How to Create a Thumbnail Image

This is a handy lesson I came across recently. You might often have a need to create a thumbnail image for use in an app or even a set of such images, this is a neat way to do it. You will create a 32 x 32 pixel thumbnail image of a control.

Select the thumbnail source

In this case I want to create a thumbnail image of a knob control on my stack.
The source will be the id of the knob group.

Select an image name

Here I want to pick a unique name for the new image. I'm going to call it "tmp" followed by a number.
I want to make sure there's no image by that name already, so I'm going to cycle through numbers until I find an available slot.

put "tmp 1" into tImageName
repeat while there is an image tImageName
add 1 to word 2 of tImageName
end repeat

Create a new image

create image tImageName
set the visible of image tImageName to true
set the resizequality of image tImageName to "best"

Take a snapshot of the source control

This is what does the work of making the image. Here pControlID is the id of the control I want to make a thumbnail from (the knob control group).

export snapshot from control id pControlID to image tImageName as PNG

Size the image to 32 x 32 pixels

Obviously you can use any size you want, but here I want a 32 x 32 pixel thumbnail image.

set the width of image tImageName to 32
set the height of image tImageName to 32

Make the new image size permanent

This step is somewhat nonintuitive. If you don't do this step then the width and height you just set will not stick, and the image will stay the original size. The next time you open the image it will spring back from the thumbnail size. You don't want this. Setting the imagedata makes the new size for the thumbnail permanent.

set the imagedata of image tImageName to the imagedata of image tImageName

Put the thumbnail in the clipboard for pasting

We don't really need to do this, but we might as well.

copy image tImageName

So here's a complete handler, ready to go. Pass the id of the control to the CreateThumbnail handler.

on mouseUp
CreateThumbnail the id of group "knob"
end mouseUp

on CreateThumbnail pControlID
    local tImageName

    -- pick a unique name for the image
    put "tmp 1" into tImageName
    repeat while there is an image tImageName
        add 1 to word 2 of tImageName
    end repeat
    -- create an image for the thumbnail
    create image tImageName
    set the visible of image tImageName to true
    set the resizequality of image tImageName to "best"
    export snapshot from control id pControlID to image tImageName as PNG
    set the width of image tImageName to 32
    set the height of image tImageName to 32
    -- make the new size permanent
    set the imagedata of image tImageName to the imagedata of image tImageName
    -- put in the clipboard for pasting
    copy image tImageName
end CreateThumbnail