Correctly calculate GLViewport with Framebuffer

[felixpalmer]

For #6552

Background

When passing a Framebuffer as a target the GLViewport calculated assumed the width and height to be equal to the GL canvas

Change List

• When a custom target is passed to the layers pass, use its width&height rather than that of the canvas
• When the moduleParameters specify a pixel ratio, respect this when calculating the GL viewport

[Pessimistress]

If I understand this approach correctly, it expects the application to calculate and pass a devicePixelRatio, which is not straightforward, yet it still doesn't account for the case where the framebuffer's aspect ratio does not match the canvas.

The canvas's aspect ratio is never wrong because we base the dimensions of viewport on the canvas's size. The same cannot be said for an external framebuffer.

I think it would be more robust to take the dimensions of the canvas and "stretch to fit" into the dimensions of the external framebuffer.

[coveralls]

Coverage increased (+0.01%) to 80.471% when pulling 57c876a on felix/layer-pass-framebuffer into da1a3ab on master.

[felixpalmer]

This does work for a external framebuffer as the dimensions (and thus aspect) are based on the viewport that is passed in. I have it working with a square framebuffer in #6554

The moduleParameters.devicePixelRatio is optional and is used when rendering into the framebuffer. I need this when rendering to a fixed size off-screen framebuffer as otherwise the y-axis inversion is wrong. If I cannot override this then it becomes difficult to construct a projectionMatrix that I can use when reading the framebuffer later.

I think it is reasonable to expect that the following works

const target = new Framebuffer(gl, {width, height});
const viewport = new WebMercatorViewport({width, height});
deck.deckRenderer.renderLayers({
  target,
  viewports: [viewport],
  moduleParameters: {devicePixelRatio: 1},
  ...
});

_getModuleParameters already accepts the passed in moduleParameters as an override and these then get passed onto the shaders. It just happens to be the case that getGLViewport ignores this and hardcodes the pixel ratio to the cssToDeviceRatio default.

I can see that the stretch-to-fit approach would work for the case of rendering the current canvas into a framebuffer, but if I just want to render a fixed view into a fixed framebuffer why should this be dependent on the canvas pixel ratio & size?

Does this make sense? Perhaps I'm misunderstanding what you mean by "stretch-to-fit"?

[Pessimistress]

Yes it makes sense. Thank you for the clarification.