Comment 29 for bug 182038

Created an attachment (id=300358)
Screenshots: scaling, opaque vs. transparent images

I have performed some science.

Results:

1: When loaded directly into Firefox, transparent images are not drawn at all, irrespective of whether they are scaled or unscaled.

2: Opaque images are always drawn. They are drawn correctly when unscaled, but wrongly when scaled.

3: Typically, scaled opaque images appear fully black; they sometimes show a portion of the picture in the wrong place (but still within the bounds of the image on the screen).

4: The incorrectly-placed portion appears when redrawing the image at a higher scale factor; as the scale factor approaches 100%, the image gradually moves to occupy its proper position.

5: There was no difference observed between GIF, PNG and JPEG images.

6: For all images, the page icon behaves erratically: it usually shows as vertical bars in some combination of black, white and the OS theme's highlight colour (!); changes when the image is redrawn or another tab is selected; and tends towards a grey square. The icon in the tab is not always the same as the icon in the location bar.

The process used to arrive at the results:

1: Four images were created using the Gimp: a transparent PNG, an opaque PNG, a transparent GIF, an opaque GIF. The opaque images were made using the Gimp's Flatten Image command; they have a red background for clarity. The transparent ones' backgrounds are completely transparent. The images are all somewhat taller than will fit in a maximised Firefox window (900 px), so Firefox will scale them they're loaded.

2: The images were all loaded into the latest trunk build of Firefox (Gecko/2008012904) running on Ubuntu Hardy (pre-release, vintage today). When the images were loaded in, the window was at a small size (to allow me to drag them in from the desktop); the content area was roughly 400 px tall.

3: The window was then maximised so the content area was about 540 px tall. This caused each image to be redrawn at a higher magnification (though still much less than 100%).

4: Screenshots were taken of each image in a maximised browser, scaled and unscaled (zoomed in by clicking). For the opaque images, screenshots were taken at two scale factors (plus unscaled): the original scale when the pictures were loaded (in an unmaximised window), and a larger scale <100% (in a maximised window).

(Some of these screenshots, and the images used to make them, are included in the attachment. I have the complete set if anyone's interested.)

5: A smaller opaque PNG image was made, that would fit inside a maximised window unscaled. This was loaded into a small browser window whose height was then varied (and the image allowed to redraw at various scales) until the image fitted unscaled inside the window.

6: A JPEG image was made, loaded and observed scaled and unscaled.