Complete Guide to the Knife Tool in Blender 3D

In this tutorial, we'll show you how to use the knife tool in Blender (4.5+) and all its options. The "Knife Topology Tool" allows you to carve new edges and vertices into the 3D mesh of an object in Blender, both to create complex shapes (using polygonal chain and holes), to subdivide and modify the mesh as needed, to improve the object's 3D topology, or to freehand create a specific shape, even a complicated one, independent of the rest of the pre-existing mesh configuration (this is different from the "Loop Cut" tool, in which the ring of edges stops at the edges and vertices that block it).

Table of contents of the tutorial:

• Activating the Knife tool: permanent session or not?
• How to use the Knife tool
• How to stop using the Knife tool and confirm the lines drawn
• How to create a closed shape (how to close a loop)
• Drawing Mode: Point-to-Point or Drag
• How to go back and delete a newly drawn edge/vertex
• Undo the entire operation, deleting the entire path of the cutting segments
• The Knife tool draws orthogonally to the user view
• The default snap of the Knife tool
• Creating multiple series of cuts (while maintaining the same session, even if we have selected the Knife as the command)
• How to delete some edges drawn by the Knife tool after confirming
• How to cut through an object using the knife tool
• Cutting across multiple objects at once
• Viewing hidden knife cutting lines with the X-ray option
• How to snap the Knife tool to the center of a segment (snap to the midpoint)
• How to ignore all snapping for the Knife tool
• How to display distance and angle measurements when carving with the Knife tool
• Drawing lines at a fixed angle increment with the knife (Angle Snapping)
• Constrain the knife's cutting line to a global axis
• Constrain the knife cutting line to a local axis
• How to make a cutting line perpendicular to a non-frontal face
• Cutting only selected parts with the Knife
• How to save settings and preferences for the Knife tool
• Problems using the Knife tool
• Applications of the Knife tool
• Alternative Tools
• Knife Tool Hotkeys

To use the Knife Tool, we must be in "Edit Mode," so for the following examples, we'll launch Blender, select the default cube by clicking on it, and press the "TAB" key to switch to Edit Mode.

There are various modes to activate the "Knife" tool, each with its own specific features.

Activating the Knife tool: permanent session or not?

There are two ways to activate the Knife tool: choosing it as the currently used tool or as a temporary command. In the first case, once you've drawn the lines with the Knife and confirmed, you can continue using the Knife tool to create other paths; in the temporary command, however, except in one specific case (by pressing the right mouse button), once you've confirmed the path, you'll return to the previously used tool.

How to permanently select the Knife tool: The Knife tool can be selected in Edit Mode from the left toolbar

(If you don't see the toolbar, make sure you're in the correct mode by clicking the ">" symbol in the top left

or pressing the "T" key.)

Once the Knife tool is selected, its function checkboxes and menus will appear both at the top of the 3D Viewport

and in the right toolbar, which can be accessed by pressing the "N" key, in the "Tool" tab

both In the "Tool" tab of the Properties window on the right,

Once the tool is activated, the mouse cursor (or the pen cursor) changes to a knife.

If we start clicking on the object in "Edit Mode,"

A list of all the tool options will appear in the bottom bar, showing the currently active ones. This list will remain visible until we finish drawing by pressing "Enter."

Activating the tool in this way will make the session permanent; that is, once you confirm and finalize the drawing by pressing "Enter," the Knife tool will still be active and available for use again.

How to temporarily select the Knife tool: If you already have another tool selected that you want to use alternately with the Knife tool, or if you want to use the Knife tool "on the fly" at any time while modeling, to activate the Knife as a temporary command, simply press the "k" key, or (still in "Edit Mode") select "Mesh > Knife Topology Tool" from the top menu.

As in the previous case, the mouse cursor icon changes to a knife-shaped image, and you can immediately click on the mesh to carve your segments, but with a few differences:
- The option settings will immediately appear in the bottom bar, even before using it.
- Since the tool isn't fully selected, the options menus won't appear, either in the top bar or in all the tabs and panels on the right, as we saw previously.
- When we finalize the drawing by pressing "Enter," you'll return directly to the previously selected tool (although we'll see that this can be avoided by right-clicking).

