If text overlays an image (photo), then in Slide Show the characters of text may be surrounded by sprinkles of colored pixels. These speckles fall mostly along horizontal and vertical lines; I speculate that they may be related to the edges of the jpeg blocks.
This problem exists in 18.104.22.168 and 22.214.171.124. It does not exist in 126.96.36.199 and 188.8.131.52. I have not tried any other versions.
I will attach a sample presentation file that shows the problem, along with a couple of screen captures that illustrate what I see.
* The speckles appear for all of the fonts that I have tried, and for normal, bold, italic, and bold-italic versions of those fonts.
* Interestingly, the speckles do not appear for outline fonts.
* The appearance of the speckles, and their color, do not depend on the color of the text characters.
* The speckles depend on the "darkness" of the image. If the text is over a white or nearly white area, there are no visible speckles. They are conspicuous over a cloudy blue sky. If the background is very dark and with much detail (like a forest scene), there may be speckles, but they won't be obvious.
Steps to Reproduce:
1. On a new slide, insert an image. I used a jpeg photo, slightly compressed but still with high quality. Enlarge the image if necessary to fill, or mostly fill, the slide.
2. Draw a rectangle over an appropriate area of the image. ("Appropriate area" means not completely white, and not absolutely smooth and featureless, but still light enough to see speckles.) Set the fill pattern of the rectangle to None, so that the image underneath is completely visible.
3. Type some text into the rectangle. Select all of the text and enlarge the point size to something big -- maybe 80 or more. If desired, change the font or color.
4. Start the slide show. In one of the affected versions, speckles should be visible around the text characters.
See screen captures, attached.
User Profile Reset: No
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36
Created attachment 136926 [details]
Impress presentation that shows the speckle problem.
In "Speckled text 2.odp", the first slide is a photo with a title in orange text. In Slide Show, purple speckles appear around all of the characters.
The second slide is identical, except that part of the text is outline characters; in Slide Show, there are no speckles around those characters.
The third slide has a different image, different font, and different color, but purple speckles still appear in Slide Show.
The fourth slide has a sky color that is nearly white, and the text title overlays both the sky and the forested mountains. In Slide Show, the speckles appear (and are not easy to see) only over the mountainsides, not over the sky.
Created attachment 136927 [details]
Screen capture from Slide Show of first slide in example presentation under version 184.108.40.206.
Purple speckles are obvious around the orange text.
Created attachment 136928 [details]
Screen capture from Slide Show of first slide in example presentation under version 220.127.116.11.
Purple speckles are obvious around the orange text. Interestingly, they are not identical to the speckles in 18.104.22.168, even though the presentation file is exactly the same.
Created attachment 136929 [details]
Screen capture from Slide Show of first slide in example presentation under version 22.214.171.124.
No speckles are visible. (This is also true under 126.96.36.199.)
You don't list Windows version or Graphics card, nor the rendering mode you've tested the presentation in (OpenGL, GPU Hardware Accelerated, CPU only) but likely issue bug 107090
We've temporarily reverted from DirectWrite Direct2D rendering to GDI based rendering for Hardware Acceleration.
Please test with an 188.8.131.52 build ( http://dev-builds.libreoffice.org/pre-releases/win/ ) or current development master.
My symptoms are quite different from bug 107090 -- cleanly formed text characters, but surrounded by scattered pixels if displayed over image -- but running under 184.108.40.206 fixed it, so I suspect you have it.
To answer your questions (apologies for omitting important info), I'm running Windows 10 Pro (10.0.15063), although LibreOffice Help About says OS Version Windows 6.2. Under Graphics Output, "Use hardware acceleration" and "Use OpenGL for all rendering" are both checked.
I'm happy to provide more info or do further testing.
Sorry. Graphics card is Intel HD Graphics. This is a plain-vanilla system.
I went back to 220.127.116.11. Turning off "Use hardware acceleration" there fixed the problem. Turning off "Use OpenGL" did not.
For what it's worth.
The "Use Hardware acceleration" and "Use OpenGL for all rendering" are mutually exclusive. Enabling one will disable the other (a UI glitch if the HA button shows checked when OpenGL is enabled).
So, it is working for you with OpenGL rendering, and also when HA is disabled (so just CPU rendering.
Thats a duplicate of bug 107090 -- poor quality rendering of the presentation canvas (bad lettering, and color artifacts like this).
Reverted in 18.104.22.168, and the 22.214.171.124 build as well as in current master.
*** This bug has been marked as a duplicate of bug 107090 ***
Thanks for the explanation, and for quickly checking on my report.