fix stencil random every stroke and not sample

engine/app_shaders.cpp

@@ -335,7 +335,7 @@ void App::initShaders()
         //"highp float rand(mediump vec2 co) { return fract(sin(dot(co.xy, vec2(12.9898, 78.233))) * 43758.5453); }\n"
         "void main(){\n"
         "   mediump vec2 uv2 = gl_FragCoord.st / resolution;\n"
-        "   mediump float stencil = 1.0 - (texture(tex_stencil, (uv2+stencil_offset) * 2.0).r * 0.9) * stencil_alpha;\n"
+        "   mediump float stencil = 1.0 - (texture(tex_stencil, (uv2+stencil_offset) * 5.0).r) * stencil_alpha;\n"
         "   mediump float brush_alpha = ( 1.0 - texture(tex, uv/q).r ) * alpha;\n"
         "   mediump vec4 fg = vec4(col.rgb, brush_alpha * stencil);\n"
 #ifdef __IOS__

engine/canvas.cpp

@@ -246,7 +246,7 @@ void ui::Canvas::stroke_draw()
             ShaderManager::u_int(kShaderUniform::TexStencil, 3); // stencil
             ShaderManager::u_vec4(kShaderUniform::Col, m_brush.m_tip_color);
             ShaderManager::u_vec2(kShaderUniform::Resolution, { m_width, m_height });
-            ShaderManager::u_vec2(kShaderUniform::StencilOffset, glm::vec2((rand()%1000)*0.001f, (rand()%1000)*0.001f));
+            ShaderManager::u_vec2(kShaderUniform::StencilOffset, stencil_offset);
             ShaderManager::u_float(kShaderUniform::StencilAlpha, m_brush.m_tip_stencil);
             ShaderManager::u_float(kShaderUniform::Wet, m_brush.m_tip_wet);
             ShaderManager::u_float(kShaderUniform::Noise, m_brush.m_tip_noise);
@@ -578,6 +578,8 @@ void ui::Canvas::stroke_start(glm::vec2 point, float pressure, const ui::Brush&
         m_show_tmp = false;
         m_commit_delayed = false;
     }
+    
+    stencil_offset = glm::vec2((rand()%1000)*0.001f, (rand()%1000)*0.001f);
 
     m_current_stroke = std::make_unique<Stroke>();
     m_current_stroke->m_camera.rot = m_cam_rot;

engine/canvas.h

@@ -80,6 +80,7 @@ public:
     static glm::vec3 m_plane_normal[6];
     static glm::vec3 m_plane_tangent[6];
     static glm::mat4 m_plane_transform[6];
+    glm::vec2 stencil_offset;
     Sampler m_sampler;
     Sampler m_sampler_brush;
     Sampler m_sampler_bg;