Shaders [SOLVED] using a variable to only draw part of an image

[Anthony Navarro]

Hello, I am having some trouble using a shader in my game.
I am relatively new to GLSL, so any help would be appreciated.
My goal: I want a shader to draw the bottom portion of a sprite, and work its way up.
I want the colors to be inverted, so the end result (for a white block) would look like
a black block is moving up to cover the white block. Hopefully this all makes sense. I
was able to make the color inversion easily enough, but I havent been able to get the inversion to rise instead of being drawn all at once. I have tried things like:

if(1.0-v_vTexCoord.y<time)
{
 //invert
}else{
 //use regular color
}

This always draws the inversion over the entire image at once, even though the "time"
uniform is decremented slowly (starting at 1). It is my understanding that v_vTexCoord coordinates are non-linear (0-1)
Maybe im wrong. Im definitely screwing something up.
Thanks in advance for any suggestions!

 

[jo-thijs]

The texture coordinates are coordinates inside a texture pages.
Multiple images are put together on a texture page.
This often causes texture coordinates to only take up a small part of the range (0-1).
You can get the range of the texture coordinates of an image by using the function sprite_get_uvs.
Texture coordinates are linear though in the sense that texels have a constant width and height in a single texture page (as far as I understand it).

 

[Anthony Navarro]

This often causes texture coordinates to only take up a small part of the range (0-1).
You can get the range of the texture coordinates of an image by using the function sprite_get_uvs.

Good to know, I will look into this and see what I come up with. Thanks!

 

[Anthony Navarro]

I was able to get it working with your advice, thanks a lot @jo-thijs