为复杂方程 manim 的部分着色

Question

I want to paint variables of MathTex element in different colors, but Manim seems to have problems with comlicated Latex expressions.

Here is my scene.

from manim import *
config.frame_width = 260 

class Find_Path(Scene):
    def construct(self):
        obj = MathTex(r"minimize \quad \sum_{start}^{end}\frac{d_{i,i+1}}{v_{i,i+1}}", 
        font_size=1000, substrings_to_isolate="d" and "v")
        obj.set_color_by_tex("d", YELLOW)
        obj.set_color_by_tex("start", GREEN)
        obj.set_color_by_tex("end", GREEN)
        obj.set_color_by_tex("v", RED)
        self.play(Write(obj))
        self.wait(3)

Here is the result.

Specifically, I want to color d_{i,i+1} in YELLOW , v_{i,i+1} in RED , start and end in GREEN .

Any advice? Frankly, I do not want to create several MathTex object in different colors and then arrange them.

Answer 1

Late answer, but I encountered a similar issue and ended up here before finding the relevant section in the documentation.

Relevant section in documentation: Using index_labels to work with complicated strings

An example with your special case:

from manim import *

config.frame_width = 8
config.frame_size = (1300, 1000)


class FindPath(Scene):
    def construct(self):

        # You can split the string in parts
        minimize = r"minimize \quad "
        summ = r"\sum_{start}^{end}"
        frac = r"\frac{d_{i,i+1}}{v_{i,i+1}}"
        tex = MathTex(minimize, summ, frac).shift(2 * UP)

        # Observe first level labels
        tex_ = tex.copy().next_to(tex, DOWN)
        self.add(index_labels(tex_, color=YELLOW))

        # Observe second level labels
        tex__ = tex_.copy().next_to(tex_, DOWN)
        for part in tex__:
            self.add(index_labels(part, color=YELLOW))

        # Finally you can color accordingly
        tex[1][0:3].set_fill(color=GREEN)
        tex[1][4:9].set_fill(color=GREEN)
        tex[2][0:6].set_fill(color=YELLOW)
        tex[2][7:13].set_fill(color=RED)

        self.add(tex, tex_, tex__)

Answer 2

Manim does a bunch of tex rewriting under the covers, and it seems that over is preferred to frac because of that rewriting.

I was able to apply the colors that you wanted (although I suspect you didn't want the sum symbol colored) with:

from manim import *

class Find_Path(Scene):
    def construct(self):
        obj1 = MathTex(r"\text{minimize}", r"\quad \sum_{\text{start}}^{\text{end}}")
        obj2 = MathTex(r"d_{i,i+1}", r"\over", r"v_{i,i+1}")
        obj1.set_color_by_tex("start", GREEN)
        obj1.set_color_by_tex("end", GREEN)
        obj2.move_to(obj1, RIGHT)
        obj2.shift(1.5 * RIGHT)
        obj2[0].set_color(YELLOW)
        obj2[2].set_color(RED)
        self.play(AnimationGroup(Write(obj1), Write(obj2)))
        self.wait(3)

but I had to resort to separate objects. Worse still, I aligned them by hand with a fudge factor.