This is a straightforward question, but one I've struggled to find a clear answer on after hours of research and experimentation. For context, I'm working on shadow maps in OpenGL (using C#).

First, I've created a framebuffer and attached a depth texture as follows:

// Generate the framebuffer.
var framebuffer = 0u;

glGenFramebuffers(1, &framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);

// Generate the depth texture.
var shadowMap = 0u;

glGenTextures(1, &shadowMap);
glBindTexture(GL_TEXTURE_2D, shadowMap);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, 1024, 1024, 0, GL_DEPTH_COMPONENT, GL_FLOAT, null);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, shadowMap, 0);

// Set the read and draw buffers.
glDrawBuffer(GL_NONE);
glReadBuffer(GL_NONE);

Later (after rendering to the shadow map and preparing the main scene), I sample from the shadow map in a GLSL fragment shader as follows:

float shadow = texture(shadowMap, shadowCoords.xy).r;

Where vec3 shadowCoords is the coordinates of the fragment from the perspective of the global directional light source (the one used to create the shadow map). The result is shown below. As expected, shadow edges are jagged due to using GL_NEAREST filtering.

To improve smoothness, I tried replaced the shadow map's filtering with GL_LINEAR, but the results haven't changed. I understand there are other avenues I could take (like Percentage-Closer Filtering), but I'd like to answer this question first, if only for my sanity. I've also noticed that other texture parameters (like GL_CLAMP_TO_EDGE rather than GL_REPEAT for wrapping) don't function for the shadow map, which hints that this may be a limitation of depth textures in general.

To reiterate: Is linear filtering possible using depth textures in OpenGL?

I'm working on shadow maps in OpenGL (using C#).

First, I've created a framebuffer and attached a depth texture as follows:

// Generate the framebuffer.
var framebuffer = 0u;

glGenFramebuffers(1, &framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);

// Generate the depth texture.
var shadowMap = 0u;

glGenTextures(1, &shadowMap);
glBindTexture(GL_TEXTURE_2D, shadowMap);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, 1024, 1024, 0, GL_DEPTH_COMPONENT, GL_FLOAT, null);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, shadowMap, 0);

// Set the read and draw buffers.
glDrawBuffer(GL_NONE);
glReadBuffer(GL_NONE);

Later (after rendering to the shadow map and preparing the main scene), I sample from the shadow map in a GLSL fragment shader as follows:

float shadow = texture(shadowMap, shadowCoords.xy).r;

Where vec3 shadowCoords is the coordinates of the fragment from the perspective of the global directional light source (the one used to create the shadow map). The result is shown below. As expected, shadow edges are jagged due to using GL_NEAREST filtering.

To improve smoothness, I tried replaced the shadow map's filtering with GL_LINEAR, but the results haven't changed. I understand there are other avenues I could take (like Percentage-Closer Filtering), but I'd like to answer this question first, if only for my sanity. I've also noticed that other texture parameters (like GL_CLAMP_TO_EDGE rather than GL_REPEAT for wrapping) don't function for the shadow map, which hints that this may be a limitation of depth textures in general.

To reiterate: Is linear filtering possible using depth textures in OpenGL?