Screen-reading shaders

Introduction

Very often it is desired to make a shader that reads from the same screen it’s writing to. 3D APIs such as OpenGL or DirectX make this very difficult because of internal hardware limitations. GPUs are extremely parallel, so reading and writing causes all sort of cache and coherency problems. As a result, not even the most modern hardware supports this properly.

The workaround is to make a copy of the screen, or a part of the screen, to a back-buffer and then read from it while drawing. Godot provides a few tools that makes this process easy!

SCREEN_TEXTURE built-in texture.

Godot Shading language has a special texture, “SCREEN_TEXTURE” (and “DEPTH_TEXTURE” for depth, in case of 3D). It takes as parameter the UV of the screen and returns a vec3 RGB with the color. A special built-in varying: SCREEN_UV can be used to obtain the UV for the current fragment. As a result, this simple 2D fragment shader:

COLOR=textureLod( SCREEN_TEXTURE, SCREEN_UV, 0.0);

results in an invisible object, because it just shows what lies behind.

The reason why textureLod must be used is because, when Godot copies back a chunk of the screen, it also does an efficient separatable gaussian blur to it’s mipmaps.

This allows for not only reading from the screen, but reading from it with different amounts of blur at no cost.

SCREEN_TEXTURE example

SCREEN_TEXTURE can be used for a lot of things. There is a special demo for Screen Space Shaders, that you can download to see and learn. One example is a simple shader to adjust brightness, contrast and saturation:

shader_type canvas_item;

uniform float brightness = 1.0;
uniform float contrast = 1.0;
uniform float saturation = 1.0;

vec3 c = textureLod(SCREEN_TEXTURE, SCREEN_UV, 0.0).rgb;

c.rgb = mix(vec3(0.0), c.rgb, brightness);
c.rgb = mix(vec3(0.5), c.rgb, contrast);
c.rgb = mix(vec3(dot(vec3(1.0), c.rgb)*0.33333), c.rgb, saturation);

COLOR.rgb = c;

Behind the scenes

While this seems magical, it’s not. The SCREEN_TEXTURE built-in, when first found in a node that is about to be drawn, does a full-screen copy to a back-buffer. Subsequent nodes that use it in shaders will not have the screen copied for them, because this ends up being very inefficient.

As a result, if shaders that use SCREEN_TEXTURE overlap, the second one will not use the result of the first one, resulting in unexpected visuals:

../../../_images/texscreen_demo1.png

In the above image, the second sphere (top right) is using the same source for SCREEN_TEXTURE as the first one below, so the first one “disappears”, or is not visible.

In 3D, this is unavoidable because copying happens when opaque rendering completes.

In 2D this can be corrected via the BackBufferCopy node, which can be instantiated between both spheres. BackBufferCopy can work by either specifying a screen region or the whole screen:

../../../_images/texscreen_bbc.png

With correct back-buffer copying, the two spheres blend correctly:

../../../_images/texscreen_demo2.png

Back-buffer logic

So, to make it clearer, here’s how the backbuffer copying logic works in Godot:

  • If a node uses the SCREEN_TEXTURE, the entire screen is copied to the back buffer before drawing that node. This only happens the first time, subsequent nodes do not trigger this.
  • If a BackBufferCopy node was processed before the situation in the point above (even if SCREEN_TEXTURE was not used), this behavior described in the point above does not happen. In other words, automatic copying of the entire screen only happens if SCREEN_TEXTURE is used in a node for the first time and no BackBufferCopy node (not disabled) was found before in tree-order.
  • BackBufferCopy can copy either the entire screen or a region. If set to only a region (not the whole screen) and your shader uses pixels not in the region copied, the result of that read is undefined (most likely garbage from previous frames). In other words, it’s possible to use BackBufferCopy to copy back a region of the screen and then use SCREEN_TEXTURE on a different region. Avoid this behavior!

DEPTH_TEXTURE

For 3D Shaders, it’s also possible to access the screen depth buffer. For this, the DEPTH_TEXTURE built-in is used. This texture is not linear, it must be converted via the inverse projection matrix.

The following code retrieves the 3D position below the pixel being drawn:

float depth = textureLod(DEPTH_TEXTURE,SCREEN_UV,0.0).r;
vec4 upos = INV_PROJECTION_MATRIX * vec4(SCREEN_UV*2.0-1.0,depth*2.0-1.0,1.0);
vec3 pixel_position = upos.xyz/upos.w;