MODEL_MATRIX cannot be modified from the vertex stage

[QbieShay]

Godot version

4.0 stable official

System information

Fedora, X11, RX 590

Issue description

When trying to assign anything to MODEL_MATRIX in my shader, it says "constants cannot be modified" but it's marked as inout in the documentation: https://docs.godotengine.org/en/stable/tutorials/shaders/shader_reference/spatial_shader.html

Steps to reproduce

Create a shader, paste this code:

// NOTE: Shader automatically converted from Godot Engine 4.0.stable's StandardMaterial3D.

shader_type spatial;
render_mode blend_mix,depth_draw_opaque,cull_back,diffuse_burley,specular_schlick_ggx;
uniform vec4 albedo : source_color;
uniform sampler2D texture_albedo : source_color,filter_linear_mipmap,repeat_enable;
uniform float point_size : hint_range(0,128);
uniform float roughness : hint_range(0,1);
uniform sampler2D texture_metallic : hint_default_white,filter_linear_mipmap,repeat_enable;
uniform vec4 metallic_texture_channel;
uniform sampler2D texture_roughness : hint_roughness_r,filter_linear_mipmap,repeat_enable;
uniform float specular;
uniform float metallic;
uniform int particles_anim_h_frames;
uniform int particles_anim_v_frames;
uniform bool particles_anim_loop;
uniform vec3 uv1_scale;
uniform vec3 uv1_offset;
uniform vec3 uv2_scale;
uniform vec3 uv2_offset;


void vertex() {
	UV=UV*uv1_scale.xy+uv1_offset.xy;
	
	MODEL_MATRIX = mat4(1.0);
	float h_frames = float(particles_anim_h_frames);
	float v_frames = float(particles_anim_v_frames);
	float particle_total_frames = float(particles_anim_h_frames * particles_anim_v_frames);
	float particle_frame = floor(INSTANCE_CUSTOM.z * float(particle_total_frames));
	if (!particles_anim_loop) {
		particle_frame = clamp(particle_frame, 0.0, particle_total_frames - 1.0);
	} else {
		particle_frame = mod(particle_frame, particle_total_frames);
	}
	UV /= vec2(h_frames, v_frames);
	UV += vec2(mod(particle_frame, h_frames) / h_frames, floor((particle_frame + 0.5) / h_frames) / v_frames);
}






void fragment() {
	vec2 base_uv = UV;
	vec4 albedo_tex = texture(texture_albedo,base_uv);
	ALBEDO = albedo.rgb * albedo_tex.rgb;
	float metallic_tex = dot(texture(texture_metallic,base_uv),metallic_texture_channel);
	METALLIC = metallic_tex * metallic;
	vec4 roughness_texture_channel = vec4(1.0,0.0,0.0,0.0);
	float roughness_tex = dot(texture(texture_roughness,base_uv),roughness_texture_channel);
	ROUGHNESS = roughness_tex * roughness;
	SPECULAR = specular;
}

Observe the error

Minimal reproduction project

see above

[clayjohn]

I think this is a documentation issue. The MODEL_MATRIX isn't used after the vertex shader and it doesn't carry over to the fragment shader. So the fact that it was previously writable was pretty misleading. I ended up making it const in #66178 to avoid the issue where users right to it and their write gets ignored

[tetrapod00]

The docs for this were changed in godotengine/godot-docs#9506 to mark MODEL_MATRIX in vertex() as in rather than inout.

So I think this is resolved now? Feel free to comment if you think this should be reopened.

I'm not sure if something needs to be done to track this in the VFX and Techart wishlist project.

I marked it as 4.3 because that's when the docs PR was done.