Make `widget_matrix` scrollable

[abonander]

I'm having a hard time thinking of how to describe it, but I think widget_matrix could be made scrollable by having two sets of dimensions and then moving it under a static mask:

It could save processing power by culling items that don't fall at least partially within the visible area, and then mask the edges so they don't bleed over. A separate UI element (slider, perhaps?) could control the scroll, setting the scroll amount on the matrix in terms of % or current out of maximum. This method could also trivially be implemented for horizontal scrolling.

[mitchmindtree]

Yep, I was thinking something along these lines also! If I find an example in the wild I'll link it in here.

This one will be pretty tricky and will require some change to the way all widget's are drawn in general.

At the moment the draw request is given to the Gl type directly in the .draw method uniquely for each rectangle/glyph. Rather than directly drawing, perhaps we should be building a single texture for each widget which we can then mask etc. This would also mean we could cache the texture and just continue to use the same texture for all widgets whose state does not change, saving a lot of processing while we're at it. I think this may also help us when it comes to dealing with the widget overlap issue (discussed a bit here #145).

I might make another issue for implementing and refactoring towards using a TextureCache.

Just a heads up, I have very little experience with graphics/textures so to anyone reading this feel free to shoot me down or improve upon my ramblings :)

#82 is related

[abonander]

Is it possible to implement masking in a fragment shader that simply doesn't change pixels that are outside the mask?

[abonander]

Alternative: render the widget_matrix's contents to a texture, apply the mask, then render the texture?

[mitchmindtree]

re Alternative: yeah that's where I was going with the whole TextureCache thing :)

re masking in a fragment shader: I've no idea sorry!

[bvssvni]

@cybergeek94 Masking through a fragment shader should be possible!

Other possibilities: Using the depth buffer or triangle vs triangle clipping. The depth buffer has the advantage that it does not require changes to the shader, while triangle clipping will work with software rasterization back-ends.

This is quite a hard problem, and we have to think about whether we want to use clipping shapes other than rectangles.

[abonander]

I did some Googling and found that OpenGL has something called a Write
Mask, but it only seems capable of blocking writes to one or several color
channels across an entire framebuffer and has no way to block individual
pixels.

I'm all for rendering to a texture but I'm wondering why it wouldn't let us
animate widgets.
On Dec 9, 2014 10:13 AM, "Sven Nilsen" notifications@github.com wrote:

@cybergeek94 https://github.com/cybergeek94 Masking through a fragment
shader should be possible!

Other possibilities: Using the depth buffer or triangle vs triangle
clipping. The depth buffer has the advantage that it does not require
changes to the shader, while triangle clipping will work with software
rasterization back-ends.

This is quite a hard problem, and we have to think about whether we want
to use other clipping shapes than rectangles.

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#207 (comment)
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[bvssvni]

The idea of rendering to texture was to save operations when widgets are not interacted with. However, if a widget is animated then the texture will stay the same.

[mitchmindtree]

This should be possible now as all conrod widgets support scrolling their children widgets. I think the neatest way to do this might be to instantiate a scrollable Canvas widget as the parent (aka "container") of a WidgetMatrix.