[Mesa-dev] sampler arrays indexed with non-constant expressions

Victor Luchitz vluchits at gmail.com
Sun Nov 17 00:24:49 PST 2013


in my opinion GLSL compiler in mesa is too restrictive when it comes to
sampler arrays. The following code can not be compiled due to the "sampler
arrays indexed with non-constant expressions is forbidden in GLSL 1.30 and

#define MAX_SHADOWS 2

varying vec4 v_ShadowProjVector[MAX_SHADOWS];

uniform sampler2DShadow u_ShadowmapTexture[MAX_SHADOWS];
# define qfShadow2D(t,v) float(shadow2D(t,v))

uniform float u_ShadowAlpha;
uniform float u_ShadowProjDistance[MAX_SHADOWS];
uniform vec4 u_ShadowmapTextureParams[MAX_SHADOWS];

void main(void)
    float finalcolor = 1.0;

    for (int i = 0; i < MAX_SHADOWS; i++)
        vec3 shadowmaptc = vec3(v_ShadowProjVector[i].xyz /

        // this keeps shadows from appearing on surfaces behind frustum's
        float d = step(v_ShadowProjVector[i].w, 0.0);

        //shadowmaptc = (shadowmaptc + vec3 (1.0)) * vec3 (0.5);
        shadowmaptc.xy = shadowmaptc.xy * u_ShadowmapTextureParams[i].xy;
// .x - texture width
        shadowmaptc.z = clamp(shadowmaptc.z, 0.0, 1.0);
        shadowmaptc.xy = vec2(clamp(shadowmaptc.x, 0.0,
u_ShadowmapTextureParams[i].x), clamp(shadowmaptc.y, 0.0,

        vec2 ShadowMap_TextureScale = u_ShadowmapTextureParams[i].zw;

        float f;

        f = qfShadow2D(u_ShadowmapTexture[i], vec3(shadowmaptc.xy *
ShadowMap_TextureScale, shadowmaptc.z));

        finalcolor *= clamp(max(max(f, d), u_ShadowAlpha), 0.0, 1.0);

    gl_FragColor = vec4(vec3(finalcolor),1.0);

Lines 159-136 of src/glsl/ast_array_index.cpp say:

    * This restriction was added in GLSL 1.30.  Shaders using earlier
    * of the language should not be rejected by the compiler front-end for
    * using this construct.  This allows useful things such as using a loop
    * counter as the index to an array of samplers.  If the loop in
    * the code should compile correctly.  Instead, emit a warning.

If compiler actually attempted to unroll the loop above, it would notice
that the does compile correctly. Instead it just emits and error and in my
opinion, contradicts the comment above but not allowing the "useful thing"
in question.

Can the compiler be changed to _first_ attempt to unroll the loop and then
check for sampler array indices being constants?

Best regards,
 Victor Luchitz
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