So, choose the mode that's most useful to you at the moment. Now we'll see how to use the basic commands of the "Knife" tool, which we'll activate in temporary mode in this example.

How to use the Knife tool :

As we've already seen, to use the Knife tool in Blender, simply press the "k" key and click the left mouse button (LMB) on the desired point on the object's geometry in "Edit Mode." (Although we can start anywhere: on a vertex, an edge, or an existing face, or even outside the mesh, as we'll see later.)

This first click establishes the starting point of the cut and

a first vertex will be created (1), while the other end of the segment will move following the knife-shaped mouse cursor (2). If the line crosses an edge, a vertex will automatically form at the intersection point with the edge (3), which will move dynamically as the temporary vertex (2) moves. As you can see, vertices are currently represented by blue dots (the color may vary depending on your Blender version).

To continue creating cutting lines, simply move the mouse cursor and left-click again at another location. Blender will automatically create a new segment connecting the previous point with the newly created one (2),

This will now be represented by a large black dot (the color and shape may vary depending on your Blender version). The previous vertices, (1) and (3), will be represented by small black dots (the importance of distinguishing the last confirmed vertex will be discussed later). A new temporary vertex (4), represented by a large blue dot, will follow the mouse cursor.

By repeating this process, we can create our shape, adding another cutting line

another vertex, and so on

until we've created the desired series of consecutive cutting segments.

How to stop using the Knife tool and confirm the lines drawn :

To save the lines drawn with the Knife tool and make the new subdivisions permanent in the mesh, simply press the "Enter" key or the "Spacebar" key.

Once you've finished creating a shape

and pressed the "Enter" key, in many cases additional segments will also be created (which will usually connect the shape to the nearest vertices),

Because an isolated shape cannot exist on top of a mesh, edges will be created to connect the new shape to the mesh's edges, also considering how shapes are formed with triangles. To better see and understand the new structure, select the entire mesh by pressing the "A" key and reveal the triangles that now compose the cube using the "Face > Triangulate Faces" command (or by pressing "Ctrl+T"), thus showing how the underlying triangles were created.

Therefore, always try to create a well-defined shape, otherwise Blender may not be able to accurately calculate the topology of the final result. For example, the result of a line that's too open and isolated

might not be what we wanted.

If you draw open shapes, it's always best to manually connect them to one of the object's edges.

How to create a closed shape (how to close a loop) :

To close a loop with the Knife tool, there are two options: either, after creating the penultimate vertex,

place the mouse cursor over the initial vertex (it will snap to it)

and press the left mouse button once to create the segment, then press the "Enter" key to confirm.

or you can double-click the left mouse button when drawing the penultimate vertex, and the connection to the starting point will be automatically created.

Then press "Enter" to confirm.

Note: Try not to have vertices aligned,

otherwise, you risk creating overlapping segments when double-clicking.

Drawing Mode: Point-to-Point or Drag :

Both methods work with the mouse and a graphics tablet pen.

Drawing Point-to-Point:

We first saw how to draw lines with the "Knife" tool from point to point: we click on a point on the surface

move the mouse to the target point (all intersection points with other segments will be represented by small blue dots)

and click on the target point (which will become a large black dot, while the intersection points will turn into small black dots),

the same happens if we choose another target point, and so on.

This method is useful for drawing segments that need to be positioned along a straight line, and you can start tracing by clicking outside the surface you're interested in

then exiting, clicking outside to create the cutting line, and then returning to the surface just by moving the mouse,

still being able to click outside on the left to create the line.

Drawing by dragging the cursor on the mesh surface:

By clicking on a point on the surface (as usual to create a starting point for the cut, after pressing the "k" key) and dragging the mouse, you can move more freely, because every time you intersect an edge or cross a vertex, a new point on the broken line will automatically be created, and from there you can change direction.

By releasing the mouse button and moving it, or by clicking and holding the left mouse button again and dragging, you can switch between the two modes.

In both cases, finalize and save the cutting lines as usual, confirming by pressing "Enter."

How to go back and delete a newly drawn edge/vertex :

To undo (before pressing "Enter") what you've drawn with the Knife tool and delete a newly drawn edge/vertex, use the classic "Ctrl+Z" key combination.
The starting point for the next cut will change accordingly.
In fact, if we are drawing (with point (3) as the last confirmed point)

Pressing "Ctrl+Z" will delete point (3) and the last confirmed point will become (2).

