ImFontAtlas: Added 'bool TexPixelsUseColors' output to help backend decide of underlying texture format. (#3369, #3844)

docs/CHANGELOG.txt

@@ -51,6 +51,8 @@ Other Changes:
 - Window: Shrink close button hit-testing region when it covers an abnormally high portion of the window visible
   area (e.g. when window is collapsed + moved in a corner) to facilitate moving the window away. (#3825)
 - Added GetAllocatorFunctions() to facilitate sharing allocators accross DLL boundaries. (#3836)
+- ImFontAtlas: Added 'bool TexPixelsUseColors' output to help backend decide of underlying texture format. (#3369)
+  This can currently only ever be set by the Freetype renderer.
 - ImDrawList: AddCircle, AddCircleFilled(): Tweaked default segment count calculation to honor MaxError
   with more accuracy. Made default segment count always even for better looking result. (#3808) [@thedmd]
 - ImDrawList: AddCircle, AddCircleFilled(): New default for style.

imgui.h

@@ -2639,11 +2639,12 @@ struct ImFontAtlas
     //-------------------------------------------
 
     // You can request arbitrary rectangles to be packed into the atlas, for your own purposes.
-    // After calling Build(), you can query the rectangle position and render your pixels.
-    // You can also request your rectangles to be mapped as font glyph (given a font + Unicode point),
+    // - After calling Build(), you can query the rectangle position and render your pixels.
+    // - If you render colored output, set 'atlas->TexPixelsUseColors = true' as this may help some backends decide of prefered texture format.
+    // - You can also request your rectangles to be mapped as font glyph (given a font + Unicode point),
     //   so you can render e.g. custom colorful icons and use them as regular glyphs.
-    // Read docs/FONTS.md for more details about using colorful icons.
-    // Note: this API may be redesigned later in order to support multi-monitor varying DPI settings.
+    // - Read docs/FONTS.md for more details about using colorful icons.
+    // - Note: this API may be redesigned later in order to support multi-monitor varying DPI settings.
     IMGUI_API int               AddCustomRectRegular(int width, int height);
     IMGUI_API int               AddCustomRectFontGlyph(ImFont* font, ImWchar id, int width, int height, float advance_x, const ImVec2& offset = ImVec2(0, 0));
     ImFontAtlasCustomRect*      GetCustomRectByIndex(int index) { IM_ASSERT(index >= 0); return &CustomRects[index]; }
@@ -2656,14 +2657,15 @@ struct ImFontAtlas
     // Members
     //-------------------------------------------
 
-    bool                        Locked;             // Marked as Locked by ImGui::NewFrame() so attempt to modify the atlas will assert.
     ImFontAtlasFlags            Flags;              // Build flags (see ImFontAtlasFlags_)
     ImTextureID                 TexID;              // User data to refer to the texture once it has been uploaded to user's graphic systems. It is passed back to you during rendering via the ImDrawCmd structure.
     int                         TexDesiredWidth;    // Texture width desired by user before Build(). Must be a power-of-two. If have many glyphs your graphics API have texture size restrictions you may want to increase texture width to decrease height.
     int                         TexGlyphPadding;    // Padding between glyphs within texture in pixels. Defaults to 1. If your rendering method doesn't rely on bilinear filtering you may set this to 0.
+    bool                        Locked;             // Marked as Locked by ImGui::NewFrame() so attempt to modify the atlas will assert.
 
     // [Internal]
     // NB: Access texture data via GetTexData*() calls! Which will setup a default font for you.
+    bool                        TexPixelsUseColors; // Tell whether our texture data is known to use colors (rather than just alpha channel), in order to help backend select a format.
     unsigned char*              TexPixelsAlpha8;    // 1 component per pixel, each component is unsigned 8-bit. Total size = TexWidth * TexHeight
     unsigned int*               TexPixelsRGBA32;    // 4 component per pixel, each component is unsigned 8-bit. Total size = TexWidth * TexHeight * 4
     int                         TexWidth;           // Texture width calculated during Build().

imgui_draw.cpp

@@ -1824,6 +1824,7 @@ void    ImFontAtlas::ClearTexData()
         IM_FREE(TexPixelsRGBA32);
     TexPixelsAlpha8 = NULL;
     TexPixelsRGBA32 = NULL;
+    TexPixelsUseColors = false;
 }
 
 void    ImFontAtlas::ClearFonts()

misc/freetype/imgui_freetype.cpp

@@ -6,6 +6,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2021/03/02: set 'atlas->TexPixelsUseColors = true' to help some backends with deciding of a prefered texture format.
 //  2021/01/28: added support for color-layered glyphs via ImGuiFreeTypeBuilderFlags_LoadColor (require Freetype 2.10+).
 //  2021/01/26: simplified integration by using '#define IMGUI_ENABLE_FREETYPE'.
 //              renamed ImGuiFreeType::XXX flags to ImGuiFreeTypeBuilderFlags_XXX for consistency with other API. removed ImGuiFreeType::BuildFontAtlas().
@@ -105,7 +106,7 @@ namespace
         FT_Int      OffsetX;            // The distance from the origin ("pen position") to the left of the glyph.
         FT_Int      OffsetY;            // The distance from the origin to the top of the glyph. This is usually a value < 0.
         float       AdvanceX;           // The distance from the origin to the origin of the next glyph. This is usually a value > 0.
-        bool        IsColored;
+        bool        IsColored;          // The glyph is colored
     };
 
     // Font parameters and metrics.
@@ -605,6 +606,7 @@ bool ImFontAtlasBuildWithFreeTypeEx(FT_Library ft_library, ImFontAtlas* atlas, u
 
     // 8. Copy rasterized font characters back into the main texture
     // 9. Setup ImFont and glyphs for runtime
+    bool tex_use_colors = false;
     for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
     {
         ImFontBuildSrcDataFT& src_tmp = src_tmp_array[src_i];
@@ -668,13 +670,15 @@ bool ImFontAtlasBuildWithFreeTypeEx(FT_Library ft_library, ImFontAtlas* atlas, u
             float v1 = (ty + info.Height) / (float)atlas->TexHeight;
             dst_font->AddGlyph(&cfg, (ImWchar)src_glyph.Codepoint, x0, y0, x1, y1, u0, v0, u1, v1, info.AdvanceX);
 
-            IM_ASSERT(dst_font->Glyphs.back().Codepoint == src_glyph.Codepoint);
+            ImFontGlyph* dst_glyph = &dst_font->Glyphs.back();
+            IM_ASSERT(dst_glyph->Codepoint == src_glyph.Codepoint);
             if (src_glyph.Info.IsColored)
-                dst_font->Glyphs.back().Colored = true;
+                dst_glyph->Colored = tex_use_colors = true;
         }
 
         src_tmp.Rects = NULL;
     }
+    atlas->TexPixelsUseColors = tex_use_colors;
 
     // Cleanup
     for (int buf_i = 0; buf_i < buf_bitmap_buffers.Size; buf_i++)