pushMatrix on an image won't pop

Programming Questions

newmiracle

pushMatrix on an image won't pop

in Programming Questions • 1 year ago

Sorry, this is a total beginner question. Please excuse my ignorance.

I'm trying to process the pixels in an image. I want to be able to make my own filters that I can apply live. But the pushMatrix and popMatrix functions are giving me some guff.

So I have a series of images inside of an array. They are called in a slideshow type functionality, and then effected live by pressing keys. I'm trying to make a image mirror itself right down the middle when I press a button, and then let go when I let go of the key. The program is pretty big (and ugly), so I'm going to only show the relevant code.

pushMatrix();
images[imageIndex].loadPixels();
int imageWidth = images[imageIndex].width;
int imageHeight = images[imageIndex].height;
// Begin loop for width
for (int x = 0; x < images[imageIndex].width; x++) {
// Begin loop for height
for (int y = 0; y < images[imageIndex].height; y++) {
images[imageIndex].pixels[y*images[imageIndex].width+x] = images[imageIndex].pixels[(images[imageIndex].width - x - 1) + y*images[imageIndex].width]; // Reversing x to mirror the image
}
}
images[imageIndex].updatePixels();
popMatrix();

The problem is that the image takes effect the way I want it to, but it doesn't "undo" itself. I press the key, it performs the mirroring, and when I let go it stays. Then when the slideshow loops itself, the next time that image comes around it is still mirrored.

I thought that putting the matrix manipulation inside push and pop would make it "undo-able", but I clearly don't have a good grasp of how push and pop work.

What am I doing wrong? Any help is greatly appreciated.

Replies(5)

calsign

Re: pushMatrix on an image won't pop

1 year ago

I'm not really sure how using pushMatrix() and popMatrix() are supposed to undo the change - besides, you aren't even calling them on the image.

I imagine that you would store the pixels in a temporary PImage and load that back into the image when you want to undo.

newmiracle

Re: pushMatrix on an image won't pop

1 year ago

Sorry, I thought that I could use it on the matrix of the image information. How would I call them on the image? All of the reference material ( http://processing.org/reference/pushMatrix_.html ) just seems to throw those lines in there. I'm so wet behind the ears that I'm not even sure what you mean by "call them on the image". How would I do that correctly?

I will probably followup with a temp PImage like you suggested, but I'm curious to see how I can use push and pop properly.

Thanks for the heads up.

PhiLho

Re: pushMatrix on an image won't pop

1 year ago

All Processing drawing functions (and some others like pushMatrix) act on the current, global, "hidden" PGraphics, named g. You can make them act on a particular PGraphics by prefixing them with the name of your graphics. Eg. if it is named someDrawing, you would call it someDrawing.pushMatrix().

Now, indeed, matrix functions act on operations like translate(), rotate(), scale(). Not on pixel operations!
If you want to undo your operation, you have two options: apply again the mirroring on the images (put the operation in a function, it will be easier to handle), or just keep an unaltered copy of the images.

newmiracle

Re: pushMatrix on an image won't pop

1 year ago

Not on pixel operations!

That's what I needed to know. It seems pretty clear that I'll need to have another PImage in there somewhere. Since my mirroring process actually mirrors one half of the image onto the other half, I can't undo it just by repeating the process (I know, it's not named very well. Kalidescope maybe?). I'll probably make a function that makes temporary PImages that can be played with and then thrown away when not needed.

But for the sake of learning, I'd like to ask some more questions about all of this (if you'll oblige me).

So what you're saying is that translate, rotate, and scale are not pixel operations? They are matrix functions. I've been trying to read about this so I have a grasp on the fundamentals of Processing, so bear with me. The difference is that matrix manipulation is really making changes in the coordinate / location system, correct? This is as opposed to the arrays (what I was thinking of as a matrix because it's an image, even though it's really an array) of information that something like a PImage would have.

Just talking it out so I know that I have the concepts right.

Thanks for your help!

::edit::

I'm having some problems with the PImage duplication, but I'll probably post any questions in a different thread.

PhiLho

Re: pushMatrix on an image won't pop

1 year ago

My currently last message in the https://forum.processing.org/topic/copy-my-screen-turn-it-etc thread shows how to draw on a PGraphics, how you can copy an image using get(), and some other tricks. The screen capture stuff can be ignored, the remainder can work with any image, of course.

" translate, rotate, and scale are not pixel operations?"
Not really. In the end, it will transform the pixel rendering of the images, but these operations just act on the transformation matrix of the PGraphics on which they are applies. That's just putting some floats in a small matrix: somehow, it is very fast (but rendering with these transformations can be slow...).
Now, what I mean exactly with "pixel operations" is more about manipulating the pixels[] array of the PGraphics (implicit, global, or of some PGraphics you created yourself) than about operations like rect() which alters pixels, but in a more "invisible" way.

So, yes (reading again your answer after mine...), your suppositions about matrix operations are right.

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