(So from this we understand the usefulness of having clearly marked the last vertex drawn.)

Note: If we previously drew a series of cutting lines by dragging the mouse, they will all be deleted at once.

Undo the entire operation, deleting the entire path of the cutting segments :

If, for some reason, we want to undo everything we've done with the "Knife" tool (before confirming with the "Enter" key), simply press the "Esc" key (before Blender 3.0, "Esc" or the right mouse button).
All cutting lines created in that tool session will be erased, and if you temporarily use the Knife tool, you'll revert to the previously used tool.

The Knife tool draws orthogonally to the user view :

The Knife tool draws according to the user view's projection, so if we draw something

and confirm,

if we rotate the object, we see that the result wasn't what we expected.

Therefore, it's always best to choose the correct viewing angle before using the tool, preferably by placing the user view orthogonal to the face on which we want to draw. Also, once the tool is activated, you can't select views with the numeric keypad; you can only use the mouse to move, zoom, and rotate.

The default snap of the Knife tool :

The Knife tool is similar to the "Extrude to Cursor or Add" tool,

which allows you to create broken lines by clicking with the mouse (Ctrl + right mouse button) to create shapes.

The "Knife" tool, however, has a built-in native snapping tool, set by default, that allows you to draw on the mesh, automatically snapping to an object's vertices, edges, and surfaces. In fact, if we draw by clicking on the mesh, the shape we create will be "smeared" across the surface;

furthermore, when we bring the knife close to a vertex, the cursor will snap to it

and when we bring the "Knife" tool close to an edge, it will take on another color (green) and, with a certain approximation, by moving along it (even without moving the cursor on the segment) a colored "placeholder" (where the new vertex will be created, if we click) will follow the edge.

The cutting line we draw on the mesh, after confirmation, will become part of that mesh, so we can also trace an image stored as a texture by an object to create decorative or structural shapes on the object itself.

Creating multiple series of cuts (while maintaining the same session, even if we have selected the Knife as the command) :

If, after creating a series of cutting lines with the Knife tool, we want to create a new series separately, even if we have selected it as a command and not as a tool, we can remain in the same session by pressing the right mouse button (or the "E" key, in versions prior to Blender 3.0). This allows you to use the knife tool to create multiple sets of cuts in sequence, without having to reactivate it after each operation, and create multiple, unconnected subdivisions.

For example, if we've created a closed loop, after snapping to the starting point and clicking the left mouse button, we'd like to create another shape elsewhere, but we see that we're creating another adjacent segment

then we press the right mouse button and can start drawing at another point on the surface,

to create another shape independently of the first.

As usual, at the end, we can press "Enter" to finalize/save the cutting lines.

If we've previously created a shape with the knife and right-click to create another set of cutting lines, the first shape will behave like the pre-existing edges on the surface, because not only will the knife snap to it be activated,

but intersection vertices will also be created if we intersect a new cutting line with it.

How to delete some edges drawn by the Knife tool after confirming :

Once we've drawn a series of cutting lines and confirmed, we may want or need to delete some segments.

In this In this case, simply choose the edge selection mode and select the segments you want to delete,

then press "Ctrl+X" or the "X" key and then choose the "Dissolve Edges" option from the context menu,

thus eliminating what we don't want.

As we said before, however, there can't be any shapes completely isolated from the edges of the surface we're drawing on, and this time we were only able to eliminate the connecting segments because two vertices of the example shape already lay on the outer edges.

Now let's look at a series of secondary functions. We'll also show how they correspond to the tool options, which we saw appearing at the top when we selected it; Therefore, although we'll talk about the knife tool as a temporarily invoked command, we'll also refer to the corresponding option to select from the top bar.

Because they're configurable options, these functions can be used in combination (and in some cases, they're designed to work together), but when using the knife as a command, the "K" key must always be pressed first, followed by the others.

To disable these options repeatedly, simply press the corresponding key once, or more than once (if there are variations).

