Apply shader to specific part of the screen

[yoinkityyoinks]

Normally I use 

mc.entityRenderer.loadShader(new ResourceLocation("shaders/post/blur.json"));

to load a shader however, this puts the shader on the entire screen. How do I apply the blur shader on only a specific part of the screen?

I tried

Field resourceManager = EntityRenderer.class.getDeclaredField(Mapping.resourceManager); /* shaderGroup = isObfuscated() ? "field_147707_" : "shaderGroup"; */
Field shaderGroup = EntityRenderer.class.getDeclaredField(Mapping.shaderGroup); /* resourceManager = isObfuscated() ? "field_147711_ac" : "resourceManager"; */
Field useShader = EntityRenderer.class.getDeclaredField(Mapping.useShader); /* useShader = isObfuscated() ? "field_175083_ad" : "useShader"; */
resourceManager.setAccessible(true);
shaderGroup.setAccessible(true);
useShader.setAccessible(true);

shaderGroup.set(mc.entityRenderer, new ShaderGroup(mc.getTextureManager(), (IResourceManager) resourceManager.get(mc.entityRenderer), mc.getFramebuffer(), new ResourceLocation("shaders/post/blur.json")));

((ShaderGroup) shaderGroup.get(mc.entityRenderer)).createBindFramebuffers(300, 300);

useShader.setBoolean(mc.entityRenderer, true);

However, this shurk my entire screen and applied the blur filter there and resizing the window causes it to reset. Is there any way I am able to only blur the part of the screen I want and not just the entire screen? 

[ChampionAsh5357]

If I'm not mistaken, there is some way to specify a portion of the screen to apply the shader to in the JSON using an intermediary layer to apply the shader to. As for how, I do not remember. I just remember there being something similar to that when I was doing research about them at the beginning of the month.

[yoinkityyoinks]

Is there way to do this in code or do I really need to create custom layers

[ChampionAsh5357]

14 hours ago, yoinkityyoinks said:

Is there way to do this in code or do I really need to create custom layers

Once again, I'm unsure. There is probably a way to do it within the program itself. However, it should be preferable to try and find a JSON solution first if possible.