[Lazarus] TPicture beginupdate or similar

Leonardo M. Ramé l.rame at griensu.com
Mon Nov 29 14:45:07 CET 2010

On 2010-11-26 23:54:32 +0100, Benito van der Zander wrote:
> >
> >Hi again, I'm doing some transformations on a TImage using
> >Canvas.CopyRect
> For changing pictures a TPaintBox is better
> >I need to do something similar to TListView's Items.BeginUpdate - Items.EndUpdate.
> That doesn't exist for images, only temporary images and the
> doublebuffered property.
> >When the user moves the mouse
> >again, before doing anithing, I paint the original area with the
> >temporary bitmap to obtain the original image, then re-draw on the new
> >area. This causes flickering, and I thing it can be avoided using
> >something like BeginUpdate at the beginning of my method and EndUpdate
> >at the end
> Draw the area around new and old mouse position in an temporary image, and
> then draw the original image as well as the redrawing on that temporary image, and
> this temporary image on the visible control.
> Then only one drawing changes the visible image and it can't flicker.
> Benito

Thanks Benito. I'm doing more or less what you say, but I can't get the
desired results.

Could you take a look at the attached example?

Basically I'm doing this:

1) Define a TRect surrounding the mouse, a 40 by 40 pixels area
2) Define a TRect to contain the zoomed area, the same rect defined in
(1) multiplied by a zoom factor.
3) Store the area defined in (2) into a temporary bitmap.
4) Using Canvas.CopyRect, I stretch the small rect (1) into (2) to get
the zoom effect.
5) The next time the user moves the mouse, the program checks if theres's a
temporary bitmap, then draw it in the same position captured in 3, and
paint it into the image. I do this to restore the original image, then
the steps 1 to 4 are repeated again.

The problem is, in steps 4 and 5, I do a Canvas.CopyRect on the original
image, and when the user moves the mouse, the temporary image is shown
just before the zoomed one, and I don't want the user note this

Leonardo M. Ramé
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