How to cut through an object using the knife tool :

To duplicate the cutting lines on the opposite face of the object,

or if you want to split it into two or more parts,

you can activate the cutting through option by pressing the "C" key (or "Z" if you're using a version prior to Blender 3.0) immediately after pressing the "K" key. This will spread the effects of the cutting lines along the user view direction through the geometry not only of the visible mesh we're working on, but also of all the mesh behind it that are within that object.

(In the examples we are using the "wireframe" view, obtained by pressing the "Shift+Z" keys and the effects of this function are mainly visible after confirming with the "Enter" key.)

The "C" key not only enables but also disables cutting through, so you can press the "K" key to start cutting lines, then press "C" to enable cutting through, then when you no longer want this function, press the "C" key again to continue cutting on the visible side only.

When we use this function, the "C" in the options bar at the bottom will "light up."

Pressing the "C" key again will turn it off to deactivate it.

If, instead of using the Knife tool as a command, we choose it from the tools, this function corresponds to the "Occlude Geometry" option, which you can find in the top bar.

Although its meaning is exactly the opposite: by default, this option is always active and means that the rest of the geometry is hidden, obstructed (occluded), and the hidden portion will not be cut, but only the portion visible to the user. If we deactivate it, we obtain the same effects as the Cut Through function.

Cutting across multiple objects at once :

We just saw that it's possible to cut across multiple meshes contained in the same object. What if these belong to different objects?
You need to bring all the affected objects into "Edit Mode" at the same time. First, select them all (for example, by holding down the "Shift" key and then selecting them) in "Object Mode" and then pressing the "Tab" key to enter "Edit Mode". Alternatively, you can enter "Edit Mode" for the first object and then select the others in the "Outliner" by activating the appropriate symbol. Then, use the knife tool as before, pressing the "K" key and then the "C" key.

Let's take an example: given three distinct objects in "Object Mode", a cube, a sphere, and a cylinder, select them while holding down the "Shift" key.

Press the "TAB" key to enter "Edit Mode" and see that they've all entered this editing mode.

In the "Outliner," you'll see that there are small symbols to the left of each selected object.

By activating and deactivating them (perhaps holding down the "Ctrl" key to keep the previous ones selected), you can include or exclude other objects from mesh editing mode.
At this point, simply press, for example, the "1" key on the numeric keypad to access the front view, press the "K" key, and immediately after that the "C" key, to draw cutting lines that propagate through the meshes, and draw.

When you press the "Enter" key to save and finalize the drawing, you'll see that it has also propagated to the meshes belonging to the other objects.

We can also use other objects as references to draw cutting lines on separate objects (for example, on the sphere, starting from the cube and cylinder).

Viewing hidden knife cutting lines with the X-ray option :

Up until now, we've used the wireframe view to view an object's hidden cutting lines, but the knife tool also has an option that allows you to show underlying cutting lines that aren't visible because they're hidden by the foreground meshes. To activate it, simply press the "V" key (in older versions of Blender, it was always active).

For example, if we activate the knife tool by pressing the "K" key, then press the "C" key (to cut through the object), immediately after pressing "V" (to show the segments drawn behind the visible geometry) and start drawing, moving the user mouse around the object will see the cuts (already drawn) transmitted to the back of the mesh, without activating "wireframe" mode.

To check if the "X-Ray" tool is active, simply look in the options at the bottom,

or, if we have the knife tool selected in the toolbar, in the top menu, where we can enable and disable this option

Blender's Knife tool offers several options for constraining the cutting process, allowing users to achieve greater precision and control.

How to snap the Knife tool to the center of a segment (snap to the midpoint) :

We've already seen that the knife tool has its own snap tool for vertices, edges, and surfaces, but you can also snap to the midpoint by pressing the "Shift" key while carving, which automatically snaps to the midpoint of the closest edge ("Ctrl" in versions prior to Blender 3.0).
In fact, if we get close to an edge

and press the "Shift" key, the placeholder for the new vertex we're going to create will automatically move to the center of the edge

and from there we can start drawing

This option is useful because it not only allows us to create cuts already possible with the "Loop Cut" tool, but also shapes that can't easily be created with it,

or it can be used as a function to facilitate the creation of topologies for the rigging

How to ignore all snapping for the Knife tool :

Conversely, if you want to temporarily disable all automatic snapping for the Knife tool, simply hold down the "Ctrl" key (the "Shift" key before Blender 3.0).
This will give you full freedom of movement, without the cursor automatically snapping to nearby mesh elements, especially if you want to draw freehand with a graphics tablet pen.

