fillGradientRectangle() needs to be refactored

[arunjose696]

Description

While looking into the the method fillGradientRectangle() in org.eclipse.swt.graphics.ImageData appears to require refactoring.

Currently, this method:

• Creates an ImageData instance with a fixed width and height.
• Constructs an Image at 100% zoom scale.
• Repeatedly scales it up/down using DPIUtil.autoScaleDown() in GC.drawImage() calls.

Expected Behavior
This method should be refactored for completenes to avoid Image constructors with ImageData (as a part of #199 )and the current impleementation is problematic as it scales images created at 100x zoom based on conditions. Since this method is part of the SWT common layer and used on all platforms (Windows, Linux, macOS), this would have to be tested on all platforms once the refactoring is complete

[HeikoKlare]

Good catch!
Another issue I see in this method is the usage of autoScale() methods to determine the width/height of the bands to draw. Due to rounding issues this may easily leads to wrongly painted areas, such as areas that are accidentially not drawn in particular at fractional scales like 125% or 175%.
May that also be the reason for the artifact lines we, e.g., see in the GEF palette at 175% when hovering over elements (@akoch-yatta)? At least that is some kind of filled gradient area, so it might be drawn by this method. If that's the case, I would vote for this to be prioritized "high" (as it visibly affected GEF), otherwise I would vote for "medium".

In any way, this method can obviously benefit from some improvements regarding scaling and multi-zoom support. And maybe this can even go beyond refactoring and actually improve the results of gradient-filled rectangles.

[akoch-yatta]

Good point. As of now I wasn't aware of any issues caused by this, but this could be caused by this. If so, I agree with high prio.

[HeikoKlare]

To test with GEF, one should check out the GEF Classic repository: https://github.com/eclipse-gef/gef-classic
Then import the org.eclipse.draw2d, org.eclipse.gef and the org.eclipse.gef.examples.logic (the latter for testing) to your Eclipse development workspace and add them to the product you are testing. With the "New" wizard, you can create a logic example to test a GEF editor:

[arunjose696]

To test with GEF, one should check out the GEF Classic repository: https://github.com/eclipse-gef/gef-classic Then import the org.eclipse.draw2d, org.eclipse.gef and the org.eclipse.gef.examples.logic (the latter for testing) to your Eclipse development workspace and add them to the product you are testing. With the "New" wizard, you can create a logic example to test a GEF editor:

I tried testing this with the logic diagram, but on Windows, the code does not pass through this function. The check returns true, which causes the method to be bypassed.

I've created a PR with the refactoring. For my own testing, I manually disabled this check on Windows and confirmed that the behavior remains the same.