Fix white gap between image and border

[cfarrow]

Fixes #218. I have manually tested this with the 'qt4.image', 'qt4.qpainter', and 'wx.image' backends on OSX.

I seen elsewhere the center-pixel adjustment used here, but I am uncertain about the details of why it is needed. For example, the ColorBar does not need this adjustment. (Is there something wrong with the kiva.agg.GraphicsContextArray used in the ImagePlot?)

@rkern, would you mind reviewing this?

[coveralls]

Coverage decreased (-0.01%) when pulling 0557606 on fix/image-draw-area into 4961533 on master.

[cfarrow]

I have manually verified this on Windows as well.

[rkern]

Can you do a quick test with a checkerboard pattern (i.e. alternating pixels black and white)? The last time I fixed this kind of thing elsewhere, that was a good visual check (ha ha) to determine if I didn't make another fencepost error. The math looks correct, though.

[cfarrow]

Good call on the test image. I was unnecessarily increasing the image size. The test image is below. There green inner border should be visible around the entire perimeter, and it is.

[coveralls]

Coverage decreased (-0.0%) when pulling 5d2bd65 on fix/image-draw-area into 4961533 on master.

[itziakos]

Is this true for all GraphicsContexts?

[cfarrow]

Empirically, it is true for the GraphicsContextArray. I rooted around the kiva source code to find a definitive answer, but gave up.

[rkern]

You will need to try other GCs to be sure that this is the right place to do this adjustment. The different backends can behave differently, albeit unintentionally. We have been fighting these off-by-(one/half) issues for a long time.

[rkern]

Sorry, I meant that literally every pixel should alternate. Each square should be exactly one pixel. If you got the boundaries correct, then you will not see any aliasing. Otherwise, you will see some neat Moiré effects.

[cfarrow]

I started going in that direction, but quit because the aliasing was making things difficult. :)

I'll try that.

[cfarrow]

This fix improves the visual appearance of image plots. The story for other graphics context varies (see below). The conclusion is that this improves on-screen image plots and off-screen renderers are no worse off with this change.

kiva.agg.GraphicsContextSystem

Before fix with no image scaling:

After fix with no image scaling:

With image scaling (before):

And after:

chaco.svg_graphics_context.SVGGraphicsContext

These were converted to png using qlmanage.

No scaling (before):

No scaling (after):

With scaling (before):

With scaling (after):

chaco.plot_graphics_context.PlotGraphicsContext

No scaling (before):

No scaling (after):

With scaling (before):

With scaling (after):

[cfarrow]

The script I used to generate the data is here.

[rkern]

The Quartz backend was the one I was most concerned with since that is the one that has tended to be off-by-half/one compared to the Agg backend. However, it also looks improved. 👍

[cfarrow]

Thanks!