How to display distance and angle measurements when carving with the Knife tool :

If you want to use the Knife tool precisely, you can view the dimensions and angles of the cutting line you're drawing by pressing the "S" key (the "A" key before Blender 3.0) after having already activated the Knife tool.

For example, if we press the "K" key and activate the knife, drawing a cutting line starting from a vertex, and press "S," the cursor's distance from that vertex and the angle relative to the bottom base segment will appear.

If we press "S" a second time, only the distance measurement will appear.

If we press "S" a third time, only the angle will appear.

Pressing "S" a fourth time will cancel this function.
As with other tools, the activation of this option will be indicated at the bottom.

And by activating the "Knife" tool in the left toolbar, you can activate the measurement by choosing one of its variants from the drop-down menu at the top, to the right of the "X-Ray" option.

The reference segment may change automatically; for example, if we draw a line starting from a vertex and move the cursor,

we note that, when exceeding a certain limit (in this case 45 degrees), the angle measurement is calculated starting from the left vertical segment.

Drawing lines at a fixed angle increment with the knife (Angle Snapping) :

To draw a cutting line that snaps to a specific angle—that is, constraining the cutting lines to multiples of that fixed angle increment—simply press the "A" key (or "C" before Blender 3.0). Here too, there are several options activated by pressing the "A" key:

Pressing the "A" key once calculates the angle relative to the user's view:

(Or, more precisely, the angle will be calculated on the screen plane, i.e., a plane orthogonal to the user's view)
If, for example, we try to draw a line with the knife, we press the "K" key and then the "A" key once to activate snapping, and we draw a line starting from a vertex of the cube, we see that the constraint starts from a line horizontal to the screen,

as also shown below, with "Angle Constraint: 0:00 (30.00) (Screen)"

Despite the The cube's edges are oblique to the user's view.

The value "30.00" is shown in parentheses in the bottom bar, because this is the current value by which the angle snap is incremented. In fact, if we move up, the line will almost immediately tilt by 30 degrees, because when angle snapping is active, the rotation angle is constrained to a multiple of this angle,

and in the bottom bar, we see that the value not in parentheses has changed to "30.00."

How do I change the snap angle increment value? While using this tool, simply type the new value (or first press the "Backspace" key to delete if a value has already been changed, then type the new value) and then move the tool (do not press "Enter" at this time, otherwise it will finalize the cutting line).

For example, if we type "15" and move the mouse, the new value will appear in the bar below

and the increment by which the line will rotate will now be 15 degrees (always relative to the screen).

If we want the knife rotation increment to be 45 degrees, we proceed in the same way: type "45" and move the mouse. The value 45 will appear in the bar below, allowing us to rotate 45, 90, 135, 180 degrees, etc.

Pressing the "A" key twice calculates the relative angle with respect to a segment:

(The angle will now be calculated on the surface of the object, starting from the nearest edge or existing cutting line.)

If, for example, we press the "K" key to activate the knife, the "A" key twice for the relative angle calculated on the surface, and the "S" key three times to display the angle measurement, we will see that a line is drawn on the surface of the cube starting from a reference segment, automatically selected and highlighted in red, according to a default angle increment (which this time corresponds to those whose measurement is displayed) (set to 30 degrees)

which is also indicated in the bottom bar

(Note that "Relative" now appears instead of "Screen"; we'll talk about the "R" key shortly.)
If we move, the angle will increase by another 30 degrees

and so on up to the 90 degrees corresponding to the vertical segment.
To change the reference segment, simply press the "R" key with the tool and the option activated. The vertical segment will turn red and the displayed measurements will be calculated relative to it.

However, while the measurement is displayed clockwise or counterclockwise to show the angle relative to the closest segment, the actual angle measurement will always be counterclockwise, so instead of 60, the bottom bar will display (360-60=) 300 degrees.

Note: This option only works if the Knife tool cursor is on the surface. If we move it away, it deactivates,

it automatically reactivates when the cursor returns to the surface.

