tuesday, October 9, 2012 2:44:43 PM UTC+2, Brian Salomon wrote:

>>> Anthony, how are you doing the initial fills to white, trans black, and
>>> blue?  When you create an SkGpuDevice, or SkDevice for that matter, the
>>> contents are uninitialized. If you call SkCanvas::clear on both canvas and
>>> canvasBG then you shouldn't see garbage pixels. If so then we have a bug.

>>> Brian

>>> On Mon, Oct 8, 2012 at 9:01 PM, Anthony Catel <a.c...@weelya.com> wrote:

>>>> Brian,

>>>> It looks like there is a problem with alpha pixel when transfering the
>>>> offscreen canvas to the other :

>>>> I create two devices and two canvas using this :

>>>>     [...]
>>>>     SkGpuDevice *devbg = new SkGpuDevice(context, target);
>>>>     SkGpuDevice *dev = new SkGpuDevice(GrContext::Create(**
>>>> kOpenGL_Shaders_GrEngine,
>>>>         (GrPlatform3DContext)**interface), SkBitmap::kARGB_8888_Config,
>>>>         width, height);
>>>>     canvasBG = new SkCanvas(dev);
>>>>     canvas = new SkCanvas(devbg);

>>>> I then fill canvasBG with SK_ColorWhite and canvas with transparent
>>>> black. I do some skia cann to canvas and then :

>>>>     canvasBG->drawBitmap(canvas->**getDevice()->accessBitmap(**false),
>>>> 0, 0, NULL);

>>>>     canvasBG->flush();

>>>> The drawing is working but all the transparent pixels from canvas are
>>>> displayed as garbage pixels (like unitialized memory) though. However if I
>>>> fill the same canvas with blue instead of transparent at the begining, it's
>>>> working well (blue color instead of garbage values). Setting it with
>>>> semi-alpha-blue displays transparent blue blended with garbage values.

>>>> I added an offset to drawBitmap to see if the garbage value are from
>>>> the background. It's not.

>>>> Here here the result for canvasBG->drawBitmap(canvas->**
>>>> getDevice()->accessBitmap(**false), 30, 30, NULL); with canvasBG to
>>>> white and canvas with semi-alpha-blue + drawRect :

>>>> http://i.imgur.com/EgIvS.png

>>>> Strange :s

>>>> On Tuesday, October 9, 2012 1:23:10 AM UTC+2, Anthony Catel wrote:

>>>>> Thanks for your patience. I'll try this solution.
>>>>> (FWIW, I've "resolved" my initial issue by unsetting the current
>>>>> shader (glUseProgram(0)). I'm now seeing my texture with some distortion
>>>>> :)).

>>>>> On Tuesday, October 9, 2012 1:02:10 AM UTC+2, Brian Salomon wrote:

>>>>>> If you create an SkGpuDevice using the constructor that takes a
>>>>>> config, width, and height then it will be FBO-backed. You can then install
>>>>>> that device on the layerCanvas using SkCanvas::setDevice. It should just
>>>>>> work and you don't need to call SkCanvas::flush when you switch from one
>>>>>> canvas to the other.
>>>>>> On Oct 8, 2012 5:44 PM, "Anthony Catel" <a.c...@weelya.com> wrote:

>>>>>>> this is the idea. But layerCanvas can't be backed by another GL
>>>>>>> device right? Or may be if :

>>>>>>> - mainCanvas is attached to a GL device on the window framebuffer
>>>>>>> - layerCanvas is attached to the same GL context but attached to an
>>>>>>> FBO
>>>>>>> - mainCanvas->drawBitmap(**layerCa**nvas->getDevice()->**accessBitma
>>>>>>> **p(false)) will tell OpenGL to draw layerCanvas texture onto
>>>>>>> mainCanvas buffer without performing CPU operation?

>>>>>>> On Monday, October 8, 2012 11:22:51 PM UTC+2, Brian Salomon wrote:

>>>>>>>> I'm not totally sure I get what you're trying to do but you can
>>>>>>>> draw from one canvas to another:

>>>>>>>>    layerCanvas->clear();
>>>>>>>>     layerCanvas->draw();
>>>>>>>>    mainCanvas->drawBitmap(**layerC****anvas->getDevice()->**
>>>>>>>> accessBitm****ap(false)).
>>>>>>>>    // layerCanvas's contents are preserved here

>>>>>>>> On Mon, Oct 8, 2012 at 5:18 PM, Anthony Catel <a.c...@weelya.com>wrote:

>>>>>>>>> Or can I do this : ?

>>>>>>>>> - create a new layer;
>>>>>>>>> - draw to the new layer
>>>>>>>>> - composite it to the root layer
>>>>>>>>> - draw on the same layer
>>>>>>>>> - compose
>>>>>>>>> - etc...

>>>>>>>>> So that the layer and its memory are not lost like on the
>>>>>>>>> saveLayer()/restore() flow.

>>>>>>>>> On Monday, October 8, 2012 11:10:01 PM UTC+2, Anthony Catel wrote:

>>>>>>>>>> Already tried to use saveLayer but the performances
>>>>>>>>>> are undermined. I have to do one saveLayer per frame.
>>>>>>>>>> Because :

>>>>>>>>>> - I need xfer modes to be applied like I'm on a transparent
>>>>>>>>>> surface
>>>>>>>>>> - I want a white background.

