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lualogic

Positioning and Resizing images in different aspect ratios keeping the same position relative to the screen

the problem I'm facing is more a matter of logic and algorithm than a specific language functionalities, i'm coding it in lua, but i believe it could be replicated in other languages with no major problems.

First of all, I'm going to show you some properties and default settings that i'm having to use to come up with a solution.

1. I have a general function that displays an image on the screen, given the X, Y position and W, H dimension, to facilitate understanding, this function is drawImage(x, y, w, h)

2. All values ​​and calculation will be based on a default resolution and aspect ratio, which in this case is the developer's. These variables will be these: DEV_SCREEN_W = 1366, DEV_SCREE_H = 768, (aspect ratio is 16:9)

3. So far, we have a function that displays an image on the screen, and default screen values ​​to which the X, Y position and W, H dimensions of a given image will be set.

4. Now, we have the CLIENT, which can be anyone, with any resolution and aspect ratio, this client will run the code on his computer.

5. Knowing this, we need to make an algorithm, so that the positions and dimensions of the image stay relatively the same regardless of the screen being used to show it.

Knowing these properties and definitions we can proceed with the problem. Let's assume that me as a developer, having a screen whose values are DEV_SCREEN_W = 1366, DEV_SCREE_H = 768 i want to set an image at position X = 352, Y = 243 with W = 900, H = 300. So At the developer screen, i'll have this:

First Image

Okay, now let's add one more image, with position and dimension X = 352, Y = 458, W = 193, H = 69

Second Image

Okay, now we need to write an algorithm that keeps the same dimension and position on the screen regardless of size, as W and H are different for each resolution, we can't use pixel points to define, my solution was to define the position between 0 and 1, so the position would represent a certain percentage of the screen, the same for the W and H.

Let's suppose i get the screen information from the client and I get CLIENT_SCREEN_W = 1280, CLIENT_SCREEN_H = 720.

Since it's the same aspect ratio, I could apply this concept to both position and dimension as it would remain perfectly proportional to the screen, so i would have:

Getting the percentage based on the DEV screen for BOTH images would be like:

X = 352/DEV_SCREEN_W * CLIENT_SCREEN_W,
Y = 243/DEV_SCREEN_H * CLIENT_SCREEN_H,
W = 900/DEV_SCREEN_W * CLIENT_SCREEN_W,
H = 300/DEV_SCREEN_H * CLIENT_SCREEN_H,

Basically, for those who didn't understand what is happening, i get the data of how many % position in pixels represents from the developer's screen (X = 352/DEV_SCREEN_W) that is (X = 352/1366 = 0.2576) and multiply this result by the W of the client screen: 0.2576 * CLIENT_SCREEN_W, that is 0.2576 * 1280 = 329. Thus, we concluded that 329 and 352 are relatively the same position in different resolutions.

Following this concept, no matter what resolution the client uses, the images are always in the same proportion, both in position and in dimension, ONLY IF IT IS IN RATIO 16:9 (the same as the developer)

And this is where the problem arises, applying this same concept to any ratio, on a 4:3 screen the both image would be stretched:

Scratch

despite keeping the same X and Y relative to the screen, the W and H had to be altered out of proportion to fit the screen, obtaining the result seen above, which cannot happen.

To avoid this, i set a proportion rate, which i get by dividing the client's screen by the dev's, thus getting how much of one represents the other, and i multiply that by W and H of both images so that both are proportionately resized to their original dimension, instead of multiplying by a relative value between 0 - 1 arbitrarily.

Getting the proportion would be like: PROPORTION_RATE = CLIENT_SCREEN_W/DEV_SCREEN_W

Applying it: W = 900*PROPORTION_RATE, H = 300*PROPORTION_RATE

Basically, this multiplication for aspect ratio, makes the image stay in the exact proportion of the screen resizing it, however, applying this, the images lose their relative position, as seen in the image below:

Fixed Dimension

As you can see, despite keeping the same proportion in W and H, the image lost its structural organization in relation to the original position defined on the developer's screen.

I've been in this problem for a while

The closest I got was to add on the Y and X axis how much a certain image has decreased, however, if i do that both images will be corrected, but they would still be out of relative position between them, as shown in the image below:

[Fixed position]

This problem of logic and algorithm is a little beyond my applicable knowledge, alone I can't find a solution, so I sincerely ask for help, or direction to the way where I can solve it.

Solution

  • Based on your comments, you want to scale both axes by the same amount, and you want to handle ratio mismatch by adding empty stripes at window edges as needed.

    First you compute the scale: scale = min(w2/w1, h2/h1), where w1,h1 is the source size, and w2,h2 is the target size.

    Next, assuming 0,0 is in the center of the screen, you can just do x2 = x1*scale, y2 = y1*scale, where x1,y1 are source coordinates and x2,y2 are converted coordinates.

    But if 0,0 is in a corner of the screen for you (which is more probable), you have to do something like:

    offset_x = (w2 - w1 * scale) / 2
offset_y = (h2 - h1 * scale) / 2

    Then:

    x2 = x1 * scale + offset_x
y2 = y1 * scale + offset_y