When we activate the "Knife" tool from the left toolbar, we can also activate the angle constraint from the top menu by choosing one of the "Angle Snapping" options

and we can also define the increment angle by adjusting the slider for the "Snap Increment" option.

Pressing the "A" key a third time deactivates the constraint.

So we've seen that with the "Knife" tool, it's possible to create vertical, horizontal, or precise cuts, which can be set by the user, facilitating the modeling of technical or architectural objects where angular precision is essential.

Constrain the knife's cutting line to a global axis :

You can constrain the drawing of a cutting line to an axis by pressing the "X" or "Y" or "Z" keys, immediately after the "K" key.

If, for example, we want to draw a cutting line along the Y axis, simply press the "K" key and immediately afterward "Y" to draw the line along the Y axis.

Note that the line corresponding to the segment we are drawing will take on the color of the parallel axis we have chosen; In fact, if after creating the first vertex (before finalizing and saving the cutting line) we press the "Z" key, the line on which the new segment lies will take on the blue color of the Z axis.

Constrain the knife cutting line to a local axis :

To constrain the cutting line to a local axis, simply double-press the corresponding X, Y, or Z axis key.

We can see a demonstration of this with a rotated cube:

Press the "TAB" key to enter "Object Mode" and rotate the cube.

Then return to "Edit Mode".

Let's repeat the same operation as in the previous chapter, pressing the "K" key to draw a cutting line trying to constrain it to the Y axis: pressing the "Y" key for the first time will constrain it to the global Y axis,

pressing the "Y" key a second time will constrain it to the local Y axis.

(Pressing the "Y" key a third time will deactivate this option.)

How to make a cutting line perpendicular to a non-frontal face :

If the object's geometry does not allow for a comfortable frontal view, for example, in the case of the lateral face of an icosphere,

we can proceed in two ways to draw a cutting line perpendicular to one side of the equilateral triangle:

A) Using relative angle constraints: Press the "K" key, then "S" three times to display the angle measures, then hold down the "Shift" key and snap to the midpoint of the triangle's side, press the "A" key twice to snap to the relative angle, choosing 90 (degrees) as the increment,

draw to the opposite side, and then rotate the sphere to see that it has correctly reached the opposite vertex (since it is an equilateral triangle).

B) Alternatively, we can select the affected face and then press "Shift+7 (numeric keypad)" or choose "View > Align View > Align View To Active > Top" from the menu to bring it into front view

and then, at this point, you can draw as if it were any front face.

Cutting only selected parts with the Knife :

To cut only the selected parts of a mesh in "Edit Mode" using the Knife tool, activate the Knife by pressing "Shift+K" instead of "K".

With typical use of the Knife tool, we can cut both the selected and deselected parts of a mesh. In some cases, however, you may want to limit the tool's use to cutting only elements within a selection, which in this case will function as a mask.

This is a useful feature, as we can localize the effects of the cut without affecting the rest of the mesh. It also makes it easier to trace from more distant areas, outside the mesh, or following external references. For example, if we want to modify only a portion of a mesh, we select it in "Edit Mode",

then, by pressing "Shift+K" instead of "K," we can conveniently draw cutting lines starting from outside the affected area,

however, only the selected portion will be modified.

The same goes for snapping to interesting vertices, but outside the part being traced.

(This is a bit of an illusion because, from the perspective of the triangles that make up the mesh, both the selected and deselected parts are affected.)

How to save settings and preferences for the Knife tool :

Sometimes we'd like certain settings for the Knife tool to be saved as default. For example, we'd like the angle constraint increment to be 10 degrees, without having to change it each time, or we might prefer that the drawn cutting line always propagate to the hidden parts of the mesh.
The most effective way to save settings is to change the tool's preferences:

- In the top menu, open the preferences window by selecting "Edit > Preferences"
- Access the "Keymap" tab
- In "3D View > Mesh > 3D View Tool: Edit Mesh, Knife" (or by searching for "knife" in the search box), you'll find the form where you can edit and save all the settings, corresponding to the options found in the top bar once you've selected it from the tools on the left.

Changes should autosave, but if this option isn't enabled, you can save from the pop-up menu, activated with the button at the bottom left.