>>>>>>>>>> I can't find a reliable solution for this.

>>>>>>>>>> The flow would be :

>>>>>>>>>> loop() { // frame loop
>>>>>>>>>>   saveLayer();
>>>>>>>>>>   Skia Call 1;
>>>>>>>>>>   Skia Call 2;
>>>>>>>>>>   ...
>>>>>>>>>>   restore() /* and compose to a white background */
>>>>>>>>>> }

>>>>>>>>>> On the second frame I want to keep my object on the screen with
>>>>>>>>>> the transparent background. The next saveLayer() would be unaware of what
>>>>>>>>>> is already on the screen unless I redraw what's on the screen after the
>>>>>>>>>> saveLayer() which seems to be very expensive. SkPicture() is maybe a
>>>>>>>>>> solution?

>>>>>>>>>> Using FBO seems less painful to me.

>>>>>>>>>> Anthony

>>>>>>>>>> On Monday, October 8, 2012 11:02:02 PM UTC+2, Brian Salomon wrote:

>>>>>>>>>>> On Mon, Oct 8, 2012 at 4:46 PM, Anthony Catel <a.c...@weelya.com
>>>>>>>>>>> > wrote:

>>>>>>>>>>>> That's what I also though.

>>>>>>>>>>>> Basically what I'm trying to achieve is :

>>>>>>>>>>>> Composing a layer with some transparent part onto a white (or
>>>>>>>>>>>> any other color) surface. Something like a browser's <canvas> element would
>>>>>>>>>>>> do on the document body surface.
>>>>>>>>>>>> I could't achieve this efficiently using skia API (I have to
>>>>>>>>>>>> keep a decent frame rate). Do you think using FBO is a good plan for this
>>>>>>>>>>>> task? Or is there a Skia API in order to manage layers I'm not aware of?

>>>>>>>>>>> Is the stuff underneath the layer also rendered by Skia? If so
>>>>>>>>>>> you can use SkCanvas's saveLayer mechanism. It switches the canvas to an
>>>>>>>>>>> offscreen target and draws it back to the main target when
>>>>>>>>>>> SkCanvas::restore is called. Check out the code comment in SkCanvas.h at
>>>>>>>>>>> line 267.

>>>>>>>>>>>> Thanks,

>>>>>>>>>>>> Anthony

>>>>>>>>>>>> On Monday, October 8, 2012 10:38:50 PM UTC+2, Brian Salomon
>>>>>>>>>>>> wrote:

>>>>>>>>>>>>> Skia will definitely mess with the OpenGL state quite a bit.
>>>>>>>>>>>>> It will change the bound textures, program, array buffers, enable/disable
>>>>>>>>>>>>> states, etc. Since you're using GL_QUADS it sounds like you're on a non-ES
>>>>>>>>>>>>> ("desktop") OpenGL. You could use glPush[Client]Attrib to save state before
>>>>>>>>>>>>> invoking Skia and then pop it afterwards. The other options are to give
>>>>>>>>>>>>> Skia its own GL context (with shared ids) or make your other GL code able
>>>>>>>>>>>>> to cope with the Skia's state changes.

>>>>>>>>>>>>> Brian

>>>>>>>>>>>>> On Mon, Oct 8, 2012 at 4:22 PM, Anthony Catel <
>>>>>>>>>>>>> a.c...@weelya.com> wrote:

>>>>>>>>>>>>>> Hey,

>>>>>>>>>>>>>> I'm trying to draw into an FBO instead of directly drawing
>>>>>>>>>>>>>> into the window frame buffer.

>>>>>>>>>>>>>> So what I've done :

>>>>>>>>>>>>>> - Generate a framebuffer object
>>>>>>>>>>>>>> - Generate a texture
>>>>>>>>>>>>>> - Attach the texture to the FBO
>>>>>>>>>>>>>> - Bind the FBO
>>>>>>>>>>>>>> - Pass the FBO id to skia : desc.fRenderTargetHandle =
>>>>>>>>>>>>>> fbo_handle;
>>>>>>>>>>>>>> [...]
>>>>>>>>>>>>>> - Do usual skia calls + SkCanvas::flush.
>>>>>>>>>>>>>> - Unbind the FBO (glBindFramebuffer(GL_**FRAMEBUF********FER,
>>>>>>>>>>>>>> 0)) + glBindTexture(GL_TEXTURE_2D, fbo_texture);
>>>>>>>>>>>>>> - Trying to draw a GL_QUADS using the texture : I got either
>>>>>>>>>>>>>> a black screen or a red screen depending on what I do with skia. But the
>>>>>>>>>>>>>> texture is not drawn.

>>>>>>>>>>>>>> Using gDebugger I can see that the FBO's texture is filled
>>>>>>>>>>>>>> with what I want but I'm unable to paint it on a rectancle. What Am I doing
>>>>>>>>>>>>>> wrong? Does skia enable some state I should disable before painting the
>>>>>>>>>>>>>> texture?

>>>>>>>>>>>>>> Here is a screen of gDebugger with my rendered texture :
>>>>>>>>>>>>>> http://p.nf/debug.png
>>>>>>>>>>>>>> FBO 1 is the one I created. Others are created by Skia I
>>>>>>>>>>>>>> guess.

>>>>>>>>>>>>>> Thanks !

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