CODEWITHSUNDEEP
openglopengl-esglslshader
Gensoukyou1337
Gensoukyou1337

Reputation: 1527

opengl - how do I change texels in a texture only within a certain rectangle area?

I know that a computationally efficient way to do if-else statements is the mix and step combo, like the following for example:

procTexel = mix(tA, tB, step(1.250, time))

How do I do it with rectangles, as in, how can I check if a texel is within my desired rectangle area within the texture coordinate (let's say, vec4(0.5, 0.5, 0.55, 0.55))? Is it a simple if-elseif-else check, or is there an even better way?

EDIT: the vec4 I referred to as the 'rectangle area' follows vec4(xmin, ymin, xmax, ymax).

Upvotes: 2

Views: 101

Answers (1)

Rabbid76
Rabbid76

Reputation: 211248

The GLSL function step(edge, x) returns 0.0 if x < edge, and 1.0 else.

This means, that the result of

step(edge, x)

is equal to 

x >= edge ? 1.0 : 0.0


The following line of code sets the variable float in_range to 1.0, if x >= a and x <= b, and sets the variable to 0.0 else:

float in_range = step(a, x) * step(x, b);

Note, this only works if a <= b.


If you want to test, if the vec2 v is in the rectangle defined by the 2 vectors vec2 v_min and vec2 v_max, the it can be done like this:

vec2  range_test = step(v_min, v) * step(v, v_max); 
float in_rect    = range_test.x * range_test.y;

Of course, this only works if v_min.x <= v_max.x and v_min.y <= v_max.y.


This means, you can write your code somehow like this:

vec2  tex_coord = ....;
vec4  rect      = vec4(xmin, ymin, xmax, ymax);

vec2  in_rect   = step(rect.xy, tex_coord) * step(tex_coord, rect.zw); 
procTexel       = mix(tA, tB, in_rect.x * in_rect.y);

Upvotes: 4

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