(In this "Keymap" tab, you can also change the hotkeys for activating the Knife tool.)

Problems using the Knife tool :

When we try to draw cutting lines with the Knife tool, the operation isn't always successful. Returning to the example of the three solids in three different objects, instead of looking in front view, we look from the opposite side (by pressing "Ctrl+1" on the numeric keypad) and draw a similar polygon by pressing the "K" key and immediately after "C".

The propagated lines didn't extend beyond the sphere, despite the distances and objects being the same.

Therefore, always try to work with the simplest geometries and spatial situations possible, especially when drawing complete propagated shapes, with cutting lines not too close to groups of adjacent edges.

Also, be careful about the topology you generate, because the Knife tool can create triangles and n-gon faces, which are not suitable for objects and characters that will later be rigged and animated.

Applications of the Knife tool :

A key advantage of the Knife tool is that it can add vertices, lines, and shapes anywhere, so you can also cut triangles.

This allows you to circumvent the known limitation of the "Loop Cut" tool,

even for solids like the cone

and draw any type of shape, the only limit being your imagination.

With the knife, we can improve the topology of a mesh

because we can easily draw lines to eliminate any triangles or n-gon polygons.

With the "Knife" tool, we can cut objects as desired,

we then confirm with the "Enter" button, press the "V" key to separate a cut, select the edge to be deleted by positioning it over it and pressing the "L" key,

press "X" to delete the edge, and then create the missing faces by selecting the opposite edges and pressing the "F" key.

This allows us to create "ruins" on our object without using the sculpting tool, thus optimizing the number of vertices, as is the case with low-poly meshes.

Another useful feature of the knife tool is the ability to draw oblique lines;

this gives us the ability to draw specific shapes on a solid, like a sci-fi panel, using angle constraints and the ability to create a new segment by right-clicking

which we can then extrude,

or we can use the "Bevel" tool to give them a two-dimensional thickness

and thus create incisions by extruding inward

or reliefs by extruding outward.

This tool has applications both in "Hard Surface," for adding detail to objects,

and in ArcViz, for modifying architectural elements,

but also in unexpected landscape environments, for example, to trace the well-defined contours of a lake.

We saw earlier that the "Knife" is like the extrusion tool activated with the "Ctrl+right mouse button" keys, which allows you to draw polygonal chains, which can be used to trace on a background image. In this case, we do this by snapping to a mesh, and the generated polygonal chain will be part of that mesh. We can then trace the image onto the mesh to create decorative or structural shapes on the mesh itself.
For example, if we load an image of a small house onto a flat surface,

wth the knife tool, we can not only trace the door and extrude its elements,

but also, if necessary, remove the parts we don't need,

and then perhaps extrude the entire building, possibly integrating the roof or wall with other textures.

Finally, another use for the knife tool could be to aid deformation. Given any mesh

we can subdivide it appropriately, unconstrained by the pre-existing topology

and then deform it with the "Proportional Editing" tool, or by associating an armature and bones.

Alternative Tools :

The "Knife" tool is very flexible, and its options simplify the task in many applications. However, in many cases, it is preferable to use other alternative functions, either because they are specialized or because they speed up the work.

"Loop cut": If you want to make one or more precise straight cuts, this is the tool for you.

However, we've seen in this tutorial that there may be limitations when mesh topology elements block the loops. One solution is to use this tool for its intended purpose and then use the "Knife" tool to complete the loops in more difficult cases.

"Bisect": If you need to cut an object with a clean, straight cut, this is the ideal tool, especially since it allows you to quickly eliminate one of the two parts;

however, it only makes straight cuts, not for example those created using a polygonal chain or even a drawing. In these cases, "Knife" solves the problem for you, as already seen.

"Knife Project": allows you to make a cut like a knife, but by projecting a pre-packaged mesh. A huge time saver.

However, the projection result isn't always perfect, so you need to adjust and retouch the resulting figure using the knife tool.

For information about the "Knife Project" tool, see the Complete Guide to the Knife Project

So we can say that, unlike true alternatives, these tools can work in symbiosis with each other and together form a great team.

Knife Tool Hotkeys

 
That's all for this tutorial on the "Knife" tool; now it's up to you to practice and practice.
Happy blending!