expand section of a texture 2D (zoom)

Question

I have a texture2d loaded and wish at a certain moment to make a sort of zoom on the texture2d.

As you can see in the image i would like to expand my red square to the resolution of the black square. The aspect ratio is the same.

update

if it isn't clear, the question is how can i make that zoom?

Answer 1

In your draw method you have a destination rectangle and source rectangle, use them.

Source is related to your texture, it would be the red box.

Destination is related to your screen.

Answer 2

That can be done with transforms (like OpenGL does for example) to your viewport coordinates. It's just 2D geometry math.

Zoom is essentially a division. A 200x200 viewport with 2x zoom is actually 100x100 size in your texture (which will be expanded to fill the 200x200 viewport, effectively zooming.) After that, you just have to apply a translation to offset your zoomed area, which is a simple sum.

If you use the same zoom for both X and Y coordinates, you'll keep the aspect ratio the same. Here's some pseudocode for zooming into a full-screen texture (hence the windowX.)

backgroundX = windowX/zoom + translationX;
backgroundY = windowY/zoom + translationY;

Bear in mind you need some kind of interpolation (graphic cards can do it for you) because 1 pixel without zoom will be split into two half-pixels with 2x zoom (1px/2 = 0.5px size each fragment.) If you use nearest neighbor for interpolation, the zoomed image will be pixelated (which may be the effect you intend.)

I always implemented this myself on shaders. Here is a section of a shader of mine that zooms a texture applied to a full-screen quad, in case it helps:

uniform ivec2 resolution; // board size (for normalization)
uniform ivec3 camera;     // xy = offset, z = zoom

vec2 p = (gl_FragCoord.xy/resolution - vec2(0.5))/camera.z + camera.xy/(float)resolution;

Now p holds my texture coordinate (in [0..1] range, that's why I need the texture resolution) for the fragment at gl_FragCoord in screen. If you're not drawing a full-screen quad, you have to replace gl_FragCoord with your texture coordinate (passed as a uniform or with glTexCoord in fixed-pipeline OpenGL.)

I substract 0.5 from the fragment coordinate to center the zoom direction on screen (otherwise you would zoom into the bottom-left corner.) You don't need this, since you could also add resolution/2 to your camera offset in your application, but I prefer using the center of the screen as the origin (for convenience.)