Need to send requestImage() some runtime generated URLs. Is this possible? - processing

Question

I'm using a remote url for a PImage. It's a jpg from a cam. I'm grabbing it and sending it to a image() every sec. Switching to a new cam every 10 secs. Every hour or so, the script crashes due to a 502 error, since the image was not successfully downloaded.
I'm attempting to setup a script that looks at the image using requestImage(), does a quick error check, and skips draw of that image if it returns a 0 or -1. Which should be simple enough. But...
How can you send a constantly updating url to requestImage() if its parameters refuse to accept anything other than a static single filename in a string and it lives in setup() / pre-process?
Anyone run into this issue before? Or am I missing something?
Here is the code. Note: cams are not active at the moment, so there are placeholders in the array...;
Thanks for looking!
PImage webImg;
PImage testImg;
int timer;
String[] camlist = {
"random_url_with_JPG_here",
"random_url_with_JPG_here",
"random_url_with_JPG_here"
};
//find length of array
int camListLength = int(random(camlist.length));
void setup() {
testImg = requestImage(webImg, "jpg");
noCursor();
fullScreen();
background(0);
}
void draw() {
if (millis() - timer >= 10000) {
camLoad();
timer = millis();
} else {
displayWebImage();
}
}
void camLoad() {
//find length of array
camListLength = int(random(camlist.length));
}
void displayWebImage() {
// load random cam url into 'webImg'
webImg = loadImage(camlist[camListLength], "jpg");
// test load
println(testImg.width);
if (testImg.width == 0) {
println("Not Loaded");
} else if (testImg.width == -1) {
println("random error");
} else {
// display 'webImage'
image(webImg, 0, 0, 800, 480);
// cache cleanup
g.removeCache(webImg);
delay(1000);
// frame count
println(frameCount+" "+g.getCache(webImg));
}
}

Answer 1

It's true that for 95% of people, it's a very bad idea to create images in the draw() function. Most programs should load all of the images at the beginning, in the setup() function.
But if you're loading images that you don't know ahead of time, then nothing is stopping you from creating images in the draw() function. You can absolutely call requestImage() from inside the draw() function.
But note that the requestImage() function runs in the background, so your code keeps executing while the image is requested. From the reference:
This function loads images on a separate thread so that your sketch doesn't freeze while images load during setup(). While the image is loading, its width and height will be 0. If an error occurs while loading the image, its width and height will be set to -1.
This means that the image won't be loaded until a couple second later, so you can't use your image variable right away. You're fixing this using a call to delay(), but you're probably better off just using the loadImage() function instead. From the reference:
If the file is not available or an error occurs, null will be returned and an error message will be printed to the console. The error message does not halt the program, however the null value may cause a NullPointerException if your code does not check whether the value returned is null.
...
Depending on the type of error, a PImage object may still be returned, but the width and height of the image will be set to -1. This happens if bad image data is returned or cannot be decoded properly. Sometimes this happens with image URLs that produce a 403 error or that redirect to a password prompt, because loadImage() will attempt to interpret the HTML as image data.
That function does not run in the background, so the code only continues after the image is fully loaded.