changing opacity/transparency of an image

[Erin]

I am trying to build an experiment where I incrementally "black out" an image displayed on a black background.  I want to be able to set the image opacity from 0% to 100% in 2% increments.  I will keep the image opacity level constant within a session, but want to be able to change it easily between sessions.  I know I could achieve this in Photoshop by putting a black background behind my image, adjusting the image's opacity, and re-saving the image.  However, I want to be able to draw randomly from a very large bank of images for each stage of the experiment, and it would be prohibitively impractical to adjust all the images manually in Photoshop for each opacity level.

I found a post in this forum on how to fade between two different colors (How to fade from slide1 to slide2 (inline?)), however I haven't been able to find any information on adjusting  opacity/transparency.  Any help would be very appreciated.  Thank you!

Erin

[Larsen, Jeff]

I don't have access to my computer for a couple weeks, but we did something similar a few years ago. Basically we made a canvas that contained the picture but randomly replaced x% of the picture's pixels with black dots with the setpixel function.

Sent from my iPhone

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[David McFarlane]

Erin,

See also http://en.wikipedia.org/wiki/Alpha_compositing for
theoretical background, from which you could derive specific
programming steps for E-Prime or any other suitable programming platform.

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David McFarlane

[Paul Groot]

Erin,

 

Since EPrime doesn't have a builtin transparency option, this would be something you will have to implement yourself. The most straightforward way would be to manipulate an offscreen bitmap using the GetPixel/SetPixel functions. However, using EBasic this way would take ages. Another option would be to have an external DLL-function to do all the work (using optimized C++ code for example), but unfortunately this requires serious programming skills.

 

best,

paul

2012/8/7 Erin <erin.s...@gmail.com>

I am trying to build an experiment where I incrementally "black out" an image displayed on a black background.  I want to be able to set the image opacity from 0% to 100% in 2% increments.  I will keep the image opacity level constant within a session, but want to be able to change it easily between sessions.  I know I could achieve this in Photoshop by putting a black background behind my image, adjusting the image's opacity, and re-saving the image.  However, I want to be able to draw randomly from a very large bank of images for each stage of the experiment, and it would be prohibitively impractical to adjust all the images manually in Photoshop for each opacity level.

I found a post in this forum on how to fade between two different colors (How to fade from slide1 to slide2 (inline?)), however I haven't been able to find any information on adjusting  opacity/transparency.  Any help would be very appreciated.  Thank you!

Erin

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