Method TableSetupDrawChannels

src/imgui/imgui_tables.cpp:2530–2567

Allocate draw channels. Called by TableUpdateLayout() - We allocate them following storage order instead of display order so reordering columns won't needlessly increase overall dormant memory cost. - We isolate headers draw commands in their own channels instead of just altering clip rects. This is in order to facilitate merging of draw commands. - After crossing FreezeRowsCount, all columns see

// - FreezeRows || FreezeColunns  --> 3+D+N*2 (unless scrolling value is zero)
// Where D is 1 if any column is clipped or hidden (dummy channel) otherwise 0.
void ImGui::TableSetupDrawChannels(ImGuiTable* table)
{
    const int freeze_row_multiplier = (table->FreezeRowsCount > 0) ? 2 : 1;
    const int channels_for_row = (table->Flags & ImGuiTableFlags_NoClip) ? 1 : table->ColumnsEnabledCount;
    const int channels_for_bg = 1 + 1 * freeze_row_multiplier;
    const int channels_for_dummy = (table->ColumnsEnabledCount < table->ColumnsCount || (memcmp(table->VisibleMaskByIndex, table->EnabledMaskByIndex, ImBitArrayGetStorageSizeInBytes(table->ColumnsCount)) != 0)) ? +1 : 0;
    const int channels_total = channels_for_bg + (channels_for_row * freeze_row_multiplier) + channels_for_dummy;
    table->DrawSplitter->Split(table->InnerWindow->DrawList, channels_total);
    table->DummyDrawChannel = (ImGuiTableDrawChannelIdx)((channels_for_dummy > 0) ? channels_total - 1 : -1);
    table->Bg2DrawChannelCurrent = TABLE_DRAW_CHANNEL_BG2_FROZEN;
    table->Bg2DrawChannelUnfrozen = (ImGuiTableDrawChannelIdx)((table->FreezeRowsCount > 0) ? 2 + channels_for_row : TABLE_DRAW_CHANNEL_BG2_FROZEN);

    int draw_channel_current = 2;
    for (int column_n = 0; column_n < table->ColumnsCount; column_n++)
    {
        ImGuiTableColumn* column = &table->Columns[column_n];
        if (column->IsVisibleX && column->IsVisibleY)
        {
            column->DrawChannelFrozen = (ImGuiTableDrawChannelIdx)(draw_channel_current);
            column->DrawChannelUnfrozen = (ImGuiTableDrawChannelIdx)(draw_channel_current + (table->FreezeRowsCount > 0 ? channels_for_row + 1 : 0));
            if (!(table->Flags & ImGuiTableFlags_NoClip))
                draw_channel_current++;
        }
        else
        {
            column->DrawChannelFrozen = column->DrawChannelUnfrozen = table->DummyDrawChannel;
        }
        column->DrawChannelCurrent = column->DrawChannelFrozen;
    }

    // Initial draw cmd starts with a BgClipRect that matches the one of its host, to facilitate merge draw commands by default.
    // All our cell highlight are manually clipped with BgClipRect. When unfreezing it will be made smaller to fit scrolling rect.
    // (This technically isn't part of setting up draw channels, but is reasonably related to be done here)
    table->BgClipRect = table->InnerClipRect;
    table->Bg0ClipRectForDrawCmd = table->OuterWindow->ClipRect;
    table->Bg2ClipRectForDrawCmd = table->HostClipRect;
    IM_ASSERT(table->BgClipRect.Min.y <= table->BgClipRect.Max.y);
}

// This function reorder draw channels based on matching clip rectangle, to facilitate merging them. Called by EndTable().
// For simplicity we call it TableMergeDrawChannels() but in fact it only reorder channels + overwrite ClipRect,